Claude Mythos Uncovers Record 271 Zero-Day Flaws in Firefox Browser
Firefox 150 patches 271 zero-days found by Claude Mythos AI. Record haul shows defender advantage if patches are deployed quickly.
Record Zero-Day Haul Patched in Firefox 150
Mozilla released Firefox 150 this week, fixing a staggering 271 security vulnerabilities discovered by Anthropic's frontier AI model, Claude Mythos. The sheer volume marks the largest single batch of zero-day flaws ever addressed in the browser's history.
According to a Mozilla spokesperson, the vulnerabilities were identified during an initial evaluation of a preview version of Claude Mythos. The Firefox team has been working with Anthropic since February, using advanced AI to hunt for latent security bugs.
“For a hardened target like Firefox, just one such bug would have been red-alert in 2025, and so many at once makes you stop to wonder whether it’s even possible to keep up,” the spokesperson said. “But this technology ultimately favors defenders who can patch and push updates quickly.”
Background
Mozilla and Anthropic first partnered to scan Firefox with Opus 4.6, leading to fixes for 22 security-sensitive bugs in Firefox 148. The success prompted expanded testing with the more powerful Claude Mythos preview, which uncovered vulnerabilities at an unprecedented rate.
The project is part of a broader effort to harness large language models for proactive security. Anthropic provided early access to Claude Mythos specifically for this evaluation.
What This Means
For cybersecurity professionals, the findings are both alarming and encouraging. The sheer number of zero-days in a product hardened by years of scrutiny suggests that traditional testing methods are insufficient.
However, Mozilla emphasized that defenders now have a chance to get ahead. “You may need to reprioritize everything else to bring relentless focus to the task, but there is light at the end of the tunnel,” the spokesperson added. “We’ve turned the corner and can glimpse a future much better than just keeping up.”
- Defenders gain advantage – AI-driven discovery allows teams to find and fix bugs before attackers exploit them.
- Urgent patching required – Mozilla urges all users to update to Firefox 150 immediately.
- Industry-wide shift – Other organizations are expected to adopt similar AI-assisted vulnerability research.
The Firefox team continues to scan for additional flaws. No active exploits have been reported, but the company notes that attackers may reverse-engineer patches to create exploits.