Mutual consent cannot be revoked and stays valid forever

[code423n4]

Lines of code

https://github.com/debtdao/Line-of-Credit/blob/e8aa08b44f6132a5ed901f8daa231700c5afeb3a/contracts/utils/MutualConsent.sol#L11-L68
https://github.com/debtdao/Line-of-Credit/blob/e8aa08b44f6132a5ed901f8daa231700c5afeb3a/contracts/modules/credit/LineOfCredit.sol#L247-L262

Vulnerability details

Impact

Contracts that inherit from the MutualConsent contract, have access to a mutualConsent modifier.
Functions that use this modifier need consent from two parties to be called successfully.

Once one party has given consent for a function call, it cannot revoke consent.
This means that the other party can call this function at any time now.

This opens the door for several exploitation paths.
Most notably though the functions LineOfCredit.setRates(), LineOfCredit.addCredit() and LineOfCredit.increaseCredit() can cause problems.

One party can use Social Engineering to make the other party consent to multiple function calls and exploit the multiple consents.

Proof of Concept

1. A borrower and lender want to change the rates for a credit.
   The borrower wants to create the possibility for himself to change the rates in the future without the lender's consent.
2. The borrower and lender agree to set dRate and fRate to 5%.
3. The lender calls the LineOfCredit.setRates() function to give his consent.
4. The borrower might now say to the lender "Let's put the rate to 5.1% instead, I will give an extra 0.1%"
5. The borrower and lender now both call the LineOfCredit.setRates() function to set the rates to 5.1%.
6. The borrower can now set the rates to 5% at any time. E.g. they might increase the rates further in the future (the borrower playing by the rules)
   and at some point the borrower can decide to set the rates to 5%.

Links:
MutualConsent contract: https://github.com/debtdao/Line-of-Credit/blob/audit/code4rena-2022-11-03/contracts/utils/MutualConsent.sol

LineOfCredit.setRates() function: https://github.com/debtdao/Line-of-Credit/blob/e8aa08b44f6132a5ed901f8daa231700c5afeb3a/contracts/modules/credit/LineOfCredit.sol#L247-L262

Tools Used

VSCode

Recommended Mitigation Steps

There are several options to fix this issue:

1. Add a function to the MutualConsent contract to revoke consent for a function call.
2. Make consent valid only for a certain amount of time.
3. Invalidate existing consents for a function when function is called with different arguments.

Option 3 requires a lot of additional bookkeeping but is probably the cleanest solution.

[c4-judge]

dmvt marked the issue as primary issue

[c4-judge]

dmvt marked the issue as selected for report

[c4-sponsor]

kibagateaux marked the issue as sponsor confirmed

[c4-judge]

dmvt marked the issue as satisfactory