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5.0.2 (2024-02-29)

  • Base64: Fix issue where dirty memory located just after the input buffer is affecting the result. (#4926)

5.0.1 (2023-12-07)

  • ERC2771Context and Context: Introduce a _contextPrefixLength() getter, used to trim extra information appended to
  • Multicall: Make aware of non-canonical context (i.e. msg.sender is not _msgSender()), allowing compatibility with ERC2771Context.

5.0.0 (2023-10-05)

Additions Summary

The following contracts and libraries were added:

  • AccessManager: A consolidated system for managing access control in complex systems.
    • AccessManaged: A module for connecting a contract to an authority in charge of its access control.
    • GovernorTimelockAccess: An adapter for time-locking governance proposals using an AccessManager.
    • AuthorityUtils: A library of utilities for interacting with authority contracts.
  • GovernorStorage: A Governor module that stores proposal details in storage.
  • ERC2771Forwarder: An ERC2771 forwarder for meta transactions.
  • ERC1967Utils: A library with ERC1967 events, errors and getters.
  • Nonces: An abstraction for managing account nonces.
  • MessageHashUtils: A library for producing digests for ECDSA operations.
  • Time: A library with helpers for manipulating time-related objects.

Removals Summary

The following contracts, libraries, and functions were removed:

  • Address.isContract (because of its ambiguous nature and potential for misuse)
  • Checkpoints.History
  • Counters
  • ERC20Snapshot
  • ERC20VotesComp
  • ERC165Storage (in favor of inheritance based approach)
  • ERC777
  • ERC1820Implementer
  • GovernorVotesComp
  • GovernorProposalThreshold (deprecated since 4.4)
  • PaymentSplitter
  • PullPayment
  • SafeMath
  • SignedSafeMath
  • Timers
  • TokenTimelock (in favor of VestingWallet)
  • All escrow contracts (Escrow, ConditionalEscrow and RefundEscrow)
  • All cross-chain contracts, including AccessControlCrossChain and all the vendored bridge interfaces
  • All presets in favor of OpenZeppelin Contracts Wizard

These removals were implemented in the following PRs: #3637, #3880, #3945, #4258, #4276, #4289

Changes by category


  • Replaced revert strings and require statements with custom errors. (#4261)
  • Bumped minimum compiler version required to 0.8.20 (#4288)
  • Use of abi.encodeCall in place of abi.encodeWithSelector and abi.encodeWithSignature for improved type-checking of parameters (#4293)
  • Replaced some uses of abi.encodePacked with clearer alternatives (e.g. bytes.concat, string.concat). (#4504) (#4296)
  • Overrides are now used internally for a number of functions that were previously hardcoded to their default implementation in certain locations: ERC1155Supply.totalSupply, ERC721.ownerOf, ERC721.balanceOf and ERC721.totalSupply in ERC721Enumerable, ERC20.totalSupply in ERC20FlashMint, and ERC1967._getImplementation in ERC1967Proxy. (#4299)
  • Removed the override specifier from functions that only override a single interface function. (#4315)
  • Switched to using explicit Solidity import statements. Some previously available symbols may now have to be separately imported. (#4399)
  • Governor, Initializable, and UUPSUpgradeable: Use internal functions in modifiers to optimize bytecode size. (#4472)
  • Upgradeable contracts now use namespaced storage (EIP-7201). (#4534)
  • Upgradeable contracts no longer transpile interfaces and libraries. (#4628)


  • Ownable: Added an initialOwner parameter to the constructor, making the ownership initialization explicit. (#4267)
  • Ownable: Prevent using address(0) as the initial owner. (#4531)
  • AccessControl: Added a boolean return value to the internal _grantRole and _revokeRole functions indicating whether the role was granted or revoked. (#4241)
  • access: Moved AccessControl extensions to a dedicated directory. (#4359)
  • AccessManager: Added a new contract for managing access control of complex systems in a consolidated location. (#4121)
  • AccessManager, AccessManaged, GovernorTimelockAccess: Ensure that calldata shorter than 4 bytes is not padded to 4 bytes. (#4624)
  • AccessManager: Use named return parameters in functions that return multiple values. (#4624)
  • AccessManager: Make schedule and execute more conservative when delay is 0. (#4644)


  • VestingWallet: Fixed revert during 1 second time window when duration is 0. (#4502)
  • VestingWallet: Use Ownable instead of an immutable beneficiary. (#4508)


  • Governor: Optimized use of storage for proposal data (#4268)
  • Governor: Added validation in ERC1155 and ERC721 receiver hooks to ensure Governor is the executor. (#4314)
  • Governor: Refactored internals to implement common queuing logic in the core module of the Governor. Added queue and _queueOperations functions that act at different levels. Modules that implement queuing via timelocks are expected to override _queueOperations to implement the timelock-specific logic. Added _executeOperations as the equivalent for execution. (#4360)
  • Governor: Added voter and nonce parameters in signed ballots, to avoid forging signatures for random addresses, prevent signature replay, and allow invalidating signatures. Add voter as a new parameter in the castVoteBySig and castVoteWithReasonAndParamsBySig functions. (#4378)
  • Governor: Added support for casting votes with ERC-1271 signatures by using a bytes memory signature instead of r, s and v arguments in the castVoteBySig and castVoteWithReasonAndParamsBySig functions. (#4418)
  • Governor: Added a mechanism to restrict the address of the proposer using a suffix in the description.
  • GovernorStorage: Added a new governor extension that stores the proposal details in storage, with an interface that operates on proposalId, as well as proposal enumerability. This replaces the old GovernorCompatibilityBravo module. (#4360)
  • GovernorTimelockAccess: Added a module to connect a governor with an instance of AccessManager, allowing the governor to make calls that are delay-restricted by the manager using the normal queue workflow. (#4523)
  • GovernorTimelockControl: Clean up timelock id on execution for gas refund. (#4118)
  • GovernorTimelockControl: Added the Governor instance address as part of the TimelockController operation salt to avoid operation id collisions between governors using the same TimelockController. (#4432)
  • TimelockController: Changed the role architecture to use DEFAULT_ADMIN_ROLE as the admin for all roles, instead of the bespoke TIMELOCK_ADMIN_ROLE that was used previously. This aligns with the general recommendation for AccessControl and makes the addition of new roles easier. Accordingly, the admin parameter and timelock will now be granted DEFAULT_ADMIN_ROLE instead of TIMELOCK_ADMIN_ROLE. (#3799)
  • TimelockController: Added a state getter that returns an OperationState enum. (#4358)
  • Votes: Use Trace208 for checkpoints. This enables EIP-6372 clock support for keys but reduces the max supported voting power to uint208. (#4539)


  • ERC2771Forwarder: Added deadline for expiring transactions, batching, and more secure handling of msg.value. (#4346)
  • ERC2771Context: Return the forwarder address whenever the of a call originating from a trusted forwarder is not long enough to contain the request signer address (i.e. is less than 20 bytes), as specified by ERC-2771. (#4481)
  • ERC2771Context: Prevent revert in _msgData() when a call originating from a trusted forwarder is not long enough to contain the request signer address (i.e. is less than 20 bytes). Return the full calldata in that case. (#4484)


  • ProxyAdmin: Removed getProxyAdmin and getProxyImplementation getters. (#3820)
  • TransparentUpgradeableProxy: Removed admin and implementation getters, which were only callable by the proxy owner and thus not very useful. (#3820)
  • ERC1967Utils: Refactored the ERC1967Upgrade abstract contract as a library. (#4325)
  • TransparentUpgradeableProxy: Admin is now stored in an immutable variable (set during construction) to avoid unnecessary storage reads on every proxy call. This removed the ability to ever change the admin. Transfer of the upgrade capability is exclusively handled through the ownership of the ProxyAdmin. (#4354)
  • Moved the logic to validate ERC-1822 during an upgrade from ERC1967Utils to UUPSUpgradeable. (#4356)
  • UUPSUpgradeable, TransparentUpgradeableProxy and ProxyAdmin: Removed upgradeTo and upgrade functions, and made upgradeToAndCall and upgradeAndCall ignore the data argument if it is empty. It is no longer possible to invoke the receive function (or send value with empty data) along with an upgrade. (#4382)
  • BeaconProxy: Reject value in initialization unless a payable function is explicitly invoked. (#4382)
  • Proxy: Removed redundant receive function. (#4434)
  • BeaconProxy: Use an immutable variable to store the address of the beacon. It is no longer possible for a BeaconProxy to upgrade by changing to another beacon. (#4435)
  • Initializable: Use the namespaced storage pattern to avoid putting critical variables in slot 0. Allow reinitializer versions greater than 256. (#4460)
  • Initializable: Use intermediate variables to improve readability. (#4576)


