- Type: Exploit
- Network: Mainnet
- Total lost: ~$80MM in various stablecoins
- Category: Reentrancy
- Exploited contracts:
- Attack transactions:
- Attack Block:: 14684814
- Date: Apr 30, 2022
- Reproduce:
forge test --match-contract Exploit_Fei -vvv
The function performs a low level call sending ETH with doTransferOut before updating the internal accounting for the borrower.
The fallback triggered by the low level call, calls Controller.exitMarket() which wipes the liquidity calculation of the borrower.
- Flashloan collateral to a factory contract
- Deploy with Create2 a borrower contract #1
- With contract #1, mint fCURRENCY equivalent to the flashloaned amount (fei currency of certain tokens)
- Then, borrow against the minted currency
- Redeem the borrowed amount to recover the collateral
- Use the factory to borrow against ETH (with fETH) more fUSDC, fUSDT and fFRAX
- Redeem all the tokens from step 6)
- Repeat steps 2 - 5.
- Redeem with the factory the fETH position
- Repay the flashloan
- Transfer the tokens back to the attacker
An old school cross-function reentrancy attack with the root cause of not respecting the checks-effects-interactions pattern. The attacker flashloaned multiple tokens wiping down the collateral draining the pool.
function borrowFresh(address payable borrower, uint borrowAmount) internal returns (uint) {
...
[ComptrollerStorage.sol L802]
// EFFECTS & INTERACTIONS
// (No safe failures beyond this point)
doTransferOut(borrower, borrowAmount);
accountBorrows[borrower].principal = vars.accountBorrowsNew;
accountBorrows[borrower].interestIndex = borrowIndex;
totalBorrows = vars.totalBorrowsNew;
emit Borrow(borrower, borrowAmount, vars.accountBorrowsNew, vars.totalBorrowsNew);
// unused function
// comptroller.borrowVerify(address(this), borrower, borrowAmount);
return uint(Error.NO_ERROR);
}
/// @notice Removes asset from sender's account liquidity calculation
/// @dev Sender must not have an outstanding borrow balance in the asset,
/// or be providing neccessary collateral for an outstanding borrow.
/// @param cTokenAddress The address of the asset to be removed
/// @return Whether or not the account successfully exited the market
function exitMarket(address cTokenAddress) external returns (uint) { ... }The issue was located in CERCImplementation.borrowFresh(), the internal function called after borrow() in the CERCDelegate Contract
The mentioned function performs the transfer via doTransferOut() before updating the internal variables.
- Respect the checks-effects-interactions to prevent cross-function and single function reentrancy
- Remember that only functions that use the reetrancy mutex of the contract that implements that modifier are protected. If different contracts declare distinct reentrancy modifiers, a cross reentrancy is still open.
