#42214 [SC-High] Leftover `WBERA` and `YEET` sent to `StakeV2` instead of to user who is claiming rewards

Submitted on Mar 21st 2025 at 18:24:40 UTC by @Oxl33 for Audit Comp | Yeet

• Report ID: #42214

• Report Type: Smart Contract

• Report severity: High

• Target: https://github.com/immunefi-team/audit-comp-yeet/blob/main/src/contracts/Zapper.sol

• Impacts:

  • Theft of unclaimed royalties

Description

Description:

When users claim rewards through one of the claim functions in StakeV2, after removing liquidity from Kodiak and swapping, commonly (if not always) there will be left unused WBERA and YEET tokens. The issue lies in the use of _msgSender() as the receiver of the leftover tokens, because when StakeV2 calls Zapper, msg.sender is not the user who initiated the transaction, but StakeV2 contract. Due to this, one of the leftover tokens (or in the case of claimRewardsInToken function - both WBERA and YEET) get sent to StakeV2 and later added to rewards that other stakers can claim, when in reality, these leftover tokens belong to the user who initiated the transaction, because they originated from Kodiak LP tokens that the user had the right to claim as reward, based on their staked YEET amount and the duration of their staking.

WBERA and/or YEET tokens can be left unused during the claiming process in these cases:

1. If Kodiak returns bigger than expected output amounts of WBERA and YEET after burning Kodiak LP tokens

2. During swaps from WBERA to YEET and vice versa, if expected input amount is smaller than actual output from Kodiak

3. During swaps from WBERA/YEET to another whitelisted token (e.g. KDK or oBERO), if expected input amount is smaller than actual output from Kodiak

Considering all these possible cases and the fact that claiming rewards is one of the main functionalities of this protocol, I believe this issue will happen commonly and will affect the majority of the users.

The issue occurs in these functions:

StakeV2::claimRewardsInNative -> Zapper::zapOutNative -> _swapToWBERA - only YEET sent to StakeV2 StakeV2::claimRewardsInToken0 -> Zapper::zapOutToToken0 - only YEET OR only WBERA sent to StakeV2 StakeV2::claimRewardsInToken1 -> Zapper::zapOutToToken1 - only YEET OR only WBERA sent to StakeV2 StakeV2::claimRewardsInToken -> Zapper::zapOut - both YEET AND WBERA sent to StakeV2

Impact:

I think the most fitting impact from all the in-scope impacts is Theft of unclaimed royalties, because the affected users lose a part of the royalties that belongs to them and other stakers benefit from them, so it is similar to theft.

This impact is meant for high severity issues and I believe it fits the overall severity of this issue, because this issue will occur frequently (high likelihood) and the funds of users are mismanaged, which leads to undeserved losses for some users and gains for others (high impact).

Proof of Concept

Proof of Concept:

1. Alice stakes her YEET tokens using StakeV2

2. A percentage of BERA from yeets of users gets sent to StakeV2

3. Manager distributes the rewards with executeRewardDistribution and executeRewardDistributionYeet functions

4. Alice wants to claim her share of the rewards and she wants to receive them in the form of e.g. oBERO tokens, so she calls StakeV2::claimRewardsInToken and specifies output token as oBERO (assuming it is whitelisted, because it is used in compound)

5. Zapper::zapOut function gets called and there are leftover tokens of WBERA and YEET, due to the reasons I mentioned in Description section

6. _clearUserDebt gets called with token0Debt and token1Debt being more than 0, and _msgSender() being StakeV2 contract

7. Leftover tokens that rightfully belong to Alice get sent to StakeV2 contract

Zapper::zapOut function gets called here: https://github.com/immunefi-team/audit-comp-yeet/blob/da15231cdefd8f385fcdb85c27258b5f0d0cc270/src/StakeV2.sol#L391

_clearUserDebt function gets called here (_msgSender() is StakeV2): https://github.com/immunefi-team/audit-comp-yeet/blob/da15231cdefd8f385fcdb85c27258b5f0d0cc270/src/contracts/Zapper.sol#L352