  • ERC20, ERC721, ERC1155: Deleted _beforeTokenTransfer and _afterTokenTransfer hooks, added a new internal _update function for customizations, and refactored all extensions using those hooks to use _update instead. (#3838, #3876, #4377)
  • ERC20: Removed Approval event previously emitted in transferFrom to indicate that part of the allowance was consumed. With this change, allowances are no longer reconstructible from events. See the code for guidelines on how to re-enable this event if needed. (#4370)
  • ERC20: Removed the non-standard increaseAllowance and decreaseAllowance functions. (#4585)
  • ERC20Votes: Changed internal vote accounting to reusable Votes module previously used by ERC721Votes. Removed implicit ERC20Permit inheritance. Note that the DOMAIN_SEPARATOR getter was previously guaranteed to be available for ERC20Votes contracts, but is no longer available unless ERC20Permit is explicitly used; ERC-5267 support is included in ERC20Votes with EIP712 and is recommended as an alternative. (#3816)
  • SafeERC20: Refactored safeDecreaseAllowance and safeIncreaseAllowance to support USDT-like tokens. (#4260)
  • SafeERC20: Removed safePermit in favor of documentation-only permit recommendations. Based on recommendations from @trust1995 (#4582)
  • ERC721: _approve no longer allows approving the owner of the tokenId. (#4377) _setApprovalForAll no longer allows setting address(0) as an operator. (#4377)
  • ERC721: Renamed _requireMinted to _requireOwned and added a return value with the current owner. Implemented ownerOf in terms of _requireOwned. (#4566)
  • ERC721Consecutive: Added a _firstConsecutiveId internal function that can be overridden to change the id of the first token minted through _mintConsecutive. (#4097)
  • ERC721URIStorage: Allow setting the token URI prior to minting. (#4559)
  • ERC721URIStorage, ERC721Royalty: Stop resetting token-specific URI and royalties when burning. (#4561)
  • ERC1155: Optimized array allocation. (#4196)
  • ERC1155: Removed check for address zero in balanceOf. (#4263)
  • ERC1155: Optimized array accesses by skipping bounds checking when unnecessary. (#4300)
  • ERC1155: Bubble errors triggered in the onERC1155Received and onERC1155BatchReceived hooks. (#4314)
  • ERC1155Supply: Added a totalSupply() function that returns the total amount of token circulating, this change will restrict the total tokens minted across all ids to 2**256-1 . (#3962)
  • ERC1155Receiver: Removed in favor of ERC1155Holder. (#4450)


  • Address: Removed the ability to customize error messages. A common custom error is always used if the underlying revert reason cannot be bubbled up. (#4502)
  • Arrays: Added unsafeMemoryAccess helpers to read from a memory array without checking the length. (#4300)
  • Arrays: Optimized findUpperBound by removing redundant SLOAD. (#4442)
  • Checkpoints: Library moved from utils to utils/structs (#4275)
  • DoubleEndedQueue: Refactored internal structure to use uint128 instead of int128. This has no effect on the library interface. (#4150)
  • ECDSA: Use unchecked arithmetic for the tryRecover function that receives the r and vs short-signature fields separately. (#4301)
  • EIP712: Added internal getters for the name and version strings (#4303)
  • Math: Makes ceilDiv to revert on 0 division even if the numerator is 0 (#4348)
  • Math: Optimized stack operations in mulDiv. (#4494)
  • Math: Renamed members of Rounding enum, and added a new rounding mode for "away from zero". (#4455)
  • MerkleProof: Use custom error to report invalid multiproof instead of reverting with overflow panic. (#4564)
  • MessageHashUtils: Added a new library for creating message digest to be used along with signing or recovery such as ECDSA or ERC-1271. These functions are moved from the ECDSA library. (#4430)
  • Nonces: Added a new contract to keep track of user nonces. Used for signatures in ERC20Permit, ERC20Votes, and ERC721Votes. (#3816)
  • ReentrancyGuard, Pausable: Moved to utils directory. (#4551)
  • Strings: Renamed toString(int256) to toStringSigned(int256). (#4330)
  • Optimized Strings.equal (#4262)

How to migrate from 4.x

ERC20, ERC721, and ERC1155

These breaking changes will require modifications to ERC20, ERC721, and ERC1155 contracts, since the _afterTokenTransfer and _beforeTokenTransfer functions were removed. Thus, any customization made through those hooks should now be done overriding the new _update function instead.

Minting and burning are implemented by _update and customizations should be done by overriding this function as well. _transfer, _mint and _burn are no longer virtual (meaning they are not overridable) to guard against possible inconsistencies.

For example, a contract using ERC20's _beforeTokenTransfer hook would have to be changed in the following way.

-function _beforeTokenTransfer(
+function _update(
   address from,
   address to,
   uint256 amount
 ) internal virtual override {
-  super._beforeTokenTransfer(from, to, amount);
   require(!condition(), "ERC20: wrong condition");
+  super._update(from, to, amount);

More about ERC721

In the case of ERC721, the _update function does not include a from parameter, as the sender is implicitly the previous owner of the tokenId. The address of this previous owner is returned by the _update function, so it can be used for a posteriori checks. In addition to to and tokenId, a third parameter (auth) is present in this function. This parameter enabled an optional check that the caller/spender is approved to do the transfer. This check cannot be performed after the transfer (because the transfer resets the approval), and doing it before _update would require a duplicate call to _ownerOf.

In this logic of removing hidden SLOADs, the _isApprovedOrOwner function was removed in favor of a new _isAuthorized function. Overrides that used to target the _isApprovedOrOwner should now be performed on the _isAuthorized function. Calls to _isApprovedOrOwner that preceded a call to _transfer, _burn or _approve should be removed in favor of using the auth argument in _update and _approve. This is showcased in ERC721Burnable.burn and in ERC721Wrapper.withdrawTo.

The _exists function was removed. Calls to this function can be replaced by _ownerOf(tokenId) != address(0).

More about ERC1155

Batch transfers will now emit TransferSingle if the batch consists of a single token, while in previous versions the TransferBatch event would be used for all transfers initiated through safeBatchTransferFrom. Both behaviors are compliant with the ERC-1155 specification.


Users that were registering EIP-165 interfaces with _registerInterface from ERC165Storage should instead do so so by overriding the supportsInterface function as seen below:

function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
  return interfaceId == type(MyInterface).interfaceId || super.supportsInterface(interfaceId);


Methods in SafeMath superseded by native overflow checks in Solidity 0.8.0 were removed along with operations providing an interface for revert strings. The remaining methods were moved to utils/Math.sol.

- import "@openzeppelin/contracts/utils/math/SafeMath.sol";
+ import "@openzeppelin/contracts/utils/math/Math.sol";

 function tryOperations(uint256 x, uint256 y) external view {
-  (bool overflowsAdd, uint256 resultAdd) = SafeMath.tryAdd(x, y);
+  (bool overflowsAdd, uint256 resultAdd) = Math.tryAdd(x, y);
-  (bool overflowsSub, uint256 resultSub) = SafeMath.trySub(x, y);
+  (bool overflowsSub, uint256 resultSub) = Math.trySub(x, y);
-  (bool overflowsMul, uint256 resultMul) = SafeMath.tryMul(x, y);
+  (bool overflowsMul, uint256 resultMul) = Math.tryMul(x, y);
-  (bool overflowsDiv, uint256 resultDiv) = SafeMath.tryDiv(x, y);
+  (bool overflowsDiv, uint256 resultDiv) = Math.tryDiv(x, y);
   // ...

Adapting Governor modules

Custom Governor modules that override internal functions may require modifications if migrated to v5. In particular, the new internal functions _queueOperations and _executeOperations may need to be used. If assistance with this migration is needed reach out via the OpenZeppelin Support Forum.

