Attacker can brick redemptions by donating a small amount

mediumCode4rena

Lines of code

https://github.com/AngleProtocol/angle-transmuter/blob/6f2ffcb1e89e3bba05c9aa2133ef94347aa42c28/contracts/transmuter/libraries/LibGetters.sol#L89

Vulnerability details

Impact

While the fix properly fixes the issue of collateralization ratio overflows (that can no longer occurs), it enables DoS attacks on the redemption mechanism:

Issue description

Consider the example that was already provided (https://github.com/code-423n4/2023-06-angle-findings/issues/9 ), where an attacker produces an overflow by donating a small amount:

Let's take an extreme example and say that stablecoinsIssued is 1 (although the attack also works with larger values). If a user now transfer 1 USD of a stablecoin to the contract, totalCollateralization will be roughly 10**18 . Therefore, the system calculates:

collatRatio = uint64(10**18 * 10**9 / 1) = uint64(10**27);

Because 10**27 > type(uint64).max, this overflows.

After the fix, any call to getCollateralRatio will revert because of the SafeCast. Therefore, any call to _quoteRedemptionCurve will also revert, meaning that redemptions fail.

Note that the attacker never directly interacted with the contract (as he donated the tokens), which means that the protection also does not work.

Recommendation

It is recommended to set collatRatio to type(uint64).max instead of reverting when an overflow happens. Then, the problem would no longer occur.

Assessed type

DoS