Signature bypass

[code423n4]

Lines of code

https://github.com/code-423n4/2022-05-cudos/blob/de39cf3cd1f1e1cf211819b06d4acf6a043acda0/solidity/contracts/Gravity.sol#L185
https://github.com/code-423n4/2022-05-cudos/blob/de39cf3cd1f1e1cf211819b06d4acf6a043acda0/solidity/contracts/Gravity.sol#L238-L240

Vulnerability details

Impact

It's possible to bypass verifySig and checkValidatorSignatures methods using empty signers.

Proof of Concept

The method ecrecover returns address(0) when the signature is wrong, so if a user use address(0) as a validator or _signer the return will be true.

_signer == ecrecover(messageDigest, _v, _r, _s);

Also, the method checkValidatorSignatures never check that the validator is repeated inside the array, so if someone it's able to specify the validators, and it's able to sign with one valid key, if he repeat the same signature multiple times, it will be computed as a different one.

Recommended Mitigation Steps

check that _signer is not empty.

[maptuhec]

Duplicate of #127