Mythos Just Rewrote the Rules of Cybersecurity: One AI Found 271 Firefox Vulnerabilities in a Month — More…

Laurent H. — CEO & Founder CYBERR & HEELR

2026-05-08

Mythos Just Rewrote the Rules of Cybersecurity: One AI Found 271 Firefox Vulnerabilities in a Month, More Than Human Teams Had in 18 Months

The numbers tell the story. In all of 2025, Mozilla shipped fixes for roughly 73 high-severity Firefox vulnerabilities. In February 2026, an earlier Anthropic model (Claude Opus 4.6) helped find and fix 22 bugs for Firefox 148.

Then Mythos arrived. In a single evaluation run, it identified 271 issues — 180 of them rated sec-high, 80 sec-moderate, and 11 sec-low. These were fixed across Firefox 150 and the rapid point releases 149.0.2, 150.0.1, and 150.0.2, contributing to a total of 423 security bugs patched in April alone.

Mozilla’s detailed follow-up post on May 7, 2026 (“Behind the Scenes: Hardening Firefox with Claude Mythos Preview”) confirms the scale and the method. The team built a custom agentic harness on top of their existing fuzzing infrastructure. Mythos didn’t just scan code — it generated reproducible proof-of-concept test cases, reasoned over complex multiprocess browser architecture, patched the source in isolated VMs to test exploits, and produced reports with near-zero false positives.

The most alarming discoveries were sandbox escape bugs.

Several of the vulnerabilities allowed a compromised content process (the sandboxed part of the browser that renders web pages) to break out and reach the privileged parent process.

https://quasa.io/media/mythos-just-rewrote-the-rules-of-cybersecurity-one-ai-found-271-firefox-vulnerabilities-in-a-month-more-than-human-teams-had-in-18-months

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