Submitted by waldenyan20, also found by ulqiorra, hansfriese, and KingNFT
Affected contract: MultiRewardStaking
When assets are withdrawn for user Alice by an approved user Bob to a receiver that is not Alice, the rewards are never accrued and the resulting staking rewards are lost forever. This is because accrueRewards is called on caller and receiver but never owner.
Third-party withdrawals are allowed by the fact that withdraw(uint256, address, address) exists in ERC4626Upgradeable and is never overwritten by a method with the same signature. Protocols composing with Popcorn will assume by the nature of this contract being an ERC4626 that this method is safe to use when it in fact costs the user significantly.
I created a test to reproduce this bug. When I included the below code within MultiRewardStaking.t.sol it passed, meaning Alice and Bob both had no rewards to claim by the end:
One can similarly create a test that doesnt expect the calls at the end to revert and that test will fail.
I reproduced the bug simply by adding a test within the existing Foundry project.
RedVeil (Popcorn) confirmed, but disagreed with severity 
