31085 - [SC - Critical] Malicious users can front-run the distribution ...
Submitted on May 12th 2024 at 14:17:54 UTC by @Ch301 for Boost | Alchemix
Report ID: #31085
Report type: Smart Contract
Report severity: Critical
Target: https://github.com/alchemix-finance/alchemix-v2-dao/blob/main/src/Voter.sol
Impacts:
Theft of unclaimed royalties
Description
Brief/Intro
The pool weight is zero but users are still able to claim bribes.
Vulnerability Details
First, let's understand two main points:
1- When a user invokes Voter.sol#vote()
[#1] will update lastVoted[_tokenId]
to block.timestamp
Users to be able to reset the voting status, need to call Voter.sol#reset()
. However, the modifier onlyNewEpoch()
will force the user to wait until a new epoch starts since the last vote. So user can call reset()
at the first second of the new epoch.
2- To distribute rewards and bribes to all gauges you need to call Voter.sol#distribute()
only once per epoch. (Normally this is done by off-chain bots)
Now, the first point updates all the related values, but the Bribe.sol#withdraw()
will create a new checkpoint X
(see: _writeCheckpoint()
[#2]) this checkpoint get considered for the new epoch not the last epoch.
Next time the user calls voter.sol#claimBribes()
. the balanceOf
of the epoch X - 1
will allow the user to claim all/part from the bribes even if he didn't help the gauge to generate more emrssions
Impact Details
1- If the malicious user is the only one who votes to a gauge, He will claim all the bribes if they exist and the pool will receive zero emissions.
2- If there are multiple users vote for that particular pool, the malicious user will steal a part from other user's bribes.
References
#1: https://github.com/alchemix-finance/alchemix-v2-dao/blob/main/src/Voter.sol#L449
#2: https://github.com/alchemix-finance/alchemix-v2-dao/blob/main/src/Bribe.sol?utm_source=immunefi#L351-L360
Proof of Concept
Foundry PoC:
Please copy the following PoC in
Voting.t.sol