fee can change without the consent of users

[code423n4]

Lines of code

https://github.com/code-423n4/2022-06-putty/blob/3b6b844bc39e897bd0bbb69897f2deff12dc3893/contracts/src/PuttyV2.sol#L240
https://github.com/code-423n4/2022-06-putty/blob/3b6b844bc39e897bd0bbb69897f2deff12dc3893/contracts/src/PuttyV2.sol#L497

Vulnerability details

Impact

Fees are applied during withdraw, but can change between the time the order is filled and its terms are agreed upon and the withdrawal time, leading to a loss of the expected funds for the concerned users.

Proof of Concept

The scenario would be:

• Alice and Bob agrees to fill an order at a time fees are 0.1%
• During the duration of the option, fees are increased to 3%
• At withdrawal they'll pay 3% of the strike, although they wouldn't have created the order in the first place with such fees

Recommended Mitigation Steps

Mitigation could be:

• Store the fees in Order and verify that they are correct when the order is filled, so they are hardcoded in the struct
• Add a timestamp: this wouldn't fully mitigate but would still be better than the current setup
• Keep past fees and fee change timestamps in memory (for example in an array) to be able to retrieve the creation time fees at withdrawal

[outdoteth]

Report: Admin can change fee at any time for existing orders

[outdoteth]

PR with fix: outdoteth/putty-v2#4