Anthropic's Mythos Preview found 271 security vulnerabilities in Firefox 150 before the browser shipped, Mozilla announced Tuesday. The model analyzed unreleased source code and flagged the bugs automatically. Anthropic has restricted Mythos to a limited group of critical industry partners, citing its power to identify cybersecurity weaknesses at scale.
The number that makes this story worth reading is not 271. It is 22. Anthropic's previous model, Opus 4.6, found only 22 security-sensitive bugs when analyzing Firefox 148 last month. That is a 12x jump in one generation, and Firefox CTO Bobby Holley claims defenders now have a decisive edge over attackers for the first time.
Holley has not disclosed the severity distribution of the 271 vulnerabilities, which is the key missing data point. Whether these are critical remote code execution flaws or low-impact edge cases changes everything about how to interpret this result. Read the original for Holley's full framing of what he calls a turning point in the attacker-defender balance.
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