ECDSA and MessageHashUtils

The ECDSA library is now focused on signer recovery. Previously it also included utility methods for producing digests to be used with signing or recovery. These utilities have been moved to the MessageHashUtils library and should be imported if needed:

 import {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
+import {MessageHashUtils} from "@openzeppelin/contracts/utils/cryptography/MessageHashUtils.sol";

 contract Verifier {
   using ECDSA for bytes32;
+  using MessageHashUtils for bytes32;

   function _verify(bytes32 data, bytes memory signature, address account) internal pure returns (bool) {
     return data
       .recover(signature) == account;

Interfaces and libraries in upgradeable contracts

The upgradeable version of the contracts library used to include a variant suffixed with Upgradeable for every contract. These variants, which are produced automatically, mainly include changes for dealing with storage that don't apply to libraries and interfaces.

The upgradeable library no longer includes upgradeable variants for libraries and interfaces. Projects migrating to 5.0 should replace their library and interface imports with their corresponding non-upgradeable version:

 // Libraries
-import {AddressUpgradeable} from '@openzeppelin/contracts-upgradeable/utils/AddressUpgradeable.sol';
+import {Address} from '@openzeppelin/contracts/utils/Address.sol';

 // Interfaces
-import {IERC20Upgradeable} from '@openzeppelin/contracts-upgradeable/interfaces/IERC20.sol';
+import {IERC20} from '@openzeppelin/contracts/interfaces/IERC20.sol';

Offchain Considerations

Some changes may affect offchain systems if they rely on assumptions that are changed along with these new breaking changes. These cases are:

Relying on revert strings for processing errors

A concrete example is AccessControl, where it was previously advised to catch revert reasons using the following regex:

/^AccessControl: account (0x[0-9a-f]{40}) is missing role (0x[0-9a-f]{64})$/

Instead, contracts now revert with custom errors. Systems that interact with smart contracts outside of the network should consider reliance on revert strings and possibly support the new custom errors.

Relying on storage locations for retrieving data

After 5.0, the storage location of some variables were changed. This is the case for Initializable and all the upgradeable contracts since they now use namespaced storaged locations. Any system relying on storage locations for retrieving data or detecting capabilities should be updated to support these new locations.

4.9.2 (2023-06-16)

  • MerkleProof: Fix a bug in processMultiProof and processMultiProofCalldata that allows proving arbitrary leaves if the tree contains a node with value 0 at depth 1.

4.9.1 (2023-06-07)

  • Governor: Add a mechanism to restrict the address of the proposer using a suffix in the description.

4.9.0 (2023-05-23)

  • ReentrancyGuard: Add a _reentrancyGuardEntered function to expose the guard status. (#3714)
  • ERC721Wrapper: add a new extension of the ERC721 token which wraps an underlying token. Deposit and withdraw guarantee that the ownership of each token is backed by a corresponding underlying token with the same identifier. (#3863)
  • EnumerableMap: add a keys() function that returns an array containing all the keys. (#3920)
  • Governor: add a public cancel(uint256) function. (#3983)
  • Governor: Enable timestamp operation for blockchains without a stable block time. This is achieved by connecting a Governor's internal clock to match a voting token's EIP-6372 interface. (#3934)
  • Strings: add equal method. (#3774)
  • IERC5313: Add an interface for EIP-5313 that is now final. (#4013)
  • IERC4906: Add an interface for ERC-4906 that is now Final. (#4012)
  • StorageSlot: Add support for string and bytes. (#4008)
  • Votes, ERC20Votes, ERC721Votes: support timestamp checkpointing using EIP-6372. (#3934)
  • ERC4626: Add mitigation to the inflation attack through virtual shares and assets. (#3979)
  • Strings: add toString method for signed integers. (#3773)
  • ERC20Wrapper: Make the underlying variable private and add a public accessor. (#4029)
  • EIP712: add EIP-5267 support for better domain discovery. (#3969)
  • AccessControlDefaultAdminRules: Add an extension of AccessControl with additional security rules for the DEFAULT_ADMIN_ROLE. (#4009)
  • SignatureChecker: Add isValidERC1271SignatureNow for checking a signature directly against a smart contract using ERC-1271. (#3932)
  • SafeERC20: Add a forceApprove function to improve compatibility with tokens behaving like USDT. (#4067)
  • ERC1967Upgrade: removed contract-wide oz-upgrades-unsafe-allow delegatecall annotation, replaced by granular annotation in UUPSUpgradeable. (#3971)
  • ERC20Wrapper: self wrapping and deposit by the wrapper itself are now explicitly forbidden. (#4100)
  • ECDSA: optimize bytes32 computation by using assembly instead of abi.encodePacked. (#3853)
  • ERC721URIStorage: Emit ERC-4906 MetadataUpdate in _setTokenURI. (#4012)
  • ShortStrings: Added a library for handling short strings in a gas efficient way, with fallback to storage for longer strings. (#4023)
  • SignatureChecker: Allow return data length greater than 32 from EIP-1271 signers. (#4038)
  • UUPSUpgradeable: added granular oz-upgrades-unsafe-allow-reachable annotation to improve upgrade safety checks on latest version of the Upgrades Plugins (starting with @openzeppelin/upgrades-core@1.21.0). (#3971)
  • Initializable: optimize _disableInitializers by using != instead of <. (#3787)
  • Ownable2Step: make acceptOwnership public virtual to enable usecases that require overriding it. (#3960)
  • UUPSUpgradeable.sol: Change visibility to the functions upgradeTo and upgradeToAndCall from external to public. (#3959)
  • TimelockController: Add the CallSalt event to emit on operation schedule. (#4001)
  • Reformatted codebase with latest version of Prettier Solidity. (#3898)
  • Math: optimize log256 rounding check. (#3745)
  • ERC20Votes: optimize by using unchecked arithmetic. (#3748)
  • Multicall: annotate multicall function as upgrade safe to not raise a flag for its delegatecall. (#3961)
  • ERC20Pausable, ERC721Pausable, ERC1155Pausable: Add note regarding missing public pausing functionality (#4007)
  • ECDSA: Add a function toDataWithIntendedValidatorHash that encodes data with version 0x00 following EIP-191. (#4063)
  • MerkleProof: optimize by using unchecked arithmetic. (#3745)

Breaking changes

  • EIP712: Addition of ERC5267 support requires support for user defined value types, which was released in Solidity version 0.8.8. This requires a pragma change from ^0.8.0 to ^0.8.8.
  • EIP712: Optimization of the cache for the upgradeable version affects the way name and version are set. This is no longer done through an initializer, and is instead part of the implementation's constructor. As a consequence, all proxies using the same implementation will necessarily share the same name and version. Additionally, an implementation upgrade risks changing the EIP712 domain unless the same name and version are used when deploying the new implementation contract.


  • ERC20Permit: Added the file IERC20Permit.sol and ERC20Permit.sol and deprecated draft-IERC20Permit.sol and draft-ERC20Permit.sol since EIP-2612 is no longer a Draft. Developers are encouraged to update their imports. (#3793)
  • Timers: The Timers library is now deprecated and will be removed in the next major release. (#4062)
  • ERC777: The ERC777 token standard is no longer supported by OpenZeppelin. Our implementation is now deprecated and will be removed in the next major release. The corresponding standard interfaces remain available. (#4066)
  • ERC1820Implementer: The ERC1820 pseudo-introspection mechanism is no longer supported by OpenZeppelin. Our implementation is now deprecated and will be removed in the next major release. The corresponding standard interfaces remain available. (#4066)

4.8.3 (2023-04-13)

  • GovernorCompatibilityBravo: Fix encoding of proposal data when signatures are missing.
  • TransparentUpgradeableProxy: Fix transparency in case of selector clash with non-decodable calldata or payable mutability. (#4154)

4.8.2 (2023-03-02)

  • ERC721Consecutive: Fixed a bug when _mintConsecutive is used for batches of size 1 that could lead to balance overflow. Refer to the breaking changes section in the changelog for a note on the behavior of ERC721._beforeTokenTransfer.

Breaking changes

  • ERC721: The internal function _beforeTokenTransfer no longer updates balances, which it previously did when batchSize was greater than 1. This change has no consequence unless a custom ERC721 extension is explicitly invoking _beforeTokenTransfer. Balance updates in extensions must now be done explicitly using __unsafe_increaseBalance, with a name that indicates that there is an invariant that has to be manually verified.

