Aamirusmani1552 - The protocol does not account for dual fee on fee on transfer token both stable fee currency and stable origin tokens are equal.

[sherlock-admin4]

Aamirusmani1552

Medium

The protocol does not account for dual fee on fee on transfer token both stable fee currency and stable origin tokens are equal.

Summary

When user want to do the defiSwap first and then stable swap, there might be situation where the stable origin token and stable fee tokens are the same tokens. And if that token is a fee on transfer token then it will cause issue:

    function _stablecoinSwap(address wallet, StablecoinSwap memory ss) internal {
        if (ss.stablecoinFeeCurrency != address(0) && ss.stablecoinFeeSafe != address(0)) {
            ERC20PermitUpgradeable(ss.stablecoinFeeCurrency).safeTransferFrom(
                wallet, ss.stablecoinFeeSafe, ss.feeAmount
            );
        }

        // Handle the transfer or burning of the origin currency:
        // If the origin is a recognized stablecoin (XYZ), burn the specified amount from the wallet.
        if (isXYZ(ss.origin)) {
            Stablecoin(ss.origin).burnFrom(wallet, ss.oAmount);
        } else {
            ERC20PermitUpgradeable(ss.origin).safeTransferFrom(wallet, ss.liquiditySafe, ss.oAmount);
        }

        // Handle the minting or transferring of the target currency:
        // If the target is a recognized stablecoin (XYZ), mint the required amount to the destination address.
        if (isXYZ(ss.target)) {
            Stablecoin(ss.target).mintTo(ss.destination, ss.tAmount);
        } else {
            // @audit so the liquidity safe is going to give approval every time?
            ERC20PermitUpgradeable(ss.target).safeTransferFrom(ss.liquiditySafe, ss.destination, ss.tAmount);
        }
    }
    ```
    
    Github: [link](https://github.com/sherlock-audit/2024-11-telcoin/blob/main/telcoin-audit/contracts/stablecoin/StablecoinHandler.sol#L148C1-L165C1)
    
    For example if user wants to perform the defi to stable swap, the balance of origin token from the defi swap will not account for fee on the transfers double time. first when fee is transferred and second when the actual tokens is transferred. So it migh cause dos.  

### Root Cause

_No response_

### Internal pre-conditions

_No response_

### External pre-conditions

_No response_

### Attack Path

_No response_

### Impact

_No response_

### PoC

I am submitting this on last minute. that is why there is no POC. will provide later if necessary.

### Mitigation

Perform the neccessary check to make it work