Submitted by Lambda
https://github.com/code-423n4/2022-12-tessera/blob/1e408ebc1c4fdcc72678ea7f21a94d38855ccc0b/src/modules/GroupBuy.sol#L455 
https://github.com/code-423n4/2022-12-tessera/blob/1e408ebc1c4fdcc72678ea7f21a94d38855ccc0b/src/modules/GroupBuy.sol#L478
The functions _verifyUnsuccessfulState and _verifySuccessfulState should always have a differing behavior with regards to reversion, i.e. when one does not revert, the other should revert. In one condition, this is not true. Namely, when we have pool.success == false and block.timestamp == pool.terminationPeriod, this check within _verifyUnsuccessfulState is false:
Similarly, this check within _verifySuccessfulState is also false:
Because this breaks a fundamental invariant of the contract, there are probably multiple ways to exploit it.
One way an attacker can exploit is by calling claim (to get his contribution back completely), bidding again with a higher value than his previous contributions (to get his contributions back again).
Lets assume we are at timestamp pool.terminationPeriod. Attacker Charlie has performed the lowest bid with quantity 10 and price 1 ETH. He calls claim to get his 10 ETH back. Now, he calls contribute with a quantity of 10 and a price of 2 ETH. Because this bid is higher than his previous one (which was the lowest one), his pendingBalances is set to 10 ETH (for the deleted entries) and his userContributions is set to 20 ETH (for this new contribution). He can now call claim again to get back his 20 ETH in userContributions, but also the 10 ETH in pendingBalances. Like that, he has stolen 10 ETH (and could use this attack pattern to drain the whole contract).
Change < in _verifySuccessfulState to <=.
HickupHH3 (judge) commented:
stevennevins (Tessera) confirmed and mitigated:
