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Weird ERC20 Tokens

This repository contains minimal example implementations in Solidity of ERC20 tokens with behaviour that may be surprising or unexpected. All the tokens in this repo are based on real tokens, many of which have been used to exploit smart contract systems in the past. It is hoped that these example implementations will be of use to developers and auditors.

The ERC20 "specification" is so loosely defined that it amounts to little more than an interface declaration, and even the few semantic requirements that are imposed are routinely violated by token developers in the wild.

This makes building smart contracts that interface directly with ERC20 tokens challenging to say the least, and smart contract developers should in general default to the following patterns when interaction with external code is required:

  1. A contract level allowlist of known good tokens.
  2. Direct interaction with tokens should be performed in dedicated wrapper contracts at the edge of the system. This allows the core to assume a consistent and known good semantics for the behaviour of external assets.

In some cases the above patterns are not practical (for example in the case of a permissionless AMM, keeping an on chain allowlist would require the introduction of centralized control or a complex governance system), and in these cases developers must take great care to make these interactions in a highly defensive manner. It should be noted that even if an onchain allowlist is not feasible, an offchain allowlist in the official UI can also protect unsophisticated users from tokens that violate the contracts expectations, while still preserving contract level permissionlessness.

Finally if you are building a token, you are strongly advised to treat the following as a list of behaviours to avoid.

Additional Resources


Reentrant Calls

Some tokens allow reentrant calls on transfer (e.g. ERC777 tokens).

This has been exploited in the wild on multiple occasions (e.g. imBTC uniswap pool drained, drained)

example: Reentrant.sol

Missing Return Values

Some tokens do not return a bool (e.g. USDT, BNB, OMG) on ERC20 methods. see here for a comprehensive (if somewhat outdated) list.

Some tokens (e.g. BNB) may return a bool for some methods, but fail to do so for others. This resulted in stuck BNB tokens in Uniswap v1 (details).

Some particulary pathological tokens (e.g. Tether Gold) declare a bool return, but then return false even when the transfer was successful (code).

A good safe transfer abstraction (example) can help somewhat, but note that the existance of Tether Gold makes it impossible to correctly handle return values for all tokens.

Two example tokens are provided:

  • MissingReturns: does not return a bool for any erc20 operation
  • ReturnsFalse: declares a bool return, but then returns false for every erc20 operation

example: MissingReturns.sol
example: ReturnsFalse.sol

Fee on Transfer

Some tokens take a transfer fee (e.g. STA, PAXG), some do not currently charge a fee but may do so in the future (e.g. USDT, USDC).

The STA transfer fee was used to drain $500k from several balancer pools (more details).

example: TransferFee.sol

Balance Modifications Outside of Transfers (rebasing / airdrops)

Some tokens may make arbitrary balance modifications outside of transfers (e.g. Ampleforth style rebasing tokens, Compound style airdrops of governance tokens, mintable / burnable tokens).

Some smart contract systems cache token balances (e.g. Balancer, Uniswap-V2), and arbitrary modifications to underlying balances can mean that the contract is operating with outdated information.

In the case of Ampleforth, some Balancer and Uniswap pools are special cased to ensure that the pool's cached balances are atomically updated as part of the rebase prodecure (details).

example: TODO: implement a rebasing token

Upgradable Tokens

Some tokens (e.g. USDC, USDT) are upgradable, allowing the token owners to make arbitrary modifications to the logic of the token at any point in time.

A change to the token semantics can break any smart contract that depends on the past behaviour.

Developers integrating with upgradable tokens should consider introducing logic that will freeze interactions with the token in question if an upgrade is detected. (e.g. the TUSD adapter used by MakerDAO).

example: Upgradable.sol

Flash Mintable Tokens

Some tokens (e.g. DAI) allow for so called "flash minting", which allows tokens to be minted for the duration of one transaction only, provided they are returned to the token contract by the end of the transaction.

This is similar to a flash loan, but does not require the tokens that are to be lent to exist before the start of the transaction. A token that can be flash minted could potentially have a total supply of max uint256.

Documentation for the MakerDAO flash mint module can be found here.

Tokens with Blocklists

Some tokens (e.g. USDC, USDT) have a contract level admin controlled address blocklist. If an address is blocked, then transfers to and from that address are forbidden.

