TimeswapConvenience.sol#borrowGivenDebt() Attacker can increase state.y to an extremely large value with a dust amount of assetOut

[code423n4]

Handle

WatchPug

Vulnerability details

https://github.com/code-423n4/2022-01-timeswap/blob/bf50d2a8bb93a5571f35f96bd74af54d9c92a210/Timeswap/Timeswap-V1-Convenience/contracts/libraries/BorrowMath.sol#L19-L53

This issue is similar to the two previous issues related to state.y manipulation. Unlike the other two issues, this function is not on TimeswapPair.sol but on TimeswapConvenience.sol, therefore this can not be solved by adding onlyConvenience modifier.

Actually, we believe that it does not make sense for the caller to specify the interest they want to pay, we recommend removing this function.

Impact

• When pool.state.y is extremely large, many core features of the protocol will malfunction, as the arithmetic related to state.y can overflow. For example:

LendMath.check(): https://github.com/code-423n4/2022-01-timeswap/blob/bf50d2a8bb93a5571f35f96bd74af54d9c92a210/Timeswap/Timeswap-V1-Core/contracts/libraries/LendMath.sol#L28-L28

BorrowMath.check(): https://github.com/code-423n4/2022-01-timeswap/blob/bf50d2a8bb93a5571f35f96bd74af54d9c92a210/Timeswap/Timeswap-V1-Core/contracts/libraries/BorrowMath.sol#L31-L31

• An attacker can set state.y to a near overflow value, then lend() to get a large amount of extra interest (as Bond tokens) with a small amount of asset tokens. This way, the attacker can steal funds from other lenders and liquidity providers.