Summary

The contract allows for voters to delegate their vote to another user, to save on gas. The signature does not hold a usage limit, so anyone with a valid signature could re-use the voters' votes, even if the voter wishes to vote for other candidates.

Vulnerability Details

The RankedChoice::rankCandidatesBySig function allows caller to re-use the same signature multiple times.
This would result in a malicious user being able to force the votes of candidates, even if the voter wishes to change it, and through different presidential elections.

Impact

If the voter wishes to change their vote, a malicious user could force the votes from the signature.

Recommendations

We need every signatures to be used only once by this function. We can use a mapping(address voter => uint ctr) ctrVotesDelegated to hold the number of times a user delegated their votes, and add a uint ctr in the rankCandidatesBySig function that checks for the actual value of ctrVotesDelegated, so the signature could not be re-used.