Did Not Approve To Zero First Causing Certain Token Transfer To Fail

[code423n4]

Lines of code

https://github.com/code-423n4/2023-02-ethos/blob/73687f32b934c9d697b97745356cdf8a1f264955/Ethos-Core/contracts/ActivePool.sol#L171
https://github.com/code-423n4/2023-02-ethos/blob/73687f32b934c9d697b97745356cdf8a1f264955/Ethos-Core/contracts/ActivePool.sol#L180
https://github.com/code-423n4/2023-02-ethos/blob/73687f32b934c9d697b97745356cdf8a1f264955/Ethos-Core/contracts/ActivePool.sol#L183

Vulnerability details

Impact

Some tokens (like USDT) do not work when changing the allowance from an existing non-zero allowance value. For example Tether (USDT)'s approve() function will revert if the current approval is not zero, to protect against front-running changes of approvals.

Proof of Concept

Allowance was not set to zero first before changing the allowance.

Some tokens (like USDT) do not work when changing the allowance from an existing non-zero allowance value. For example Tether (USDT)'s approve() function will revert if the current approval is not zero, to protect against front-running changes of approvals.

in ActivePool.sendCollateral, in lines 180 and 183, before calling safeIncreaseAllowance, must be approved by zero first, and then the safeIncreaseAllowance function can be called. Otherwise, the ActivePool.sendCollateral function will revert every time it handles such kinds of tokens.

The same problem can be seen in the line below:
https://github.com/code-423n4/2023-02-ethos/blob/73687f32b934c9d697b97745356cdf8a1f264955/Ethos-Core/contracts/ActivePool.sol#L279
https://github.com/code-423n4/2023-02-ethos/blob/73687f32b934c9d697b97745356cdf8a1f264955/Ethos-Core/contracts/BorrowerOperations.sol#L506

Tools Used

Manually

Sample report approved by the C4 judges team:
code-423n4/2022-06-connext-findings#154

Recommended Mitigation Steps

It is recommended to set the allowance to zero before increasing the allowance

[code423n4]

Withdrawn by SaeedAlipoor01988