Small print.
Wide open.

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Mozilla I=

Customer since 2025

Finding the vulnerabilities

attackers haven’t

“We have many years of experience picking apart the
work of the world’s best security researchers, and Mythos
Preview is every bit as capable. So far we’ve found no
category or complexity of vulnerability that humans can

find that this model can’t.”

Bobby Holley, Chief Technology Officer at Firefox

Results

22 Vulnerabilities found by Claude
Opus 4.6, and resolved as of
Firefox 148

Vulnerabilities found by Claude
Mythos Preview, and resolved as
of Firefox 150

271

Background

Mozilla is the creator and maintainer of Firefox, a leading
open-source browser used by hundreds of millions of
people globally. For more than two decades, Firefox has
been one of the most heavily scrutinized open-source
codebases on the web. Its engineering team was an early
adopter of Claude Code, which today is widely used across
Firefox engineering.

Impact

Mozilla has long used advanced security techniques to
protect Firefox users, which is why Anthropic’s Frontier Red
Team approached the company to put Claude to the test on
cyberdefense. In a two-week evaluation, Claude Opus 4.6
uncovered 22 previously unknown vulnerabilities in
Firefox’s most critical code — 14 of them high-severity.
Mozilla’s engineers began landing fixes within hours and
shipped them in Firefox 148. Two months later, Mozilla
shipped fixes for an additional 271 vulnerabilities identified
by Claude Mythos Preview.

For Mozilla, the asymmetry that has long favored attackers
is starting to reverse: defects that once waited years to
surface can now be found and fixed individually by the
experienced defenders who own the code.

“We view these tools as clear evidence that large-scale, AI-
assisted analysis is a powerful new addition in security
engineers’ toolbox. Firefox has undergone some of the most
extensive testing, static analysis, and regular security
review over decades. Despite this, the model was able to
reveal many previously unknown bugs,” said Mozilla.