4.8.1 (2023-01-12)

  • ERC4626: Use staticcall instead of call when fetching underlying ERC-20 decimals. (#3943)

4.8.0 (2022-11-08)

  • TimelockController: Added a new admin constructor parameter that is assigned the admin role instead of the deployer account. (#3722)
  • Initializable: add internal functions _getInitializedVersion and _isInitializing (#3598)
  • ERC165Checker: add supportsERC165InterfaceUnchecked for consulting individual interfaces without the full ERC165 protocol. (#3339)
  • Address: optimize functionCall by calling functionCallWithValue directly. (#3468)
  • Address: optimize functionCall functions by checking contract size only if there is no returned data. (#3469)
  • Governor: make the relay function payable, and add support for EOA payments. (#3730)
  • GovernorCompatibilityBravo: remove unused using statements. (#3506)
  • ERC20: optimize _transfer, _mint and _burn by using unchecked arithmetic when possible. (#3513)
  • ERC20Votes, ERC721Votes: optimize getPastVotes for looking up recent checkpoints. (#3673)
  • ERC20FlashMint: add an internal _flashFee function for overriding. (#3551)
  • ERC4626: use the same decimals() as the underlying asset by default (if available). (#3639)
  • ERC4626: add internal _initialConvertToShares and _initialConvertToAssets functions to customize empty vaults behavior. (#3639)
  • ERC721: optimize transfers by making approval clearing implicit instead of emitting an event. (#3481)
  • ERC721: optimize burn by making approval clearing implicit instead of emitting an event. (#3538)
  • ERC721: Fix balance accounting when a custom _beforeTokenTransfer hook results in a transfer of the token under consideration. (#3611)
  • ERC721: use unchecked arithmetic for balance updates. (#3524)
  • ERC721Consecutive: Implementation of EIP-2309 that allows batch minting of ERC721 tokens during construction. (#3311)
  • ReentrancyGuard: Reduce code size impact of the modifier by using internal functions. (#3515)
  • SafeCast: optimize downcasting of signed integers. (#3565)
  • ECDSA: Remove redundant check on the v value. (#3591)
  • VestingWallet: add releasable getters. (#3580)
  • VestingWallet: remove unused library Math.sol. (#3605)
  • VestingWallet: make constructor payable. (#3665)
  • Create2: optimize address computation by using assembly instead of abi.encodePacked. (#3600)
  • Clones: optimized the assembly to use only the scratch space during deployments, and optimized predictDeterministicAddress to use fewer operations. (#3640)
  • Checkpoints: Use procedural generation to support multiple key/value lengths. (#3589)
  • Checkpoints: Add new lookup mechanisms. (#3589)
  • Arrays: Add unsafeAccess functions that allow reading and writing to an element in a storage array bypassing Solidity's "out-of-bounds" check. (#3589)
  • Strings: optimize toString. (#3573)
  • Ownable2Step: extension of Ownable that makes the ownership transfers a two step process. (#3620)
  • Math and SignedMath: optimize function max by using > instead of >=. (#3679)
  • Math: Add log2, log10 and log256. (#3670)
  • Arbitrum: Update the vendored arbitrum contracts to match the nitro upgrade. (#3692)

Breaking changes

  • ERC721: In order to add support for batch minting via ERC721Consecutive it was necessary to make a minor breaking change in the internal interface of ERC721. Namely, the hooks _beforeTokenTransfer and _afterTokenTransfer have one additional argument that may need to be added to overrides:
 function _beforeTokenTransfer(
     address from,
     address to,
     uint256 tokenId,
+    uint256 batchSize
 ) internal virtual override
  • ERC4626: Conversion from shares to assets (and vice-versa) in an empty vault used to consider the possible mismatch between the underlying asset's and the vault's decimals. This initial conversion rate is now set to 1-to-1 irrespective of decimals, which are meant for usability purposes only. The vault now uses the assets decimals by default, so off-chain the numbers should appear the same. Developers overriding the vault decimals to a value that does not match the underlying asset may want to override the _initialConvertToShares and _initialConvertToAssets to replicate the previous behavior.

  • TimelockController: During deployment, the TimelockController used to grant the TIMELOCK_ADMIN_ROLE to the deployer and to the timelock itself. The deployer was then expected to renounce this role once configuration of the timelock is over. Failing to renounce that role allows the deployer to change the timelock permissions (but not to bypass the delay for any time-locked actions). The role is no longer given to the deployer by default. A new parameter admin can be set to a non-zero address to grant the admin role during construction (to the deployer or any other address). Just like previously, this admin role should be renounced after configuration. If this param is given address(0), the role is not allocated and doesn't need to be revoked. In any case, the timelock itself continues to have this role.


  • EIP712: Added the file EIP712.sol and deprecated draft-EIP712.sol since the EIP is no longer a Draft. Developers are encouraged to update their imports. (#3621)
-import "@openzeppelin/contracts/utils/cryptography/draft-EIP712.sol";
+import "@openzeppelin/contracts/utils/cryptography/EIP712.sol";
  • ERC721Votes: Added the file ERC721Votes.sol and deprecated draft-ERC721Votes.sol since it no longer depends on a Draft EIP (EIP-712). Developers are encouraged to update their imports. (#3699)
-import "@openzeppelin/contracts/token/ERC721/extensions/draft-ERC721Votes.sol";
+import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Votes.sol";

ERC-721 Compatibility Note

ERC-721 integrators that interpret contract state from events should make sure that they implement the clearing of approval that is implicit in every transfer according to the EIP. Previous versions of OpenZeppelin Contracts emitted an explicit Approval event even though it was not required by the specification, and this is no longer the case.

With the new ERC721Consecutive extension, the internal workings of ERC721 are slightly changed. Custom extensions to ERC721 should be reviewed to ensure they remain correct. The internal functions that should be considered are _ownerOf (new), _beforeTokenTransfer, and _afterTokenTransfer.

ERC-4626 Upgrade Note

Existing ERC4626 contracts that are upgraded to 4.8 must initialize a new variable that holds the vault token decimals. The recommended way to do this is to use a reinitializer:

function migrateToV48() public reinitializer(2) {

4.7.3 (2022-08-10)

Breaking changes

  • ECDSA: recover(bytes32,bytes) and tryRecover(bytes32,bytes) no longer accept compact signatures to prevent malleability. Compact signature support remains available using recover(bytes32,bytes32,bytes32) and tryRecover(bytes32,bytes32,bytes32).

4.7.2 (2022-07-25)

  • LibArbitrumL2, CrossChainEnabledArbitrumL2: Fixed detection of cross-chain calls for EOAs. Previously, calls from EOAs would be classified as cross-chain calls. (#3578)
  • GovernorVotesQuorumFraction: Fixed quorum updates so they do not affect past proposals that failed due to lack of quorum. (#3561)
  • ERC165Checker: Added protection against large returndata. (#3587)

4.7.1 (2022-07-18)

  • SignatureChecker: Fix an issue that causes isValidSignatureNow to revert when the target contract returns ill-encoded data. (#3552)
  • ERC165Checker: Fix an issue that causes supportsInterface to revert when the target contract returns ill-encoded data. (#3552)

4.7.0 (2022-06-29)

  • TimelockController: Migrate _call to _execute and allow inheritance and overriding similar to Governor. (#3317)
  • CrossChainEnabledPolygonChild: replace the require statement with the custom error NotCrossChainCall. (#3380)
  • ERC20FlashMint: Add customizable flash fee receiver. (#3327)
  • ERC4626: add an extension of ERC20 that implements the ERC4626 Tokenized Vault Standard. (#3171)
  • SafeERC20: add safePermit as mitigation against phantom permit functions. (#3280)
  • Math: add a mulDiv function that can round the result either up or down. (#3171)
  • Math: Add a sqrt function to compute square roots of integers, rounding either up or down. (#3242)
  • Strings: add a new overloaded function toHexString that converts an address with fixed length of 20 bytes to its not checksummed ASCII string hexadecimal representation. (#3403)
  • EnumerableMap: add new UintToUintMap map type. (#3338)
  • EnumerableMap: add new Bytes32ToUintMap map type. (#3416)
  • SafeCast: add support for many more types, using procedural code generation. (#3245)
  • MerkleProof: add multiProofVerify to prove multiple values are part of a Merkle tree. (#3276)
  • MerkleProof: add calldata versions of the functions to avoid copying input arrays to memory and save gas. (#3200)
  • ERC721, ERC1155: simplified revert reasons. (#3254, (#3438))
  • ERC721: removed redundant require statement. (#3434)
  • PaymentSplitter: add releasable getters. (#3350)
  • Initializable: refactored implementation of modifiers for easier understanding. (#3450)
  • Proxies: remove runtime check of ERC1967 storage slots. (#3455)

Breaking changes

  • Initializable: functions decorated with the modifier reinitializer(1) may no longer invoke each other.

