Stricter preconditions for payInterest #623
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This update for the `master` branch contains the changes made to address findings discovered during a security audit: * [Fix] Stricter preconditions for payInterest (#623) * [Fix] Fix offset in comments (#624) * [Fix] Use fixed lower call gas limit (#627) * [Fix] Separate XDaiForeignBridge contract with compound and GSN support (#626) * [Fix] Update GSN interface (#628) * [Fix] Block ERC20 selectors in AMB requests (#630) * [Other] Bump package and contracts interfaces version prior to 6.0.0 (#629)
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The
InterestReceiverStakeBuybackandInterestReceiverSwapToETHcontracts both utilize Uniswap in order to swap tokens. There is some slippage protection, but no protection against price manipulation is built in.Because the
payInterestfunction in theInterestConnectoris external and permissionless, anyone can call it at any time. Therefore, once enough interest accrues, one could take out a flash loan to manipulate the relevant Uniswap pools and callpayInterestto make a profit.Additionally, even if
payInterestweren't permissionless, it is possible for the Uniswap call inonInterestReceivedto fail, leaving the tokens sitting in theInterestReceivercontract. Here, theonInterestReceivedfunction is external and permissionless, meaning it's once again prone to being attacked via price manipulation.Furthermore, assuming the interest payment was not callable by untrusted parties at all, it would still be possible to manipulate the price by sandwiching that transaction between two others, either by chance or by collusion with miners.
The fix is to disable ability of storing tokens on the interest receiver contract and introduces extra EOA-only check to payInterest method.
This reduces the risk of possible price manipulations, since it is no longer possible to use flash-loans to perform an attack.
Front-running of the transaction is considered less dangerous, as it requires a much higher attack cost (at least 2 Uniswap transactions gas fees + 2 slippage and swap fees).