Firefox's Security Revolution: How AI Discovered 271 Hidden Vulnerabilities

In a groundbreaking collaboration with Anthropic, the Firefox team deployed an early version of Claude Mythos Preview—a frontier AI model—to scan for latent security flaws in the browser. The result? A staggering 271 zero-day vulnerabilities were identified and fixed in Firefox 150. This Q&A explores the implications of this achievement, the technology behind it, and what it means for the future of browser security.

How many zero-day vulnerabilities did Claude Mythos discover in Firefox?

Claude Mythos Preview identified a total of 271 zero-day vulnerabilities during its initial evaluation of Firefox. This number is extraordinary, especially considering that for a hardened target like Firefox, even a single such bug would have been considered a red-alert event as recently as 2025. The findings were integrated into the Firefox 150 release, which includes patches for all 271 issues. The volume of discoveries highlights both the power of AI-driven security analysis and the hidden depth of vulnerabilities that traditional methods might miss.

What is Claude Mythos and how was it used in this collaboration?

Claude Mythos is a cutting-edge AI model developed by Anthropic, designed to excel at complex analytical tasks, including security auditing. In this collaboration, the Firefox team applied an early version of Claude Mythos Preview to systematically scan Firefox's codebase for security weaknesses. The model worked around the clock, analyzing code patterns and behaviors to uncover vulnerabilities that would be extremely difficult for human reviewers to find. This automated approach allowed the team to discover and fix issues at an unprecedented scale—something that would have taken months or years using traditional methods.

What was the previous collaboration with Anthropic that led to the current effort?

Earlier, the Firefox team partnered with Anthropic to use an older AI model, Opus 4.6, for security scanning. That initial collaboration resulted in fixes for 22 security-sensitive bugs in Firefox 148. Building on that success, the team decided to apply the more advanced Claude Mythos Preview to the same task. The dramatic increase from 22 to 271 vulnerabilities demonstrates the rapid evolution of AI capabilities in cybersecurity. This progression shows how frontier AI models are becoming more effective at identifying subtle, previously unknown flaws in complex software.

How did the Firefox team react to discovering 271 zero-days at once?

The team experienced what they described as vertigo—a moment of overwhelming realization when the sheer number of vulnerabilities became apparent. For a highly secure product like Firefox, finding even one such bug would normally trigger an emergency response. To find 271 at once was almost paralyzing. However, the team quickly shook off that feeling and reprioritized their entire workflow. They brought relentless, single-minded focus to fixing the issues. The experience was challenging but ultimately hopeful, proving that with the right tools and determination, defenders can stay ahead of threats.

Does this technology favor defenders or attackers in cybersecurity?

According to the Firefox team, this technology favorably shifts the balance toward defenders—but only if they can patch and push updates quickly. AI models like Claude Mythos empower security teams to identify vulnerabilities at a speed and depth previously impossible. As long as defenders can integrate these findings into rapid release cycles and get fixes to users promptly, they gain a decisive advantage. Attackers may also use similar AI tools, but the defensive side benefits from coordinated patching and the ability to close many holes at once. The key is speed of response: the faster patches are deployed, the less opportunity attackers have to exploit discovered flaws.

What does the future look like for software security defenders after this achievement?

The Firefox team sees a bright future where defenders can finally win decisively. They acknowledge that the work is not finished—but they have turned the corner. The success with Claude Mythos suggests that AI-driven security audits can scale to find hundreds of vulnerabilities in a single product, reducing the risk of zero-day exploits. Teams that embrace these tools and reorganize their priorities to focus on patching will be able to move beyond just keeping up with threats. Instead, they can proactively eliminate entire classes of vulnerabilities, making software significantly more secure. This is a hopeful message for the entire cybersecurity community.

How did the Firefox team manage to fix all 271 vulnerabilities for the Firefox 150 release?

Fixing 271 vulnerabilities required extraordinary effort and reprioritization. The team paused other projects and dedicated their full attention to analyzing and patching each issue identified by Claude Mythos. They worked around the clock, leveraging the AI's detailed reports to understand each vulnerability's root cause and develop fixes. The process was streamlined by the model's ability to categorize bugs and suggest potential patch approaches. Despite the intensity, the team is proud of their accomplishment. They believe that any organization facing a similar challenge can succeed with focused effort and the right tools—though it may feel overwhelming at first.