#42039 [SC-High] When calling `StakeV2::claimRewardsInNative()` surplus $YEET are send to the StakeV2 contract instead of the user

Submitted on Mar 20th 2025 at 08:26:24 UTC by @BenR for Audit Comp | Yeet

• Report ID: #42039

• 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 yield

Description

Brief/Intro

When a user calls StakeV2::claimRewardsInNative to claim his rewards in $BERA tokens, surplus $YEET tokens are send to the StakeV2 contract instead of the user. This results in a loss of the surplus tokens for the user.

Vulnerability Details

When staking $YEET in the StakeV2 contract, users are earning rewards in form of vault shares from the Trifecta vault. If a user wants to claim his rewards in $BERA tokens he calls StakeV2::claimRewardsInNative(). The function checks if the user has earned the provided amount of shares and calls Zapper:: zapOutNative(). This function

• withdraws the provided amount of share amount from the Trifecta vault

• unstakes the received LP tokens from the $YEET/$WBERA KodiakVault and receives the corresponding $YEET and $WBERA tokens

• swaps the $YEET tokens to $WBERA tokens based on swap parameters provided by the user

• unwraps the $WBERA to $BERA and sends it to the user

• transfers any surplus $YEET tokens not consumed by the swap using the _clearUserDebt function

_clearUserDebt(token0, token1, token0Debt, token1Debt, _msgSender());

The issue arises from the fact that the receiver of the surplus $YEET tokens is set to _msgSender() which is the StakeV2 contract and not the user.

Impact Details

Because the surplus $YEET tokens are send to _msgSender() which is the StakeV2 contract, their value is lost for the user.

References

https://github.com/immunefi-team/audit-comp-yeet/blob/da15231cdefd8f385fcdb85c27258b5f0d0cc270/src/contracts/Zapper.sol#L622

Proof of Concept

POC (step by step)

• user want to claim his rewards in BERA and calls StakeV2::claimRewardsInNative with the amount of 100 shares

• the 100 shares are withdrawn from the TrifectaVault returning 100 LP tokens of the $YEET/$WBERA KodiakVault

• the 100 LP tokens are unstaked from the $YEET/$WBERA KodiakVault and the Zapper contract receives 500$WBERA and 500,000 $YEET

• based on the parameters provided by the user 400,000 $YEET are swapped to 400 $WBERA

• all 900 $WBERA are unwrapped to $BERA and send to the user

• the surplus 100.000 $YEET are send to the StakingV2 contract instead of the user resulting in a loss of 100.000 $YEET for the user