For well over a year, versions of Bitcoin Core — Bitcoin’s leading software implementation — contained a severe software bug. The bug was fixed with Bitcoin Core 0.16.3 (and 0.17.0rc4), released this week, and the status of the Bitcoin network now appears to be safe, with no harm done. The Bitcoin Core project has released a full disclosure report, revealing that the bug was even worse than previously thought.
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These are the good, the bad and the ugly details about one of Bitcoin Core’s nastiest bugs to date. (But not in that order.)
The Bad
The bad, of course, is the bug itself, now documented as CVE-2018-17144 in the Common Vulnerabilities and Exposures databank.
The bug was introduced as part of a block relay-related performance upgrade deployed in Bitcoin Core 0.14.0, officially released in March of 2017. In short, the bug would fail to reject a block containing a transaction that spends the same coins (“inputs”) multiple times. Indeed, it would allow for an (irregular) form of double-spending: arguably the very thing Bitcoin was designed to prevent[5].
It posed a serious problem, which might have manifested in several ways.
First, Bitcoin Core versions 0.14.0 through 0.14.2 (and, in some cases, newer versions), would have accepted the block but, at the same time, recognized that something was wrong. However, they wouldn’t be able to tell what was wrong, exactly. As a result, the node would stop operating altogether and shut down. If an invalid block caused by this bug had made its way to such nodes, they would have, in effect, crashed. That’s bad.
But it gets much worse.
Bitcoin Core versions 0.15.0 through 0.16.2 included another performance improvement, making it such that, in