4.6.0 (2022-04-26)

  • crosschain: Add a new set of contracts for cross-chain applications. CrossChainEnabled is a base contract with instantiations for several chains and bridges, and AccessControlCrossChain is an extension of access control that allows cross-chain operation. (#3183)
  • AccessControl: add a virtual _checkRole(bytes32) function that can be overridden to alter the onlyRole modifier behavior. (#3137)
  • EnumerableMap: add new AddressToUintMap map type. (#3150)
  • EnumerableMap: add new Bytes32ToBytes32Map map type. (#3192)
  • ERC20FlashMint: support infinite allowance when paying back a flash loan. (#3226)
  • ERC20Wrapper: the decimals() function now tries to fetch the value from the underlying token instance. If that calls revert, then the default value is used. (#3259)
  • draft-ERC20Permit: replace immutable with constant for _PERMIT_TYPEHASH since the keccak256 of string literals is treated specially and the hash is evaluated at compile time. (#3196)
  • ERC1155: Add a _afterTokenTransfer hook for improved extensibility. (#3166)
  • ERC1155URIStorage: add a new extension that implements a _setURI behavior similar to ERC721's _setTokenURI. (#3210)
  • DoubleEndedQueue: a new data structure that supports efficient push and pop to both front and back, useful for FIFO and LIFO queues. (#3153)
  • Governor: improved security of onlyGovernance modifier when using an external executor contract (e.g. a timelock) that can operate without necessarily going through the governance protocol. (#3147)
  • Governor: Add a way to parameterize votes. This can be used to implement voting systems such as fractionalized voting, ERC721 based voting, or any number of other systems. The params argument added to _countVote method, and included in the newly added _getVotes method, can be used by counting and voting modules respectively for such purposes. (#3043)
  • Governor: rewording of revert reason for consistency. (#3275)
  • Governor: fix an inconsistency in data locations that could lead to invalid bytecode being produced. (#3295)
  • Governor: Implement IERC721Receiver and IERC1155Receiver to improve token custody by governors. (#3230)
  • TimelockController: Implement IERC721Receiver and IERC1155Receiver to improve token custody by timelocks. (#3230)
  • TimelockController: Add a separate canceller role for the ability to cancel. (#3165)
  • Initializable: add a reinitializer modifier that enables the initialization of new modules, added to already initialized contracts through upgradeability. (#3232)
  • Initializable: add an Initialized event that tracks initialized version numbers. (#3294)
  • ERC2981: make royaltyInfo public to allow super call in overrides. (#3305)

Upgradeability notice

  • TimelockController: (Action needed) The upgrade from <4.6 to >=4.6 introduces a new CANCELLER_ROLE that requires set up to be assignable. After the upgrade, only addresses with this role will have the ability to cancel. Proposers will no longer be able to cancel. Assigning cancellers can be done by an admin (including the timelock itself) once the role admin is set up. To do this, we recommend upgrading to the TimelockControllerWith46MigrationUpgradeable contract and then calling the migrateTo46 function.

Breaking changes

  • Governor: Adds internal virtual _getVotes method that must be implemented; this is a breaking change for existing concrete extensions to Governor. To fix this on an existing voting module extension, rename getVotes to _getVotes and add a bytes memory argument. (#3043)
  • Governor: Adds params parameter to internal virtual _countVote method; this is a breaking change for existing concrete extensions to Governor. To fix this on an existing counting module extension, add a bytes memory argument to _countVote. (#3043)
  • Governor: Does not emit VoteCast event when params data is non-empty; instead emits VoteCastWithParams event. To fix this on an integration that consumes the VoteCast event, also fetch/monitor VoteCastWithParams events. (#3043)
  • Votes: The internal virtual function _getVotingUnits was made view (which was accidentally missing). Any overrides should now be updated so they are view as well.

4.5.0 (2022-02-09)

  • ERC2981: add implementation of the royalty standard, and the respective extensions for ERC721 and ERC1155. (#3012)
  • GovernorTimelockControl: improve the state() function to have it reflect cases where a proposal has been canceled directly on the timelock. (#2977)
  • Preset contracts are now deprecated in favor of Contracts Wizard. (#2986)
  • Governor: add a relay function to help recover assets sent to a governor that is not its own executor (e.g. when using a timelock). (#2926)
  • GovernorPreventLateQuorum: add new module to ensure a minimum voting duration is available after the quorum is reached. (#2973)
  • ERC721: improved revert reason when transferring from wrong owner. (#2975)
  • Votes: Added a base contract for vote tracking with delegation. (#2944)
  • ERC721Votes: Added an extension of ERC721 enabled with vote tracking and delegation. (#2944)
  • ERC2771Context: use immutable storage to store the forwarder address, no longer an issue since Solidity >=0.8.8 allows reading immutable variables in the constructor. (#2917)
  • Base64: add a library to parse bytes into base64 strings using encode(bytes memory) function, and provide examples to show how to use to build URL-safe tokenURIs. (#2884)
  • ERC20: reduce allowance before triggering transfer. (#3056)
  • ERC20: do not update allowance on transferFrom when allowance is type(uint256).max. (#3085)
  • ERC20: add a _spendAllowance internal function. (#3170)
  • ERC20Burnable: do not update allowance on burnFrom when allowance is type(uint256).max. (#3170)
  • ERC777: do not update allowance on transferFrom when allowance is type(uint256).max. (#3085)
  • ERC777: add a _spendAllowance internal function. (#3170)
  • SignedMath: a new signed version of the Math library with max, min, and average. (#2686)
  • SignedMath: add an abs(int256) method that returns the unsigned absolute value of a signed value. (#2984)
  • ERC1967Upgrade: Refactor the secure upgrade to use ERC1822 instead of the previous rollback mechanism. This reduces code complexity and attack surface with similar security guarantees. (#3021)
  • UUPSUpgradeable: Add ERC1822 compliance to support the updated secure upgrade mechanism. (#3021)
  • Some more functions have been made virtual to customize them via overrides. In many cases this will not imply that other functions in the contract will automatically adapt to the overridden definitions. People who wish to override should consult the source code to understand the impact and if they need to override any additional functions to achieve the desired behavior.

Breaking changes

  • ERC1967Upgrade: The function _upgradeToAndCallSecure was renamed to _upgradeToAndCallUUPS, along with the change in security mechanism described above.
  • Address: The Solidity pragma is increased from ^0.8.0 to ^0.8.1. This is required by the account.code.length syntax that replaces inline assembly. This may require users to bump their compiler version from 0.8.0 to 0.8.1 or later. Note that other parts of the code already include stricter requirements.

