gm gm here we go again.
This morning somebody abused a bug in ApeStr to swap 21 apes for 21 ApeStr-owned apes.
This was a known exploit (see the SQUIGSTR exploit from ~1 year ago) but incentive to run it was never there.
The bug lies in buyTargetNFT: it forces a strategy contract to send arbitrary calldata to any target. Afterwards it checks only three things: its BAYC balance rose by exactly 1, it owns expectedId, and "cost" equals its ETH balance drop.
This was patched in later NFTStrategy releases, but affects the original 6 (PunkSTR not affected).
The attacker bought 2 BAYC to use as bait. Using a helper contract, they prep the strategy contract by selling an ape to it for 5 wei and immediately buying it back for 6 wei. That primes the contract's ethToTwap with 6 wei.
They then call buyTargetNFT and pass the calldata to transfer an ape from ApeSTR to the helper using safeTransferFrom. This in turn hands execution over to the helper (remember, safeTransferFrom is NOT safe), which transfer TWO apes back to ApeSTR to satisfy the balance check (1 out, 2 in leaves ApeSTR at +1). It also calls processTokenTwap, which converts the 6 wei, setting the cost of one of the newly deposited apes to 7 wei (6 * 120%). The helper contract buys it back in the same tx.
example flow:
- Attacker has BAYC
#1 and
#2. ApeSTR has BAYC
#3.
- Attacker "sells" BAYC
#1 to ApeSTR for 6 wei and sends BAYC
#2 to it for free, but steals BAYC
#3 in the process.
- ApeSTR relists BAYC
#1 for 7 wei, which the attacker buys.
- Attacker now has BAYC
#1 and BAYC
#3, and ApeSTR has BAYC
#2 (but doesn't even know it).
The attacker ran this for all 21 BAYC in the ApeSTR contract, resulting in them being able to claim packs for all of those BAYC. Total net proceeds: $14k.
This previously affected SQUIGSTR because of their shared contract - any art blocks NFT sent in would satisfy the balance check, allowing the attacker to drain the contract of squiggles in exchange for basically valueless art blocks NFTs.
It was known to be possible on ApeSTR as well, so I guess it was just a matter of time before we saw it happen.
The unfortunate consequence is that now, ApeSTR has 21 apes that it is completely unaware of, so they cannot be bought from the contract. And 210 attacker-owned mudwater packs were dumped into bids.
