<!-- canonical: https://0xsimao.com/findings/tokemak-convex-rewards-stuck-dos -->

# Aura/Convex rewards are stuck after DOS

Crit/High · Sherlock · Liquidity provisioning · 17th July 2023

Finding H-16 of the Tokemak competition.

- Protocol: https://audits.sherlock.xyz/contests/101
- Codebase: https://github.com/0xsimao/2023-06-tokemak/tree/5d8e902ce33981a6506b1b5fb979a084602c6c9a
- Source: https://github.com/sherlock-audit/2023-06-tokemak-judging/issues/738

---

Since `_claimRewards` accounts for rewards with balanceBefore/After, and anyone can claim Convex rewards, then attacker can DOS the rewards and make them stuck in the LiquidationRow contract.

## Vulnerability Detail

Anyone can claim Convex rewards for any account.

https://etherscan.io/address/0x0A760466E1B4621579a82a39CB56Dda2F4E70f03#code

```solidity
function getReward(address _account, bool _claimExtras) public updateReward(_account) returns(bool){
    uint256 reward = earned(_account);
    if (reward > 0) {
        rewards[_account] = 0;
        rewardToken.safeTransfer(_account, reward);
        IDeposit(operator).rewardClaimed(pid, _account, reward);
        emit RewardPaid(_account, reward);
    }

    //also get rewards from linked rewards
    if(_claimExtras){
        for(uint i=0; i < extraRewards.length; i++){
            IRewards(extraRewards[i]).getReward(_account);
        }
    }
    return true;
}
```

In ConvexRewardsAdapter, the rewards are accounted for by using balanceBefore/after.

```solidity
function _claimRewards(
    address gauge,
    address defaultToken,
    address sendTo
) internal returns (uint256[] memory amounts, address[] memory tokens) {

		uint256[] memory balancesBefore = new uint256[](totalLength);
    uint256[] memory amountsClaimed = new uint256[](totalLength);
...

		for (uint256 i = 0; i < totalLength; ++i) {
        uint256 balance = 0;
        // Same check for "stash tokens"
        if (IERC20(rewardTokens[i]).totalSupply() > 0) {
            balance = IERC20(rewardTokens[i]).balanceOf(account);
        }

        amountsClaimed[i] = balance - balancesBefore[i];

	return (amountsClaimed, rewardTokens);
```

Adversary can call the external convex contract's  `getReward(tokemakContract)`. After this, the reward tokens are transferred to Tokemak without an accounting hook.

Now, when Tokemak calls claimRewards, then no new rewards are transferred, because the attacker already transferred them. `amountsClaimed` will be 0.

## Impact

Rewards are stuck in the LiquidationRow contract and not queued to the MainRewarder.

## Code Snippet

```solidity
// get balances after and calculate amounts claimed
for (uint256 i = 0; i < totalLength; ++i) {
    uint256 balance = 0;
    // Same check for "stash tokens"
    if (IERC20(rewardTokens[i]).totalSupply() > 0) {
        balance = IERC20(rewardTokens[i]).balanceOf(account);
    }

    amountsClaimed[i] = balance - balancesBefore[i];

    if (sendTo != address(this) && amountsClaimed[i] > 0) {
        IERC20(rewardTokens[i]).safeTransfer(sendTo, amountsClaimed[i]);
    }
}
```

https://github.com/sherlock-audit/2023-06-tokemak/blob/5d8e902ce33981a6506b1b5fb979a084602c6c9a/v2-core-audit-2023-07-14/src/destinations/adapters/rewards/ConvexRewardsAdapter.sol/#L102

## Tool used

Manual Review

## Recommendation

Don't use balanceBefore/After. You could consider using `balanceOf(address(this))` after claiming to see the full amount of tokens in the contract. This assumes that only the specific rewards balance is in the contract.

---

Related findings:

- [Attackers will reset `avgStart` of any user making rewards stuck for longer and get lost to savings](https://0xsimao.com/findings/exactly-protocol-update-staking-contract-ii-avg-rewards-stuck-longer): Exactly Protocol Update - Staking Contract
- [Having no deposits in `StakedEXA` will lead to stuck rewards when harvesting](https://0xsimao.com/findings/exactly-protocol-update-staking-contract-ii-deposits-stuck-rewards-harvesting): Exactly Protocol Update - Staking Contract