Mozilla fixes 271 Firefox flaws that Claude Mythos Preview found
Mozilla says that Firefox 150 fixes 271 vulnerabilities that an early version of Claude Mythos Preview identified in an initial evaluation: 180 rated high, 80 moderate and 11 low.
2for an organization
Mozilla fixed 271 vulnerabilities that the model identified, 180 of them rated high, in one release.
Not ratedfor society
No outcome for Firefox users beyond the fixes is documented, and no source reports that any of the flaws was exploited.
Mozilla began working with Anthropic in February 2026. A scan of Firefox with Claude Opus 4.6 led to fixes for 22 security-sensitive bugs in Firefox 148. Mozilla then applied an early version of Claude Mythos Preview to Firefox. Firefox 150, released on 21 April 2026, fixes 271 vulnerabilities identified during that initial evaluation. In May, Mozilla’s engineers wrote that 180 of the 271 were rated high, 80 moderate and 11 low, and that the model’s reports produced few false positives. The same post says that Mozilla fixed 423 security bugs in its April releases in all, since its security team also hunts for bugs by fuzzing, by manual inspection and with other models, and that further fixes from the evaluation shipped in 149.0.2, 150.0.1 and 150.0.2. Anthropic cites the result in its Project Glasswing update. Privacy Guides reported the announcement on 22 April 2026. The count of 271 is Mozilla’s own, and Mozilla has not described most of the individual bugs.
Mozilla says that the model identified the 271 vulnerabilities and that its reports produced few false positives. Mozilla’s engineers triaged the reports, wrote the fixes and shipped them. The count is Mozilla’s own.
Limits
The count of 271 is Mozilla’s own, and Mozilla has not described most of the individual bugs. Its team also hunts for bugs by fuzzing, by inspection and with other models, and fixed 423 security bugs in April in all, so the model’s share of Firefox’s hardening is not separated.
As part of our continued collaboration with Anthropic, we had the opportunity to apply an early version of Claude Mythos Preview to Firefox. This week's release of Firefox 150 includes fixes for 271 vulnerabilities identified during this initial evaluation.
which led to fixes for 22 security-sensitive bugs in Firefox 148
Of the 271 bugs we announced for Firefox 150: 180 were sec-high, 80 were sec-moderate, and 11 were sec-low.
That's because our security team hunts for new bugs every day by attacking Firefox with a combination of (a) fuzzing systems (b) manual inspection and (c) this new agentic pipeline across a variety of models.
Mozilla has announced that Firefox 150 (set to be released this week) will include fixes for 271 bugs found using Anthropic's
Details
AI system
Reported configuration
Early version of Claude Mythos Preview
Workflow
A large language model applied to a browser’s code in an agentic pipeline that finds bugs, writes test cases and describes their impact. A maintainer whose security team deduplicated, triaged and fixed the reports.
Follow-up
Conditions for replication
The maintainer must triage reports and ship fixes before attackers find the same flaws. The pipeline must deduplicate against known issues and produce few false positives.
Mozilla applied Claude Mythos Preview to Firefox’s code to find security vulnerabilities before attackers do.
Cite this benefit
Peter H. Vartanian. (2026). Mozilla fixes 271 Firefox flaws that Claude Mythos Preview found (AIBD-1237). Artificial Intelligence Benefits Database. https://w3id.org/aibd/AIBD-1237 (archived at https://doi.org/10.57967/hf/11470)