MochiVault.flashFee() May Truncate Result

[code423n4]

Handle

leastwood

Vulnerability details

Impact

The flashFee() function differs to the implementation found in usdm.flashFee() due to a missing SCALE variable which ensures the result is not truncated. The end result for MochiVault.flashFee() is that users could end up paying a slightly lower fee than intended if _amount is somewhat small.

This could abused by calling flashLoan() with a small _amount input and continually reentering the contract before using the actual funds. This enables a user to pay marginally less in fees as compared to a typical user.

If the asset being held in the vault is configured with a low decimals value, the degree of truncation could be significant, potentially resulting in little to no fees paid back into MochiVault.sol.

Proof of Concept

https://github.com/code-423n4/2021-10-mochi/blob/main/projects/mochi-core/contracts/vault/MochiVault.sol#L345-L354

Tools Used

Manual code review

Recommended Mitigation Steps

Consider updating return (_amount * 1337) / 1000000; in MochiVault.flashLoan() to return (_amount * ((1337 * SCALE) / 1000000)) / SCALE; where SCALE = 1e18.

[r2moon]

return (_amount * 1337) / 1000000; and return (_amount * ((1337 * SCALE) / 1000000)) / SCALE; could return same amount i think.

[ghoul-sol]

I see a risk of rounding errors but I think this is low risk since small amount flash loans are inpractical