Mozilla Says Claude Mythos Helped Find 271 Firefox Bugs

Mozilla says its "zero-days are numbered" after identifying 271 security‑sensitive Firefox vulnerabilities using Anthropic's Claude Mythos Preview model.

In security terms, a zero‑day is a vulnerability that developers do not yet know about and therefore have had "zero days" to fix. Mozilla caught this slate of vulnerabilities by having Claude Mythos scan Firefox's codebase ahead of Tuesday's Firefox 150 release, spotting flaws that human reviewers had not yet reported. Engineers then confirmed these issues and wrote patches before the affected code reached general users.

Claude Mythos Preview is an unreleased frontier‑class model that Anthropic is testing under its restricted Project Glasswing program. Mozilla first ran a trial with Anthropic's Claude Opus 4.6 model, which scanned thousands of C++ files and found dozens of security‑relevant reports in Firefox 148. That experiment convinced Mozilla that LLMs can help security teams find real‑world ...