Immunefi Audit Competitions
Alchemix V3

58768 sc high mytsharesdeposited is not updated during liquidations breaking core accounting

Submitted on Nov 4th 2025 at 12:38:53 UTC by @auditagent for Audit Comp | Alchemix V3

Description

Brief/Intro

The _mytSharesDeposited state variable is intended to track the total amount of myt tokens held by the AlchemistV3 contract.

However, during the liquidation process, myt tokens are transferred out, but the _mytSharesDeposited variable is never decremented to reflect this outflow. This occurs in both the _forceRepay and _doLiquidation

Vulnerability Details

_mytSharesDeposited feeds both depositCap enforcement and TVL via _getTotalUnderlyingValue(). When MYT exits the contract without decrementing this counter, TVL and cap appear higher than reality.

function _forceRepay(uint256 accountId, uint256 amount) internal returns (uint256) {
    // ...
    if (account.collateralBalance > protocolFeeTotal) {
        account.collateralBalance -= protocolFeeTotal;
        // Transfer the protocol fee to the protocol fee receiver
        TokenUtils.safeTransfer(myt, protocolFeeReceiver, protocolFeeTotal); // myt transferred out
    }
    if (creditToYield > 0) {
        // Transfer the repaid tokens from the account to the transmuter.
        TokenUtils.safeTransfer(myt, address(transmuter), creditToYield); // myt transferredout
        // @audit: `_mytSharesDeposited` is not decremented for MYT transferred out
    }
    return creditToYield;
}

function _doLiquidation(...) internal returns (...) {
    // ...
    // send liquidation amount - fee to transmuter
    TokenUtils.safeTransfer(myt, transmuter, amountLiquidated - feeInYield); // myt transferred out
    // send base fee to liquidator if available
    if (feeInYield > 0 && account.collateralBalance >= feeInYield) {
        TokenUtils.safeTransfer(myt, msg.sender, feeInYield); // myt transferred out
        // @audit: `_mytSharesDeposited` is not decremented for MYT transferred out

    }
    // ...
}

Root cause

  • In _forceRepay, MYT is sent to the transmuter and the protocol fee receiver, but _mytSharesDeposited is never decremented by those outflows.
  • In _doLiquidation, MYT is sent to the transmuter and to the liquidator, but _mytSharesDeposited is not decremented.
  • Other flows correctly update the counter.

Impact Details

  • _getTotalUnderlyingValue() overstates collateral.

References

Proof of Concept

Proof of Concept

Add the following test in src/test/AlchemistV3.t.sol and run using forge test --match-test test_mismatch

    function test_mismatch() external {
        uint256 depositAmount = 1_000e18;
        vm.startPrank(address(0xbeef));
        SafeERC20.safeApprove(address(vault), address(alchemist), depositAmount);
        alchemist.deposit(depositAmount, address(0xbeef), 0);
        uint256 tokenId = AlchemistNFTHelper.getFirstTokenId(address(0xbeef), address(alchemistNFT));
        alchemist.mint(tokenId, alchemist.totalValue(tokenId) * FIXED_POINT_SCALAR / minimumCollateralization, address(0xbeef));
        vm.stopPrank();

        uint256 initialVaultSupply = IERC20(address(mockStrategyYieldToken)).totalSupply();
        uint256 modifiedVaultSupply = (initialVaultSupply * 7000 / 10_000) + initialVaultSupply;
        IMockYieldToken(mockStrategyYieldToken).updateMockTokenSupply(modifiedVaultSupply);

        vm.startPrank(externalUser);
        alchemist.liquidate(tokenId);
        vm.stopPrank();

        uint256 shares = IERC20(address(vault)).balanceOf(address(alchemist));
        // @audit: TVL derived from _mytSharesDeposited should equal underlyingFromShares but they are not
        assertNotEq(alchemist.getTotalUnderlyingValue(), alchemist.convertYieldTokensToUnderlying(shares));
    }