diff --git a/.changeset/warm-geese-dance.md b/.changeset/warm-geese-dance.md
new file mode 100644
index 000000000..5deb7a3e6
--- /dev/null
+++ b/.changeset/warm-geese-dance.md
@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': patch
+---
+
+`Base64`: Fix issue where dirty memory located just after the input buffer is affecting the result.
diff --git a/contracts/mocks/Base64DirtyUpgradeable.sol b/contracts/mocks/Base64DirtyUpgradeable.sol
new file mode 100644
index 000000000..1b0c41909
--- /dev/null
+++ b/contracts/mocks/Base64DirtyUpgradeable.sol
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.20;
+
+import {Base64} from "@openzeppelin/contracts/utils/Base64.sol";
+import {Initializable} from "../proxy/utils/Initializable.sol";
+
+contract Base64DirtyUpgradeable is Initializable {
+    struct A {
+        uint256 value;
+    }
+
+    function __Base64Dirty_init() internal onlyInitializing {
+    }
+
+    function __Base64Dirty_init_unchained() internal onlyInitializing {
+    }
+    function encode(bytes memory input) public pure returns (string memory) {
+        A memory unused = A({value: type(uint256).max});
+        // To silence warning
+        unused;
+
+        return Base64.encode(input);
+    }
+}
diff --git a/contracts/mocks/WithInit.sol b/contracts/mocks/WithInit.sol
index 04c53936d..763566af2 100644
--- a/contracts/mocks/WithInit.sol
+++ b/contracts/mocks/WithInit.sol
@@ -142,6 +142,13 @@ contract AuthoritiyObserveIsConsumingUpgradeableWithInit is AuthoritiyObserveIsC
         __AuthoritiyObserveIsConsuming_init();
     }
 }
+import "./Base64DirtyUpgradeable.sol";
+
+contract Base64DirtyUpgradeableWithInit is Base64DirtyUpgradeable {
+    constructor() payable initializer {
+        __Base64Dirty_init();
+    }
+}
 import "./CallReceiverMockUpgradeable.sol";
 
 contract CallReceiverMockUpgradeableWithInit is CallReceiverMockUpgradeable {
diff --git a/lib/openzeppelin-contracts b/lib/openzeppelin-contracts
index d4ec2782b..26b4b6099 160000
--- a/lib/openzeppelin-contracts
+++ b/lib/openzeppelin-contracts
@@ -1 +1 @@
-Subproject commit d4ec2782b724be9e56bf33aa91ff599df185c0b0
+Subproject commit 26b4b6099936fc785309f3da118ec8607b6716ed
diff --git a/package-lock.json b/package-lock.json
index 0f4f9f55e..cf544edf5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "openzeppelin-solidity",
-  "version": "4.9.2",
+  "version": "5.0.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "openzeppelin-solidity",
-      "version": "4.9.2",
+      "version": "5.0.1",
       "license": "MIT",
       "devDependencies": {
         "@changesets/changelog-github": "^0.4.8",
diff --git a/test/utils/Base64.test.js b/test/utils/Base64.test.js
index dfff0b0d0..c5d7add0e 100644
--- a/test/utils/Base64.test.js
+++ b/test/utils/Base64.test.js
@@ -1,8 +1,9 @@
 const { expect } = require('chai');
 
 const Base64 = artifacts.require('$Base64');
+const Base64Dirty = artifacts.require('$Base64Dirty');
 
-contract('Strings', function () {
+contract('Base64', function () {
   beforeEach(async function () {
     this.base64 = await Base64.new();
   });
@@ -30,4 +31,13 @@ contract('Strings', function () {
       expect(await this.base64.$encode([])).to.equal('');
     });
   });
+
+  it('Encode reads beyond the input buffer into dirty memory', async function () {
+    const mock = await Base64Dirty.new();
+    const buffer32 = Buffer.from(web3.utils.soliditySha3('example').replace(/0x/, ''), 'hex');
+    const buffer31 = buffer32.slice(0, -2);
+
+    expect(await mock.encode(buffer31)).to.equal(buffer31.toString('base64'));
+    expect(await mock.encode(buffer32)).to.equal(buffer32.toString('base64'));
+  });
 });