BSC Token Hub Hack

Reported loss $586M
Bnb Smart Chain
Forged IAVL Merkle range proof

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.

Technical root cause

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.

How it happened

  1. The bridge relied on relayers to submit cross-chain packages and an IAVL Merkle proof to show that a Beacon Chain message was genuine.
  2. The attacker first registered as a relayer, then supplied a crafted range proof for an arbitrary withdrawal payload.
  3. In the affected IAVL verifier, a proof node with a populated left child did not include its right child in the root-hash calculation.
  4. That let the attacker preserve the expected root while inserting a forged right-side payload.
  5. The Token Hub treated the forged message as valid and issued 1 million BNB twice.

Protocol details

Classification Protocol Logic / Bridge & Cross-Chain
Protocol Type Exploit/Other
Category Bridge Hack
Protocol links Website @BNBCHAIN
Attacker Address 0x489A8756C18C0b8B24EC2a2b9FF3D4d447F79BEc

Funds Recovery

79.4%

Recovered

$465.5M

Net Loss

$120,716,000

Evidence

Understand the attack patterns

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Cross-Chain Bridge Attacks

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