SignalCapabilitySG-0002
Mozilla fixed 271 Firefox bugs found by Claude Mythos Preview in a single pass
Mozilla said it patched 271 security bugs in Firefox 150 after running the browser's code through Claude Mythos Preview, about twelve times as many as an earlier Claude model had found. Mozilla classed 180 of them as high severity. SecurityWeek noted that only a few of the fixes were credited as individual CVEs, so many were likely hardening changes rather than exploitable flaws.
Why it matters
Automated bug-finding at this scale changes the balance for defenders and attackers alike. It also shows how quickly a model can map the weak points of a large system, which matters when that system is the sandbox the model runs in.
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4 sources. Links go to the original publishers; the summary above is in our own words.
- primaryThe zero-days are numberedMozilla · April 21, 2026blog.mozilla.org/en/firefox/privacy-security/ai-security-zero-day-vulnerabiliti…
- primaryBehind the Scenes Hardening Firefox with Claude Mythos PreviewMozilla Hacks · May 7, 2026hacks.mozilla.org/2026/05/behind-the-scenes-hardening-firefox/
- newsClaude Mythos finds 271 Firefox vulnerabilitiesSecurityWeek · 2026securityweek.com/news/claude-mythos-finds-271-firefox-vulnerabilities/
- news271 Firefox flaws closed thanks to Mythos AI: Breakthrough for IT security?heise online · 2026heise.de/-11267732
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Paperclip Index. “Mozilla fixed 271 Firefox bugs found by Claude Mythos Preview in a single pass.” Signal SG-0002. Reported April 21, 2026; updated Oct. 7, 2026. https://paperclipindex.com/signal/SG-0002