Submitted by ShadowyNoobDev, also found by 0xw4rd3n, CertoraInc, ckksec, Czar102, defsec, Alex the Entreprenerd, Heartless, IllIllI, Jujic, kirk-baird, leastwood, pauliax, peritoflores, Randyyy, reassor, Rhynorater, Sleepy, SolidityScan, and wuwe1
ConcurRewardPool.sol#L34
Since the reward tokens are transferred before the balances are set to 0, it is possible to perform a reentrancy attack if the reward token has some kind of call back functionality e.g. ERC777. pBTC is an ERC777 token that is currently available on Convex. A similar attack occurred with imBTC on uniswap v1.
You can also find a walkthrough replicating a similar attack here.
ryuheimat (Concur) confirmed
Alex the Entreprenerd (judge) commented:
