Removing an allowed token permanently bricks existing LP redemptions
Summary
The owner can call setAllowedToken(token, false), which deletes the only mapping from the underlying token to its AssetToken. Existing LP shares and underlying remain in the old AssetToken, but redeem now rejects the token as not allowed. Re-allowing deploys a new AssetToken and does not recover the old one.
Root cause and evidence
ThunderLoan.setAllowedToken at lines 238-242 executes delete s_tokenToAssetToken[token]. isAllowedToken at lines 260-262 derives the allowed status solely from that mapping, and redeem at lines 161-168 uses revertIfNotAllowedToken. No separate registry retains the legacy AssetToken address.
Impact
A mistaken, compromised, or malicious owner action can permanently lock all underlying deposited for a token and make every existing LP receipt unredeemable.
Minimal patch
Separate token status from the immutable token-to-AssetToken association. Keep the mapping entry forever and store an independent isAllowed flag that can stop new deposits/loans while still permitting redemption. At minimum, refuse removal while the AssetToken has nonzero supply or underlying.
Regression test
Deposit, disable the token, and assert that new deposits and loans are blocked but redemption of existing shares still succeeds. Re-enabling must reuse the original AssetToken address rather than deploy a new one.
Lead Judging Commences
[M-01] 'ThunderLoan::setAllowedToken' can permanently lock liquidity providers out from redeeming their tokens
## Description If the 'ThunderLoan::setAllowedToken' function is called with the intention of setting an allowed token to false and thus deleting the assetToken to token mapping; nobody would be able to redeem funds of that token in the 'ThunderLoan::redeem' function and thus have them locked away without access. ## Vulnerability Details If the owner sets an allowed token to false, this deletes the mapping of the asset token to that ERC20. If this is done, and a liquidity provider has already deposited ERC20 tokens of that type, then the liquidity provider will not be able to redeem them in the 'ThunderLoan::redeem' function. ```solidity function setAllowedToken(IERC20 token, bool allowed) external onlyOwner returns (AssetToken) { if (allowed) { if (address(s_tokenToAssetToken[token]) != address(0)) { revert ThunderLoan__AlreadyAllowed(); } string memory name = string.concat("ThunderLoan ", IERC20Metadata(address(token)).name()); string memory symbol = string.concat("tl", IERC20Metadata(address(token)).symbol()); AssetToken assetToken = new AssetToken(address(this), token, name, symbol); s_tokenToAssetToken[token] = assetToken; emit AllowedTokenSet(token, assetToken, allowed); return assetToken; } else { AssetToken assetToken = s_tokenToAssetToken[token]; @> delete s_tokenToAssetToken[token]; emit AllowedTokenSet(token, assetToken, allowed); return assetToken; } } ``` ```solidity function redeem( IERC20 token, uint256 amountOfAssetToken ) external revertIfZero(amountOfAssetToken) @> revertIfNotAllowedToken(token) { AssetToken assetToken = s_tokenToAssetToken[token]; uint256 exchangeRate = assetToken.getExchangeRate(); if (amountOfAssetToken == type(uint256).max) { amountOfAssetToken = assetToken.balanceOf(msg.sender); } uint256 amountUnderlying = (amountOfAssetToken * exchangeRate) / assetToken.EXCHANGE_RATE_PRECISION(); emit Redeemed(msg.sender, token, amountOfAssetToken, amountUnderlying); assetToken.burn(msg.sender, amountOfAssetToken); assetToken.transferUnderlyingTo(msg.sender, amountUnderlying); } ``` ## Impact The below test passes with a ThunderLoan\_\_NotAllowedToken error. Proving that a liquidity provider cannot redeem their deposited tokens if the setAllowedToken is set to false, Locking them out of their tokens. ```solidity function testCannotRedeemNonAllowedTokenAfterDepositingToken() public { vm.prank(thunderLoan.owner()); AssetToken assetToken = thunderLoan.setAllowedToken(tokenA, true); tokenA.mint(liquidityProvider, AMOUNT); vm.startPrank(liquidityProvider); tokenA.approve(address(thunderLoan), AMOUNT); thunderLoan.deposit(tokenA, AMOUNT); vm.stopPrank(); vm.prank(thunderLoan.owner()); thunderLoan.setAllowedToken(tokenA, false); vm.expectRevert(abi.encodeWithSelector(ThunderLoan.ThunderLoan__NotAllowedToken.selector, address(tokenA))); vm.startPrank(liquidityProvider); thunderLoan.redeem(tokenA, AMOUNT_LESS); vm.stopPrank(); } ``` ## Recommendations It would be suggested to add a check if that assetToken holds any balance of the ERC20, if so, then you cannot remove the mapping. ```diff function setAllowedToken(IERC20 token, bool allowed) external onlyOwner returns (AssetToken) { if (allowed) { if (address(s_tokenToAssetToken[token]) != address(0)) { revert ThunderLoan__AlreadyAllowed(); } string memory name = string.concat("ThunderLoan ", IERC20Metadata(address(token)).name()); string memory symbol = string.concat("tl", IERC20Metadata(address(token)).symbol()); AssetToken assetToken = new AssetToken(address(this), token, name, symbol); s_tokenToAssetToken[token] = assetToken; emit AllowedTokenSet(token, assetToken, allowed); return assetToken; } else { AssetToken assetToken = s_tokenToAssetToken[token]; + uint256 hasTokenBalance = IERC20(token).balanceOf(address(assetToken)); + if (hasTokenBalance == 0) { delete s_tokenToAssetToken[token]; emit AllowedTokenSet(token, assetToken, allowed); + } return assetToken; } } ```
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