Steadefi

Summary

Attacker can first deposit small amount of asset token to get shares, and front-run other depositors' transactions and inflate shares price by large "donation", thus attacker can withdraw more loan tokens than he initially owned.

Vulnerability Details

User can get share by depositing asset to Lending vault, the amount of minted shares is calculated as:

Let's assume:
1.Alice
2.Bob sees Alice's transaction in mempool and front-runs by first depositing 1 wei to the vault and then get 1 share;
3.Bob then transfers 1 weth directly to the vault, inflates pool token price to (1 weth + 1);
4.Alice's deposit transaction gets confirmed and Alice get 1 share;
5.Bob withdraw from vault and get 1.5 weth back, making 0.5 ether profit.

Impact

User's deposited loan tokens may be stolen by attacker.

Tools Used

Recommendations

Consider minting a minimal amount of pool tokens during the first deposit and sending them to zero address, this increases the cost of the attack. Uniswap V2 uses the value 1000 as it is small enough to don't hurt the first minter, while still increasing the cost of this attack by 1000x.
https://github.com/Uniswap/v2-core/blob/master/contracts/UniswapV2Pair.sol#L119-L121