What happened
On October 6, 2022, an attacker exploited BSC Token Hub, the native bridge between BNB Beacon Chain and BNB Smart Chain. The attacker forged cross-chain proof messages that the bridge accepted and caused two unauthorized 1 million BNB transfers, for 2 million BNB in total. BNB Chain validators paused the network and coordinated a patch, which prevented much of the minted BNB from being moved further.
The IAVL range-proof verifier did not bind every proof node and payload value into the computed root hash. In the affected path, a populated right child could be changed without changing the calculated root, so the bridge accepted an arbitrary cross-chain transfer payload.
Case & protocol details
Attack Timeline
The bridge relied on relayers to submit cross-chain packages and an IAVL Merkle proof to show that a Beacon Chain message was genuine. The attacker first registered as a relayer, then supplied a crafted range proof for an arbitrary withdrawal payload. In the affected IAVL verifier, a proof node with a populated left child did not include its right child in the root-hash calculation.
That let the attacker preserve the expected root while inserting a forged right-side payload. The Token Hub treated the forged message as valid and issued 1 million BNB twice.
Evidence & learning
Sources and on-chain records
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