withdraw timelock can be circumvented

[code423n4]

Handle

cmichel

Vulnerability details

One can withdraw the entire PrizePool deposit by circumventing the timelock.
Assume the user has no credits for ease of computation:

• user calls withdrawWithTimelockFrom(user, amount=userBalance) with their entire balance. This "mints" an equivalent amount of timelock and resets _unlockTimestamps[user] = timestamp = blockTime + lockDuration.
• user calls withdrawWithTimelockFrom(user, amount=0) again but this time withdrawing 0 amount. This will return a lockDuration of 0 and thus unlockTimestamp = blockTime. The inner _mintTimelock now resets _unlockTimestamps[user] = unlockTimestamp
• As if (timestamp <= _currentTime()) is true, the full users amount is now transferred out to the user in the _sweepTimelockBalances call.

Impact

Users don't need to wait for their deposit to contribute their fair share to the prize pool.
They can join before the awards and leave right after without a penalty which leads to significant issues for the protocol.
It's the superior strategy but it leads to no investments in the strategy to earn the actual interest.

Recommended Mitigation Steps

The unlock timestamp should be increased by duration each time, instead of being reset to the duration.

[asselstine]

Mitigation:

If a user's timelock balance is non-zero, the prize strategy rejects the ticket burn.