Seaman

title	Seaman
description	Forcing contracts to trade at manipulated token prices
type	Exploit
network	binance smart chain
date	2022-11-29
loss_usd	7000
returned_usd	0
tags	business logic price manipulation
subcategory
vulnerable_contracts	0x6bc9b4976ba6f8c9574326375204ee469993d038
tokens_lost	GVC
attacker_addresses	0x4b1f47be1f678076f447585beba025e3a046a9fa 0x0E647d34c4caF61D9E377a059A01b5C85AB1d82a
malicious_token
attack_block	23467516
reproduction_command	forge test --match-contract Exploit_Seaman -vvv
attack_txs	0x6f1af27d08b10caa7e96ec3d580bf39e29fd5ece00abda7d8955715403bf34a8
sources	title url Beosin Alert https://twitter.com/BeosinAlert/status/1597535796621631489 title url Source Code https://bscscan.com/address/0x6bc9b4976ba6f8c9574326375204ee469993d038#code

Step-by-step

1. Flashloan some USDC
2. Use the flashloan to buy all GVC in a pool
3. Call transfer() so contract buys GVC at current high price
4. Sell your GVC
5. Return flashloan

Detailed Description

This is very similar to the attack on MBC. It actually involves the same method, swapAndLiquifyV1.

Every time someone called _transfer on Seaman, the swapAndLiquify calls where made. These methods would exchange the accumulated fee on the contract for GVC, passing through the BUSD pool.

    function _transfer(
        address from,
        address to,
        uint256 amount
    ) internal override {
         // ...
         if( uniswapV2Pair.totalSupply() > 0 && balanceOf(address(this)) > balanceOf(address(uniswapV2Pair)).div(10000) && to == address(uniswapV2Pair)){
            if (
                !swapping &&
                _tokenOwner != from &&
                _tokenOwner != to &&
               !ammPairs[from] &&
                !(from == address(uniswapV2Router) && !ammPairs[to])&&
                swapAndLiquifyEnabled
            ) {
                swapping = true;
                swapAndLiquifyV3();
                swapAndLiquifyV1();
                swapping = false;
            }
        }
       // ...
    }

    function swapAndLiquifyV1() public {
        uint256 canlpAmount = lpAmount.sub(lpTokenAmount);
        uint256 amountT = balanceOf(address(uniswapV2Pair)).div(10000);
        if(balanceOf(address(this)) >= canlpAmount && canlpAmount >= amountT){
            if(canlpAmount >= amountT.mul(5))
                canlpAmount = amountT.mul(5);
            lpTokenAmount = lpTokenAmount.add(canlpAmount);
            uint256 beflpBal = lpToken.balanceOf(address(this));
            swapTokensFor(canlpAmount,address(lpToken),address(this));
            uint256 newlpBal = lpToken.balanceOf(address(this)).sub(beflpBal);
            lpDivTokenAmount = lpDivTokenAmount.add(newlpBal);
            isLpProc = true;
        }
    }

    function swapAndLiquifyV3() public {
        uint256 canhAmount = hAmount.sub(hTokenAmount);
        uint256 amountT = balanceOf(address(uniswapV2Pair)).div(10000);
        if(balanceOf(address(this)) >= canhAmount && canhAmount >= amountT){
            if(canhAmount >= amountT.mul(5))
                canhAmount = amountT.mul(5);
            hTokenAmount = hTokenAmount.add(canhAmount);
            uint256 befhBal = hToken.balanceOf(address(this));
            swapTokensFor(canhAmount,address(hToken),address(this));
            uint256 newhBal = hToken.balanceOf(address(this)).sub(befhBal);
            hDivTokenAmount = hDivTokenAmount.add(newhBal);
            isHProc = true;
        }
    }

This makes it possible for an attacker to manipulate the price and force the contract to buy tokens at a high price. In this case, the attacker influenced the price of GVC by requesting a flashloan of BUSD and buying up all the available liquidity of GVC in the pool and then calling transfer() on Seaman.

Possible mitigations

1. Prevent users to manipulate contract balances via low liquidity pair interactions.
2. Do not automatically perform trades without a sanity check on the prices

Related

• MBC Token - Same swapAndLiquify pattern exploited via price manipulation
• BVaults - Price manipulation through unprotected swap functions
• Four Meme - Preemptive pool price manipulation on BSC