Malicious or compromised token owners can trap funds in a contract by adding the contract address to the blocklist. This could potentially be the result of regulatory action against the contract itself, against a single user of the contract (e.g. a Uniswap LP), or could also be a part of an extortion attempt against users of the blocked contract.

example: BlockList.sol

Pausable Tokens

Some tokens can be paused by an admin (e.g. BNB, ZIL).

Similary to the blocklist issue above, an admin controlled pause feature opens users of the token to risk from a malicious or compromised token owner.

example: Pausable.sol

Approval Race Protections

Some tokens (e.g. USDT, KNC) do not allow approving an amount M > 0 when an existing amount N > 0 is already approved. This is to protect from an ERC20 attack vector described here.

This PR shows some in the wild problems caused by this issue.

example: Approval.sol

Revert on Approval To Zero Address

Some tokens (e.g. OpenZeppelin) will revert if trying to approve the zero address to spend tokens (i.e. a call to approve(address(0), amt)).

Integrators may need to add special cases to handle this logic if working with such a token.

example: ApprovalToZero.sol

Revert on Zero Value Transfers

Some tokens (e.g. LEND) revert when transfering a zero value amount.

example: RevertZero.sol

Multiple Token Addresses

Some proxied tokens have multiple addresses. As an example consider the following snippet. rescueFunds is intended to allow the contract owner to return non pool tokens that were accidentaly sent to the contract. However, it assumes a single address per token and so would allow the owner to steal all funds in the pool.

mapping isPoolToken(address => bool);
constructor(address tokenA, address tokenB) public {
  isPoolToken[tokenA] = true;
  isPoolToken[tokenB] = true;
function rescueFunds(address token, uint amount) external nonReentrant onlyOwner {
    require(!isPoolToken[token], "access denied");
    token.transfer(msg.sender, amount);

example: Proxied.sol

Low Decimals

Some tokens have low decimals (e.g. USDC has 6). Even more extreme, some tokens like Gemini USD only have 2 decimals.

This may result in larger than expected precision loss.

example: LowDecimals.sol

High Decimals

Some tokens have more than 18 decimals (e.g. YAM-V2 has 24).

This may trigger unexpected reverts due to overflow, posing a liveness risk to the contract.

example: HighDecimals.sol

transferFrom with src == msg.sender

Some token implementations (e.g. DSToken) will not attempt to decrease the caller's allowance if the sender is the same as the caller. This gives transferFrom the same semantics as transfer in this case. Other implementations (e.g. OpenZeppelin, Uniswap-v2) will attempt to decrease the caller's allowance from the sender in transferFrom even if the caller and the sender are the same address, giving transfer(dst, amt) and transferFrom(address(this), dst, amt) a different semantics in this case.


Examples of both semantics are provided:

Non string metadata

Some tokens (e.g. MKR) have metadata fields (name / symbol) encoded as bytes32 instead of the string prescribed by the ERC20 specification.

This may cause issues when trying to consume metadata from these tokens.

example: Bytes32Metadata.sol

Revert on Transfer to the Zero Address

Some tokens (e.g. openzeppelin) revert when attempting to transfer to address(0).

This may break systems that expect to be able to burn tokens by transfering them to address(0).

example: RevertToZero.sol

No Revert on Failure

Some tokens do not revert on failure, but instead return false (e.g. ZRX).

While this is technicaly compliant with the ERC20 standard, it goes against common solidity coding practices and may be overlooked by developers who forget to wrap their calls to transfer in a require.

example: NoRevert.sol

Revert on Large Approvals & Transfers

Some tokens (e.g. UNI, COMP) revert if the value passed to approve or transfer is larger than uint96.

Both of the above tokens have special case logic in approve that sets allowance to type(uint96).max if the approval amount is uint256(-1), which may cause issues with systems that expect the value passed to approve to be reflected in the allowances mapping.

example: Uint96.sol

Code Injection Via Token Name

Some malicious tokens have been observed to include malicious javascript in their name attribute, allowing attackers to extract private keys from users who choose to interact with these tokens via vulnerable frontends.

This has been used to exploit etherdelta users in the wild (reference).

Unusual Permit Function

Some tokens (DAI, RAI, GLM, STAKE, CHAI, HAKKA, USDFL, HNY) have a permit() implementation that does not follow EIP2612. Tokens that do not support permit may not revert, which could lead to the execution of later lines of code in unexpected scenarios. Uniswap's Permit2 may provide a more compatible alternative.

example: DaiPermit.sol