4.4.2 (2022-01-11)


  • GovernorCompatibilityBravo: Fix error in the encoding of calldata for proposals submitted through the compatibility interface with explicit signatures. (#3100)

4.4.1 (2021-12-14)

  • Initializable: change the existing initializer modifier and add a new onlyInitializing modifier to prevent reentrancy risk. (#3006)

Breaking change

It is no longer possible to call an initializer-protected function from within another initializer function outside the context of a constructor. Projects using OpenZeppelin upgradeable proxies should continue to work as is, since in the common case the initializer is invoked in the constructor directly. If this is not the case for you, the suggested change is to use the new onlyInitializing modifier in the following way:

 contract A {
-    function initialize() public   initializer { ... }
+    function initialize() internal onlyInitializing { ... }
 contract B is A {
     function initialize() public initializer {

4.4.0 (2021-11-25)

  • Ownable: add an internal _transferOwnership(address). (#2568)
  • AccessControl: add internal _grantRole(bytes32,address) and _revokeRole(bytes32,address). (#2568)
  • AccessControl: mark _setupRole(bytes32,address) as deprecated in favor of _grantRole(bytes32,address). (#2568)
  • AccessControlEnumerable: hook into _grantRole(bytes32,address) and _revokeRole(bytes32,address). (#2946)
  • EIP712: cache address(this) to immutable storage to avoid potential issues if a vanilla contract is used in a delegatecall context. (#2852)
  • Add internal _setApprovalForAll to ERC721 and ERC1155. (#2834)
  • Governor: shift vote start and end by one block to better match Compound's GovernorBravo and prevent voting at the Governor level if the voting snapshot is not ready. (#2892)
  • GovernorCompatibilityBravo: consider quorum an inclusive rather than exclusive minimum to match Compound's GovernorBravo. (#2974)
  • GovernorSettings: a new governor module that manages voting settings updatable through governance actions. (#2904)
  • PaymentSplitter: now supports ERC20 assets in addition to Ether. (#2858)
  • ECDSA: add a variant of toEthSignedMessageHash for arbitrary length message hashing. (#2865)
  • MerkleProof: add a processProof function that returns the rebuilt root hash given a leaf and a proof. (#2841)
  • VestingWallet: new contract that handles the vesting of Ether and ERC20 tokens following a customizable vesting schedule. (#2748)
  • Governor: enable receiving Ether when a Timelock contract is not used. (#2849)
  • GovernorTimelockCompound: fix ability to use Ether stored in the Timelock contract. (#2849)

4.3.3 (2021-11-08)

  • ERC1155Supply: Handle totalSupply changes by hooking into _beforeTokenTransfer to ensure consistency of balances and supply during IERC1155Receiver.onERC1155Received calls.

4.3.2 (2021-09-14)

  • UUPSUpgradeable: Add modifiers to prevent upgradeTo and upgradeToAndCall being executed on any contract that is not the active ERC1967 proxy. This prevents these functions being called on implementation contracts or minimal ERC1167 clones, in particular.

4.3.1 (2021-08-26)

  • TimelockController: Add additional isOperationReady check.

4.3.0 (2021-08-17)

  • ERC2771Context: use private variable from storage to store the forwarder address. Fixes issues where _msgSender() was not callable from constructors. (#2754)
  • EnumerableSet: add values() functions that returns an array containing all values in a single call. (#2768)
  • Governor: added a modular system of Governor contracts based on GovernorAlpha and GovernorBravo. (#2672)
  • Add an interfaces folder containing solidity interfaces to final ERCs. (#2517)
  • ECDSA: add tryRecover functions that will not throw if the signature is invalid, and will return an error flag instead. (#2661)
  • SignatureChecker: Reduce gas usage of the isValidSignatureNow function for the "signature by EOA" case. (#2661)

4.2.0 (2021-06-30)

  • ERC20Votes: add a new extension of the ERC20 token with support for voting snapshots and delegation. (#2632)
  • ERC20VotesComp: Variant of ERC20Votes that is compatible with Compound's Comp token interface but restricts supply to uint96. (#2706)
  • ERC20Wrapper: add a new extension of the ERC20 token which wraps an underlying token. Deposit and withdraw guarantee that the total supply is backed by a corresponding amount of underlying token. (#2633)
  • Enumerables: Improve gas cost of removal in EnumerableSet and EnumerableMap.
  • Enumerables: Improve gas cost of lookup in EnumerableSet and EnumerableMap.
  • Counter: add a reset method. (#2678)
  • Tokens: Wrap definitely safe subtractions in unchecked blocks.
  • Math: Add a ceilDiv method for performing ceiling division.
  • ERC1155Supply: add a new ERC1155 extension that keeps track of the totalSupply of each tokenId. (#2593)
  • BitMaps: add a new BitMaps library that provides a storage efficient datastructure for uint256 to bool mapping with contiguous keys. (#2710)

Breaking Changes

  • ERC20FlashMint is no longer a Draft ERC. (#2673))

How to update: Change your import paths by removing the draft- prefix from @openzeppelin/contracts/token/ERC20/extensions/draft-ERC20FlashMint.sol.

See Releases and Stability: Drafts.

4.1.0 (2021-04-29)

  • IERC20Metadata: add a new extended interface that includes the optional name(), symbol() and decimals() functions. (#2561)
  • ERC777: make reception acquirement optional in _mint. (#2552)
  • ERC20Permit: add a _useNonce to enable further usage of ERC712 signatures. (#2565)
  • ERC20FlashMint: add an implementation of the ERC3156 extension for flash-minting ERC20 tokens. (#2543)
  • SignatureChecker: add a signature verification library that supports both EOA and ERC1271 compliant contracts as signers. (#2532)
  • Multicall: add abstract contract with multicall(bytes[] calldata data) function to bundle multiple calls together (#2608)
  • ECDSA: add support for ERC2098 short-signatures. (#2582)
  • AccessControl: add an onlyRole modifier to restrict specific function to callers bearing a specific role. (#2609)
  • StorageSlot: add a library for reading and writing primitive types to specific storage slots. (#2542)
  • UUPS Proxies: add UUPSUpgradeable to implement the UUPS proxy pattern together with EIP1967Proxy. (#2542)

Breaking changes

This release includes two small breaking changes in TimelockController.

  1. The onlyRole modifier in this contract was designed to let anyone through if the role was granted to address(0), allowing the possibility to to make a role "open", which can be used for EXECUTOR_ROLE. This modifier is now replaced by AccessControl.onlyRole, which does not have this ability. The previous behavior was moved to the modifier TimelockController.onlyRoleOrOpenRole.
  2. It was possible to make PROPOSER_ROLE an open role (as described in the previous item) if it was granted to address(0). This would affect the schedule, scheduleBatch, and cancel operations in TimelockController. This ability was removed as it does not make sense to open up the PROPOSER_ROLE in the same way that it does for EXECUTOR_ROLE.

4.0.0 (2021-03-23)

  • Now targeting the 0.8.x line of Solidity compilers. For 0.6.x (resp 0.7.x) support, use version 3.4.0 (resp 3.4.0-solc-0.7) of OpenZeppelin.
  • Context: making _msgData return bytes calldata instead of bytes memory (#2492)
  • ERC20: removed the _setDecimals function and the storage slot associated to decimals. (#2502)
  • Strings: addition of a toHexString function. (#2504)
  • EnumerableMap: change implementation to optimize for key → value lookups instead of enumeration. (#2518)
  • GSN: deprecate GSNv1 support in favor of upcoming support for GSNv2. (#2521)
  • ERC165: remove uses of storage in the base ERC165 implementation. ERC165 based contracts now use storage-less virtual functions. Old behavior remains available in the ERC165Storage extension. (#2505)
  • Initializable: make initializer check stricter during construction. (#2531)
  • ERC721: remove enumerability of tokens from the base implementation. This feature is now provided separately through the ERC721Enumerable extension. (#2511)
  • AccessControl: removed enumerability by default for a more lightweight contract. It is now opt-in through AccessControlEnumerable. (#2512)
  • Meta Transactions: add ERC2771Context and a MinimalForwarder for meta-transactions. (#2508)
  • Overall reorganization of the contract folder to improve clarity and discoverability. (#2503)
  • ERC20Capped: optimize gas usage by enforcing the check directly in _mint. (#2524)
  • Rename UpgradeableProxy to ERC1967Proxy. (#2547)
  • ERC777: optimize the gas costs of the constructor. (#2551)
  • ERC721URIStorage: add a new extension that implements the _setTokenURI behavior as it was available in 3.4.0. (#2555)
  • AccessControl: added ERC165 interface detection. (#2562)
  • ERC1155: make uri public so overloading function can call it using super. (#2576)

Bug fixes for beta releases

  • AccessControlEnumerable: Fixed renounceRole not updating enumerable set of addresses for a role. (#2572)

How to upgrade from 3.x

Since this version has moved a few contracts to different directories, users upgrading from a previous version will need to adjust their import statements. To make this easier, the package includes a script that will migrate import statements automatically. After upgrading to the latest version of the package, run:

npx openzeppelin-contracts-migrate-imports

Make sure you're using git or another version control system to be able to recover from any potential error in our script.

How to upgrade from 4.0-beta.x

Some further changes have been done between the different beta iterations. Transitions made during this period are configured in the migrate-imports script. Consequently, you can upgrade from any previous 4.0-beta.x version using the same script as described in the How to upgrade from 3.x section.

