ecrecover can return undefined data if signature does not verify

Impact

the ecrecover precompile does not fill the output buffer if the signature does not verify, see https://github.com/ethereum/go-ethereum/blob/b058cf454b3bdc7e770e2b3cec83a0bcb48f55ee/core/vm/contracts.go#L188. however, the ecrecover builtin will still return whatever is at memory location 0.

this means that the if the compiler has been convinced to write to the 0 memory location with specially crafted data (generally, this can happen with a hashmap access or immutable read) just before the ecrecover, a signature check might pass on an invalid signature.

A contract search was performed. Most uses of ecrecover are used for erc2612-style permit implementations, which typically look like:

    assert _owner != empty(address)
    assert block.timestamp <= _deadline
                  
    nonce: uint256 = self.nonces[_owner]
    digest: bytes32 = keccak256(
        concat(   
            b"\x19\x01",
            self.DOMAIN_SEPARATOR,
            keccak256(_abi_encode(PERMIT_TYPEHASH, _owner, _spender, _value, nonce, _deadline))
        )         
    )             
    assert ecrecover(digest, convert(_v, uint256), convert(_r, uint256), convert(_s, uint256)) == _owner

in this case, the immutable PERMIT_TYPEHASH is loaded into ecrecover's output buffer right before ecrecover(), and so the output of ecrecover() here when the signature is invalid will be the value of PERMIT_TYPEHASH. in this case, since PERMIT_TYPEHASH is not a valid address, it will never compare == to _owner, and so the behaviour is exactly the same as if ecrecover() returned 0 in this case.

in general, a contract could have unexpected behavior (i.e. mistakenly pass this style of signature check) if an immutable representing a real address (ex. OWNER) was read right before the ecrecover operation.

Patches

v0.3.10 (with 019a37ab98ff53f04fecfadf602b6cd5ac748f7f and #3586)

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

References

Are there any links users can visit to find out more?