Make ERC2771Context return original sender address if msg.data.length <= 20

.changeset/unlucky-beans-obey.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': patch
+---
+
+`ERC2771Context`: Return the forwarder address whenever `msg.data.length` is not appended with the request signer address (i.e. length is less than 20 bytes).

contracts/metatx/ERC2771Context.sol

@@ -22,7 +22,7 @@ abstract contract ERC2771Context is Context {
     }
 
     function _msgSender() internal view virtual override returns (address sender) {
-        if (isTrustedForwarder(msg.sender)) {
+        if (isTrustedForwarder(msg.sender) && msg.data.length >= 20) {
             // The assembly code is more direct than the Solidity version using `abi.decode`.
             /// @solidity memory-safe-assembly
             assembly {

test/metatx/ERC2771Context.test.js

@@ -12,6 +12,8 @@ const ContextMockCaller = artifacts.require('ContextMockCaller');
 const { shouldBehaveLikeRegularContext } = require('../utils/Context.behavior');
 
 contract('ERC2771Context', function (accounts) {
+  const [, anotherAccount] = accounts;
+
   const MAX_UINT48 = web3.utils.toBN(1).shln(48).subn(1).toString();
 
   beforeEach(async function () {
@@ -79,6 +81,15 @@ contract('ERC2771Context', function (accounts) {
         const { tx } = await this.forwarder.execute(req);
         await expectEvent.inTransaction(tx, ERC2771ContextMock, 'Sender', { sender: this.sender });
       });
+
+      it('returns the original sender when calldata length is less than 20 bytes (address length)', async function () {
+        // The forwarder doesn't produce calls with calldata length less than 20 bytes
+        const recipient = await ERC2771ContextMock.new(anotherAccount);
+
+        const { tx } = await recipient.msgSender({ from: anotherAccount });
+
+        await expectEvent.inTransaction(tx, ERC2771ContextMock, 'Sender', { sender: anotherAccount });
+      });
     });
 
     describe('msgData', function () {