Stratax Contracts
First Flight #57
Beginner FriendlyDeFi
100 EXP
View all submissions
Previous Next
Submission Details
Impact: medium
Likelihood: medium

Wrong Asset Passed to 1inch Swap Fallback in Open Flow Causes DoS

Author Revealed upon completion

Wrong Asset Passed to 1inch Swap Fallback in Open Flow Causes DoS

Description

  • In _executeOpenOperation, the swap converts borrowed tokens to collateral (e.g., WETH → USDC). The _call1InchSwap function expects the second parameter to be the output token (the asset being swapped to), used when the 1inch router returns no data to compute returnAmount via balanceOf(_asset).

  • The open flow incorrectly passes flashParams.borrowToken (the input token) instead of _asset (the collateral/output token). When the router returns no data (result.length == 0), the fallback reads balanceOf(borrowToken), which is ~0 after the swap. This yields returnAmount ≈ 0 and causes the transaction to revert even though the swap succeeded and the contract received sufficient collateral.

// Step 3: Swap borrowed tokens via 1inch to get back the collateral token
IERC20(flashParams.borrowToken).approve(address(oneInchRouter), flashParams.borrowAmount);
​
// Execute swap via 1inch
@> uint256 returnAmount =
@> _call1InchSwap(flashParams.oneInchSwapData, flashParams.borrowToken, flashParams.minReturnAmount);
// Decode the return amount from the swap
if (result.length > 0) {
(returnAmount,) = abi.decode(result, (uint256, uint256));
} else {
// If no return data, check balance
@> returnAmount = IERC20(_asset).balanceOf(address(this));
}

Risk

Likelihood (medium):

  • The 1inch API returns calldata for multiple router versions and swap paths. Some paths (e.g., simple pool swaps, certain aggregator routes) use functions that do not return (returnAmount, spentAmount) and leave result empty.

  • Router upgrades or API changes can alter which paths return data. Integrators cannot guarantee that all swap routes will return data.

  • The unwind flow correctly passes _asset (line 586); only the open flow has the bug, indicating an oversight rather than a design choice.

Impact (medium):

  • DoS for users attempting to open leveraged positions when the chosen swap path returns no data.

  • The revert message ("Insufficient funds to repay flash loan" or "Insufficient return amount from swap") is misleading—the swap succeeded but the fallback used the wrong token.

  • No fund loss; the swap executes correctly and tokens are received. The failure occurs in post-swap validation.

Severity: Medium

Proof of Concept

// Scenario: Open flow with swap path that returns no data (result.length == 0)
//
// CONTEXT: _call1InchSwap uses the second parameter only when the 1inch router
// returns empty result. In that case it computes returnAmount = balanceOf(_asset).
// The second param must be the OUTPUT token (what we receive), not the INPUT token
// (what we spend). The open flow incorrectly passes borrowToken (input).
//
// 1. Flash loan gives _amount of USDC (_asset)
// 2. Supply USDC to Aave, borrow WETH (flashParams.borrowToken)
// 3. Swap WETH → USDC via 1inch; swap succeeds, router returns empty result
// (some 1inch paths do not return (returnAmount, spentAmount))
// 4. _call1InchSwap fallback triggers: returnAmount = IERC20(borrowToken).balanceOf(this)
// = IERC20(WETH).balanceOf(this) = 0 (we swapped all WETH away)
// 5. require(returnAmount >= totalDebt) → REVERTS (0 < totalDebt)
// 6. Transaction reverts despite successful swap and sufficient USDC received
//
// CORRECT BEHAVIOR: With _asset passed, fallback would use balanceOf(USDC),
// which equals the swap output. returnAmount would be correct and the tx would succeed.

Recommended Mitigation

Pass the output token (the asset being swapped to) as the second argument to _call1InchSwap. In the open flow, the swap is borrowToken → collateral; the collateral equals _asset (the flash loan token). The second parameter is used when the router returns no data to compute returnAmount via balanceOf(_asset)—it must be the token the contract receives from the swap, not the token it spends.

// Step 3: Swap borrowed tokens via 1inch to get back the collateral token
IERC20(flashParams.borrowToken).approve(address(oneInchRouter), flashParams.borrowAmount);
​
// Execute swap via 1inch
uint256 returnAmount =
- _call1InchSwap(flashParams.oneInchSwapData, flashParams.borrowToken, flashParams.minReturnAmount);
+ _call1InchSwap(flashParams.oneInchSwapData, _asset, flashParams.minReturnAmount);

Note: _asset and flashParams.collateralToken are identical in the open flow (both are the flash loan token). Either is correct; _asset is preferred for consistency with the unwind flow (line 586).

Location: src/Stratax.sol:514-516

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!