58209 sc medium lack of slippage protection in transmuter claimredemption and alchemistv3 withdraw leads to user yield losses

Submitted on Oct 31st 2025 at 11:37:36 UTC by @Smartkelvin for Audit Comp | Alchemix V3

• Report ID: #58209

• Report Type: Smart Contract

• Report severity: Medium

• Target: https://github.com/alchemix-finance/v3-poc/blob/immunefi_audit/src/Transmuter.sol

• Impacts:

  • Smart contract unable to operate due to lack of token funds

Description

Brief/Intro

The Transmuter.claimRedemption function and it contract associate (AlchemistV3.withdraw) lacks any minimum output protection , allowing dynamic system state changes—such as concurrent claims, bad debt ratio spikes, etc—to cause users to receive significantly less yield than expected after burning their position NFT

Vulnerability Details

The claimRedemption function in the Transmuter contract processes user redemptions by calculating time-pro-rata transmuted synthetics, applying bad debt scaling, conditionally redeeming from the Alchemist, and distributing yield tokens. However, it performs no verification that the final claimYield meets a user-specified minimum, exposing outputs to variance from external and concurrent state changes.

Impact Details

Direct User Loss

MEV Exploitation

Mitigation

Introduce a minYieldOut parameter to both functions and revert check immediately before transfers.

Proof of Concept

Proof of Concept

  function testClaimRedemption_BadDebtSlippage_UserGetsLess() external {
    uint256 user1Deposit = 2000e18;
    uint256 user1Mint = 1000e18;
    
    // ============================================
    //  User creates transmuter position
    // ============================================
    vm.startPrank(address(0xbeef));
    SafeERC20.safeApprove(address(vault), address(alchemist), user1Deposit);
    alchemist.deposit(user1Deposit, address(0xbeef), 0);
    uint256 tokenId = AlchemistNFTHelper.getFirstTokenId(address(0xbeef), address(alchemistNFT));
    
    // Mint synthetic tokens
    alchemist.mint(tokenId, user1Mint, address(0xbeef));
    
    // Create transmuter redemption
    SafeERC20.safeApprove(address(alToken), address(transmuterLogic), user1Mint);
    transmuterLogic.createRedemption(user1Mint);
    vm.stopPrank();
    
    uint256 redemptionId = 1;
    vm.roll(block.number + transmuterLogic.timeToTransmute() + 1);
    uint256 totalSyntheticsBefore = alchemist.totalSyntheticsIssued();
    uint256 totalUnderlyingBefore = alchemist.getTotalUnderlyingValue();
    uint256 transmuterBalance = IERC20(address(vault)).balanceOf(address(transmuterLogic));
    uint256 transmuterUnderlyingValue = alchemist.convertYieldTokensToUnderlying(transmuterBalance);
    
    uint256 denominator = totalUnderlyingBefore + transmuterUnderlyingValue > 0 
        ? totalUnderlyingBefore + transmuterUnderlyingValue 
        : 1;
    
    uint256 badDebtRatioBefore = totalSyntheticsBefore * 1e18 / denominator;
    
    console.log("\n=== SYSTEM STATE BEFORE CLAIM ===");
    console.log("Total Synthetics Issued:", totalSyntheticsBefore);
    console.log("Total Underlying in Alchemist:", totalUnderlyingBefore);
    console.log("Transmuter Balance:", transmuterBalance);
    console.log("Total Backing (denominator):", denominator);
    console.log("Bad Debt Ratio:", badDebtRatioBefore);
    
    // Calculate what user EXPECTS to receive (1:1 conversion)
    uint256 expectedYieldTokens = alchemist.convertDebtTokensToYield(user1Mint);
    console.log("\nUser expects yield tokens:", expectedYieldTokens);
    // ============================================
    //  User claims redemption
    // ============================================
    vm.startPrank(address(0xbeef));
    
    uint256 yieldBalanceBefore = IERC20(address(vault)).balanceOf(address(0xbeef));
    
    transmuterLogic.claimRedemption(redemptionId);
    
    uint256 yieldBalanceAfter = IERC20(address(vault)).balanceOf(address(0xbeef));
    uint256 actualYieldReceived = yieldBalanceAfter - yieldBalanceBefore;
    
    vm.stopPrank();
    
    console.log("\n=== CLAIM RESULTS ===");
    console.log("Expected yield tokens:", expectedYieldTokens);
    console.log("Actually received:", actualYieldReceived);

    
    console.log("\n=== THE ISSUE ===");
    console.log("User has NO slippage protection!");
    console.log("If bad debt existed, they would receive less");
    console.log("Transaction would still succeed - no revert");
    console.log("No way to specify minYieldTokens parameter");
    
    // Test passes, demonstrating lack of protection
    assertTrue(actualYieldReceived > 0, "User received some yield");
    
}