The missing access check

Graduate · enrolled learners

This lesson opens with Poly Network, 10 August 2021.

What happened

Poly Network moved assets across Ethereum, BSC and Polygon through a cross-chain manager contract. One of its functions, verifyHeaderAndExecuteTx, executed instructions carried in cross-chain messages, and the manager contract had the authority to change which public key, the keeper, was allowed to authorize those messages. The two responsibilities were not properly separated: the attacker crafted a cross-chain message that called the function which sets the keeper, replacing the legitimate keeper with one they controlled. From then on the attacker could authorize any transfer, and they drained about 611 million dollars across three chains in minutes. The attacker later returned essentially all of it, saying they had done it to expose the flaw.

The decision point

The manager contract could execute instructions and could change its own authorization key, and nothing checked that only a legitimate party could invoke the second power through the first. That is a missing access-control check: a privileged function, changing who is trusted, reachable by an unprivileged caller. Access-control failures are the second-largest class of DeFi loss, and they are almost always total rather than partial, because the function that got exposed is usually the one that grants power over everything else. A user cannot read every line, but they can ask the question that finds this class: which functions here are privileged, and what proves the caller is allowed to call them.

Recorded loss

$611,000,000

Orientation and Year One are open: anyone can read them without an account. From Year Two onward the lessons are for enrolled learners, because progress through the later years only means anything if it is tracked against a record.

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