The domainSeperator is not recalculated after a hard fork happens

[code423n4]

Handle

shw

Vulnerability details

Impact

The variable domainSeperator in EIP712Base is cached in the contract storage and will not change after the contract is initialized. However, if a hard fork happens after the contract deployment, the domainSeperator would become invalid on one of the forked chains due to the block.chainid has changed.

Proof of Concept

Referenced code:
EIP712Base.sol#L25-L44

Recommended Mitigation Steps

Consider using the implementation from OpenZeppelin, which recalculates the domain separator if the current block.chainid is not the cached chain ID.

[Splidge]

The OpenZeppelin implementation can't be used because of the contracts uses a proxy pattern and so can't use the constructor in the OpenZeppelin version.
Instead I have taken the same method OpenZeppelin use to get a new domainSeperator, here

[Splidge]

I've noticed that there is an upgradable version of the OpenZeppelin metaTx contracts. I'll try and work on using them instead of the fix used above.