3.4.2 (2021-07-24)

  • TimelockController: Add additional isOperationReady check.

3.4.1 (2021-03-03)

  • ERC721: made _approve an internal function (was private).

3.4.0 (2021-02-02)

  • BeaconProxy: added new kind of proxy that allows simultaneous atomic upgrades. (#2411)
  • EIP712: added helpers to verify EIP712 typed data signatures on chain. (#2418)
  • ERC20Permit: added an implementation of the ERC20 permit extension for gasless token approvals. (#2237)
  • Presets: added token presets with preminted fixed supply ERC20PresetFixedSupply and ERC777PresetFixedSupply. (#2399)
  • Address: added functionDelegateCall, similar to the existing functionCall. (#2333)
  • Clones: added a library for deploying EIP 1167 minimal proxies. (#2449)
  • Context: moved from contracts/GSN to contracts/utils. (#2453)
  • PaymentSplitter: replace usage of .transfer() with Address.sendValue for improved compatibility with smart wallets. (#2455)
  • UpgradeableProxy: bubble revert reasons from initialization calls. (#2454)
  • SafeMath: fix a memory allocation issue by adding new SafeMath.tryOp(uint,uint)→(bool,uint) functions. SafeMath.op(uint,uint,string)→uint are now deprecated. (#2462)
  • EnumerableMap: fix a memory allocation issue by adding new EnumerableMap.tryGet(uint)→(bool,address) functions. EnumerableMap.get(uint)→string is now deprecated. (#2462)
  • ERC165Checker: added batch getSupportedInterfaces. (#2469)
  • RefundEscrow: beneficiaryWithdraw will forward all available gas to the beneficiary. (#2480)
  • Many view and pure functions have been made virtual to customize them via overrides. In many cases this will not imply that other functions in the contract will automatically adapt to the overridden definitions. People who wish to override should consult the source code to understand the impact and if they need to override any additional functions to achieve the desired behavior.

Security Fixes

  • ERC777: fix potential reentrancy issues for custom extensions to ERC777. (#2483)

If you're using our implementation of ERC777 from version 3.3.0 or earlier, and you define a custom _beforeTokenTransfer function that writes to a storage variable, you may be vulnerable to a reentrancy attack. If you're affected and would like assistance please write to Read more in the pull request.

3.3.0 (2020-11-26)

  • Now supports both Solidity 0.6 and 0.7. Compiling with solc 0.7 will result in warnings. Install the solc-0.7 tag to compile without warnings.
  • Address: added functionStaticCall, similar to the existing functionCall. (#2333)
  • TimelockController: added a contract to augment access control schemes with a delay. (#2354)
  • EnumerableSet: added Bytes32Set, for sets of bytes32. (#2395)

3.2.2-solc-0.7 (2020-10-28)

  • Resolve warnings introduced by Solidity 0.7.4. (#2396)

3.2.1-solc-0.7 (2020-09-15)

  • ERC777: Remove a warning about function state visibility in Solidity 0.7. (#2327)

3.2.0 (2020-09-10)

New features

  • Proxies: added the proxy contracts from OpenZeppelin SDK. (#2335)

Proxy changes with respect to OpenZeppelin SDK

Aside from upgrading them from Solidity 0.5 to 0.6, we've changed a few minor things from the proxy contracts as they were found in OpenZeppelin SDK.

  • UpgradeabilityProxy was renamed to UpgradeableProxy.
  • AdminUpgradeabilityProxy was renamed to TransparentUpgradeableProxy.
  • Proxy._willFallback was renamed to Proxy._beforeFallback.
  • UpgradeabilityProxy._setImplementation and AdminUpgradeabilityProxy._setAdmin were made private.


  • Address.isContract: switched from extcodehash to extcodesize for less gas usage. (#2311)

Breaking changes

  • ERC20Snapshot: switched to using _beforeTokenTransfer hook instead of overriding ERC20 operations. (#2312)

This small change in the way we implemented ERC20Snapshot may affect users who are combining this contract with other ERC20 flavors, since it no longer overrides _transfer, _mint, and _burn. This can result in having to remove Solidity override(...) specifiers in derived contracts for these functions, and to instead have to add it for _beforeTokenTransfer. See Using Hooks in the documentation.

3.1.0 (2020-06-23)

New features

  • SafeCast: added functions to downcast signed integers (e.g. toInt32), improving usability of SignedSafeMath. (#2243)
  • functionCall: new helpers that replicate Solidity's function call semantics, reducing the need to rely on call. (#2264)
  • ERC1155: added support for a base implementation, non-standard extensions and a preset contract. (#2014, #2230)


  • ReentrancyGuard: reduced overhead of using the nonReentrant modifier. (#2171)
  • AccessControl: added a RoleAdminChanged event to _setAdminRole. (#2214)
  • Made all public functions in the token preset contracts virtual. (#2257)


  • SafeERC20: deprecated safeApprove. (#2268)

3.0.2 (2020-06-08)


  • Added SPX license identifier to all contracts. (#2235)

3.0.1 (2020-04-27)


  • ERC777: fixed the _approve internal function not validating some of their arguments for non-zero addresses. (#2213)

3.0.0 (2020-04-20)

New features

  • AccessControl: new contract for managing permissions in a system, replacement for Ownable and Roles. (#2112)
  • SafeCast: new functions to convert to and from signed and unsigned values: toUint256 and toInt256. (#2123)
  • EnumerableMap: a new data structure for key-value pairs (like mapping) that can be iterated over. (#2160)

Breaking changes

  • ERC721: burn(owner, tokenId) was removed, use burn(tokenId) instead. (#2125)
  • ERC721: _checkOnERC721Received was removed. (#2125)
  • ERC721: _transferFrom and _safeTransferFrom were renamed to _transfer and _safeTransfer. (#2162)
  • Ownable: removed _transferOwnership. (#2162)
  • PullPayment, Escrow: withdrawWithGas was removed. The old withdraw function now forwards all gas. (#2125)
  • Roles was removed, use AccessControl as a replacement. (#2112)
  • ECDSA: when receiving an invalid signature, recover now reverts instead of returning the zero address. (#2114)
  • Create2: added an amount argument to deploy for contracts with payable constructors. (#2117)
  • Pausable: moved to the utils directory. (#2122)
  • Strings: moved to the utils directory. (#2122)
  • Counters: moved to the utils directory. (#2122)
  • SignedSafeMath: moved to the math directory. (#2122)
  • ERC20Snapshot: moved to the token/ERC20 directory. snapshot was changed into an internal function. (#2122)
  • Ownable: moved to the access directory. (#2120)
  • Ownable: removed isOwner. (#2120)
  • Secondary: removed from the library, use Ownable instead. (#2120)
  • Escrow, ConditionalEscrow, RefundEscrow: these now use Ownable instead of Secondary, their external API changed accordingly. (#2120)
  • ERC20: removed _burnFrom. (#2119)
  • Address: removed toPayable, use payable(address) instead. (#2133)
  • ERC777: _send, _mint and _burn now use the caller as the operator. (#2134)
  • ERC777: removed _callsTokensToSend and _callTokensReceived. (#2134)
  • EnumerableSet: renamed get to at. (#2151)
  • ERC165Checker: functions no longer have a leading underscore. (#2150)
  • ERC721Metadata, ERC721Enumerable: these contracts were removed, and their functionality merged into ERC721. (#2160)
  • ERC721: added a constructor for name and symbol. (#2160)
  • ERC20Detailed: this contract was removed and its functionality merged into ERC20. (#2161)
  • ERC20: added a constructor for name and symbol. decimals now defaults to 18. (#2161)
  • Strings: renamed fromUint256 to toString (#2188)

2.5.1 (2020-04-24)


  • ERC777: fixed the _send and _approve internal functions not validating some of their arguments for non-zero addresses. (#2212)

2.5.0 (2020-02-04)

New features

  • SafeCast.toUintXX: new library for integer downcasting, which allows for safe operation on smaller types (e.g. uint32) when combined with SafeMath. (#1926)
  • ERC721Metadata: added baseURI, which can be used for dramatic gas savings when all token URIs share a prefix (e.g.<id>). (#1970)
  • EnumerableSet: new library for storing enumerable sets of values. Only AddressSet is supported in this release. (#2061)
  • Create2: simple library to make usage of the CREATE2 opcode easier. (#1744)


