Lines of code

Vulnerability details

Impact

Attackers could drain rewards meant for other transcoders.

Proof of Concept

The key vulnerable code is in the reward() function:<https://github.com/code-423n4/2023-08-livepeer/blob/a3d801fa4690119b6f96aeb5508e58d752bda5bc/contracts/bonding/BondingManager.sol#L881-L882&gt;
This uses the transcoder’s total stake (earningsPool.totalStake) and the total active stake (currentRoundTotalActiveStake) to determine the transcoder’s portion of the rewards.

An attacker who is a transcoder could temporarily increase their stake before calling reward() by bonding more tokens or transferring bonds from another account they control. This would increase earningsPool.totalStake and get them a larger portion of the totalRewardTokens.

Then in updateTranscoderWithRewards(), the attacker’s cumulative rewards are increased by the full amount of _rewards: t.cumulativeRewards = t.cumulativeRewards.add(transcoderRewardStakeRewards).add(transcoderCommissionRewards);
After claiming the rewards, the attacker could decrease their stake back to the original amount. This would allow them to unfairly claim a larger portion of rewards than they should based on their true stake

Tools Used

Manual

Recommended Mitigation Steps

A snapshot of the total active stake should be taken before reward() is called, and that snapshot value should be used in the reward calculation instead of currentRoundTotalActiveStake.

Assessed type

Other

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