CertiK: Analysis of PolyNetwork Attack Eventso

event review
On August 10, 2021, PolyNetwork suffered a cross-chain attack, and $600 million in encrypted assets were transferred (the attackers began to return the stolen assets one after another). The attacker conducted attack transactions on multiple public chains, and completed the attack through cross-chain management contracts and repeater components.

To explain with the example of the property above, the hacker used the fake homeowner proof (invalid transaction on the source chain) to get the real key (the signed Merkle proof on the Alliance Chain) from the property (repeater).



Attack Analysis
1. The hacker initiated an attack transaction that should have been invalid on the source chain.

2. The attack transaction is written into the source chain without being fully checked, and is then included in the Merkle tree of the Alliance Chain by the repeater and signed, and then published in the Alliance Chain block.

3. The hacker uses the valid Merkle proof of step 2 on the target chain to call the ECCM contract of Poly Network and change the keepers to the public key controlled by the hacker.

4. After obtaining the keepers permission, hackers can unlock assets arbitrarily on multiple public chains.

It is worth noting here that Poly Network’s relayers on some chains did not pass the attack transaction, so even if the smart contracts are similar, some assets on the target chain are not affected.