  • ERC777: _burn is now internal, providing more flexibility and making it easier to create tokens that deflate. (#1908)
  • ReentrancyGuard: greatly improved gas efficiency by using the net gas metering mechanism introduced in the Istanbul hardfork. (#1992, #1996)
  • ERC777: improve extensibility by making _send and related functions internal. (#2027)
  • ERC721: improved revert reason when transferring tokens to a non-recipient contract. (#2018)

Breaking changes

  • ERC165Checker now requires a minimum Solidity compiler version of 0.5.10. (#1829)

2.4.0 (2019-10-29)

New features

  • Address.toPayable: added a helper to convert between address types without having to resort to low-level casting. (#1773)
  • Facilities to make metatransaction-enabled contracts through the Gas Station Network. (#1844)
  • Address.sendValue: added a replacement to Solidity's transfer, removing the fixed gas stipend. (#1962)
  • Added replacement for functions that don't forward all gas (which have been deprecated): (#1976)
    • PullPayment.withdrawPaymentsWithGas(address payable payee)
    • Escrow.withdrawWithGas(address payable payee)
  • SafeMath: added support for custom error messages to sub, div and mod functions. (#1828)


  • Address.isContract: switched from extcodesize to extcodehash for less gas usage. (#1802)
  • ERC20 and ERC777 updated to throw custom errors on subtraction overflows. (#1828)


  • Deprecated functions that don't forward all gas: (#1976)
    • PullPayment.withdrawPayments(address payable payee)
    • Escrow.withdraw(address payable payee)

Breaking changes

  • Address now requires a minimum Solidity compiler version of 0.5.5. (#1802)
  • SignatureBouncer has been removed from drafts, both to avoid confusions with the GSN and GSNRecipientSignature (previously called GSNBouncerSignature) and because the API was not very clear. (#1879)

How to upgrade from 2.4.0-beta

The final 2.4.0 release includes a refactor of the GSN contracts that will be a breaking change for 2.4.0-beta users.

  • The default empty implementations of _preRelayedCall and _postRelayedCall were removed and must now be explicitly implemented always in custom recipients. If your custom recipient didn't include an implementation, you can provide an empty one.
  • GSNRecipient, GSNBouncerBase, and GSNContext were all merged into GSNRecipient.
  • GSNBouncerSignature and GSNBouncerERC20Fee were renamed to GSNRecipientSignature and GSNRecipientERC20Fee.
  • It is no longer necessary to inherit from GSNRecipient when using GSNRecipientSignature and GSNRecipientERC20Fee.

For example, a contract using GSNBouncerSignature would have to be changed in the following way.

-contract MyDapp is GSNRecipient, GSNBouncerSignature {
+contract MyDapp is GSNRecipientSignature {

Refer to the table below to adjust your inheritance list.

2.4.0-beta 2.4.0
GSNRecipient, GSNBouncerSignature GSNRecipientSignature
GSNRecipient, GSNBouncerERC20Fee GSNRecipientERC20Fee
GSNBouncerBase GSNRecipient

2.3.0 (2019-05-27)

New features

  • ERC1820: added support for interacting with the ERC1820 registry contract (IERC1820Registry), as well as base contracts that can be registered as implementers there. (#1677)
  • ERC777: support for the ERC777 token, which has multiple improvements over ERC20 (but is backwards compatible with it) such as built-in burning, a more straightforward permission system, and optional sender and receiver hooks on transfer (mandatory for contracts!). (#1684)
  • All contracts now have revert reason strings, which give insight into error conditions, and help debug failing transactions. (#1704)


  • Reverted the Solidity version bump done in v2.2.0, setting the minimum compiler version to v0.5.0, to prevent unexpected build breakage. Users are encouraged however to stay on top of new compiler releases, which usually include bugfixes. (#1729)


  • PostDeliveryCrowdsale: some validations where skipped when paired with other crowdsale flavors, such as AllowanceCrowdsale, or MintableCrowdsale and ERC20Capped, which could cause buyers to not be able to claim their purchased tokens. (#1721)
  • ERC20._transfer: the from argument was allowed to be the zero address, so it was possible to internally trigger a transfer of 0 tokens from the zero address. This address is not a valid destinatary of transfers, nor can it give or receive allowance, so this behavior was inconsistent. It now reverts. (#1752)

2.2.0 (2019-03-14)

New features

  • ERC20Snapshot: create snapshots on demand of the token balances and total supply, to later retrieve and e.g. calculate dividends at a past time. (#1617)
  • SafeERC20: ERC20 contracts with no return value (i.e. that revert on failure) are now supported. (#1655)
  • ERC20: added internal _approve(address owner, address spender, uint256 value), allowing derived contracts to set the allowance of arbitrary accounts. (#1609)
  • ERC20Metadata: added internal _setTokenURI(string memory tokenURI). (#1618)
  • TimedCrowdsale: added internal _extendTime(uint256 newClosingTime) as well as TimedCrowdsaleExtended(uint256 prevClosingTime, uint256 newClosingTime) event allowing to extend the crowdsale, as long as it hasn't already closed.


  • Upgraded the minimum compiler version to v0.5.2: this removes many Solidity warnings that were false positives. (#1606)
  • ECDSA: recover no longer accepts malleable signatures (those using upper-range values for s, or 0/1 for v). (#1622)
  • ERC721's transfers are now more gas efficient due to removal of unnecessary SafeMath calls. (#1610)
  • Fixed variable shadowing issues. (#1606)


  • (minor) SafeERC20: safeApprove wasn't properly checking for a zero allowance when attempting to set a non-zero allowance. (#1647)

Breaking changes in drafts

  • TokenMetadata has been renamed to ERC20Metadata. (#1618)
  • The library Counter has been renamed to Counters and its API has been improved. See an example in ERC721, lines 17 and 204. (#1610)

2.1.3 (2019-02-26)

  • Backported SafeERC20.safeApprove bugfix. (#1647)

2.1.2 (2019-01-17)

  • Removed most of the test suite from the npm package, except PublicRole.behavior.js, which may be useful to users testing their own Roles.

2.1.1 (2019-01-04)

  • Version bump to avoid conflict in the npm registry.

2.1.0 (2019-01-04)

New features

  • Now targeting the 0.5.x line of Solidity compilers. For 0.4.24 support, use version 2.0 of OpenZeppelin.
  • WhitelistCrowdsale: a crowdsale where only whitelisted accounts (WhitelistedRole) can purchase tokens. Adding or removing accounts from the whitelist is done by whitelist admins (WhitelistAdminRole). Similar to the pre-2.0 WhitelistedCrowdsale. (#1525, #1589)
  • RefundablePostDeliveryCrowdsale: replacement for RefundableCrowdsale (deprecated, see below) where tokens are only granted once the crowdsale ends (if it meets its goal). (#1543)
  • PausableCrowdsale: allows for pausers (PauserRole) to pause token purchases. Other crowdsale operations (e.g. withdrawals and refunds, if applicable) are not affected. (#832)
  • ERC20: transferFrom and _burnFrom now emit Approval events, to represent the token's state comprehensively through events. (#1524)
  • ERC721: added _burn(uint256 tokenId), replacing the similar deprecated function (see below). (#1550)
  • ERC721: added _tokensOfOwner(address owner), allowing to internally retrieve the array of an account's owned tokens. (#1522)
  • Crowdsales: all constructors are now public, meaning it is not necessary to extend these contracts in order to deploy them. The exception is FinalizableCrowdsale, since it is meaningless unless extended. (#1564)
  • SignedSafeMath: added overflow-safe operations for signed integers (int256). (#1559, #1588)


  • The compiler version required by Array was behind the rest of the library so it was updated to v0.4.24. (#1553)
  • Now conforming to a 4-space indentation code style. (1508)
  • ERC20: more gas efficient due to removed redundant requires. (#1409)
  • ERC721: fixed a bug that prevented internal data structures from being properly cleaned, missing potential gas refunds. (#1539 and #1549)
  • ERC721: general gas savings on transferFrom, _mint and _burn, due to redundant requires and SSTOREs. (#1549)


Breaking changes


  • ERC721._burn(address owner, uint256 tokenId): due to the owner parameter being unnecessary. (#1550)
  • RefundableCrowdsale: due to trading abuse potential on crowdsales that miss their goal. (#1543)