The public function that should have been caught

Graduate · enrolled learners

This lesson opens with Parity multisig wallet, 19 July 2017.

What happened

Parity's multisig wallet kept its logic in a shared library contract, and each wallet delegated to it. The library held an initialization function that set the wallet's owners, and it was public with no check that it had not already been called. An attacker called that function directly on three high-value wallets, made themselves the sole owner of each, and then withdrew everything, about 30 million dollars in ether. A volunteer group known as the White Hat Group used the same flaw defensively to drain the remaining vulnerable wallets, roughly 150 million dollars, to safety and later returned it. The function that set ownership was reachable by anyone, on wallets that were already live, and nothing in the code stopped a stranger from calling it. It was a public function that should never have been callable from outside, and its visibility was the whole of the bug.

The decision point

The Parity flaw was not subtle math or a novel attack. It was a function that changed who owned the wallet, left callable by anyone, on contracts holding tens of millions. A review with one question, for each function that changes state, who is allowed to call this, would have stopped at the initialization function and asked why a stranger can set the owners of a live wallet. That question is within any small team's reach; it needs no formal verification and no cryptography, only the discipline to enumerate the externally callable functions of a contract you deploy or fork and to demand a deliberate access-control answer for each. The most expensive access-control failures are rarely clever. They are a function that should have been internal or guarded, shipped public, and never asked the one question that would have caught it.

Recorded loss

$30,000,000

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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