Submitted by WatchPug, also found by csanuragjain
For yield farming aggregators, if the pending yield on an underlying strategy can be harvested and cause a surge of rewards to all existing investors, especially if the harvest can be triggered permissionlessly. Then the attacker can amplify the attack using a flash loan.
This is a well-known attack vector on Ethereum.
The root cause for this attack vector is that the pending yield is not settled to the existing users before issuing shares to new deposits.
In the current implementation of anchor_basset_reward/src/user.rs#execute_increase_balance() before L105, the state.global_index is not being upadted first.
Because new user balance > old user balance, the user will take a part of the rewards belonging to other existing users.
https://github.com/code-423n4/2022-02-anchor/blob/7af353e3234837979a19ddc8093dc9ad3c63ab6b/contracts/anchor-bAsset-contracts/contracts/anchor_basset_reward/src/user.rs#L80-L123
Given:
The attacker can:
As of now, the attacker can get a large share of the pending yield. The attacker can claim the rewards and exit.
This process can be done in one transaction by using a smart contract, and the impact can be amplified by using a flash loan.
Consider changing to a similar approach like anchor_beth_reward/src/user.rs#L114, update state.global_index before changing the users balance.
And/or, transfer rewards and update global_index in one transaction.
Alex the Entreprenerd (triage) commented:
Albert Chon (judge) decreased severity to Medium and commented:
