apegurus/poly-network

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Poly Network (2021)

Overview

In August 2021, Poly Network, a cross-chain interoperability protocol, was exploited for approximately $611 million across Ethereum, Binance Smart Chain, and Polygon. The attacker was able to manipulate the protocol's "keeper" role, allowing them to sign and execute arbitrary cross-chain transactions.

Root Cause

The vulnerability was in the EthCrossChainManager contract. The contract had a function crossChain that could call any contract on the target chain. The attacker used this to call the EthCrossChainData contract's putCurEpochConPubKeyBytes function. This function was intended to update the public keys of the "keepers" (the entities that sign cross-chain messages). Because the EthCrossChainManager was the owner of the EthCrossChainData contract, the call was authorized, allowing the attacker to replace the official keeper keys with their own.

Attack Flow

1. Attacker crafted a cross-chain message on a source chain (e.g., Ontology).
2. The message was designed to trigger a call to putCurEpochConPubKeyBytes on the target chain (Ethereum/BSC/Polygon).
3. The EthCrossChainManager received the message and, because it was the owner of the data contract, executed the call.
4. The attacker's public key was now registered as the only valid keeper key.
5. The attacker then crafted and signed withdrawal transactions for the bridge's assets using their own key.
6. The bridge accepted these transactions as valid and released the funds.

Impact

• Loss: ~$611M (Most was later returned by the attacker)
• Protocol: Poly Network
• Chain: Ethereum, BSC, Polygon
• Date: 2021-08-10

Key Transactions

• Attack tx (Ethereum): 0xb1f3535b698f3a0917a219673e7c0e1501c35f9bb8a2811b7a781363bd23c228

Detection Heuristics

• Pattern 1: Cross-chain managers that can call arbitrary functions on internal data or configuration contracts.
• Pattern 2: Lack of strict access control on functions that modify critical system roles (like keepers or validators).

Remediation

• Fix 1: Implement a whitelist of allowed functions that can be called via cross-chain messages.
• Fix 2: Ensure that critical configuration functions (like updating keeper keys) require multi-signature authorization or a time-lock, and cannot be triggered by a single cross-chain call.

References

• rekt.news/polynetwork-rekt/
• slowmist.medium.com/the-analysis-and-q-a-of-poly-network-hack-8112a353e439