Claude Mythos Has Found 271 Zero-Days in Firefox

That’s a lot . No, it’s an extraordinary number: Since February, the Firefox team has been working around the clock using frontier AI models to find and fix latent security vulnerabilities in the browser. We wrote previously about our collaboration with Anthropic to scan Firefox with Opus 4.6, which led to fixes for 22 security-sensitive bugs in Firefox 148. 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 i

The report was published by Schneier on Security with a current urgency rating of 6/10 for this queue. Security teams should treat this as a timely item to review, especially where the affected products, vendors, or techniques overlap with their environment.

What We Know So Far

The source item was published at 2026-04-29T10:12:17+00:00 and is being tracked from Schneier on Security. The available RSS summary indicates that the story is relevant because it published in the last 6 hours; contains high-urgency security terms; affects a major vendor or platform. Review the original report for full technical context, affected versions, and any vendor-specific remediation details.

Where a CVE, patch advisory, active exploitation note, or public proof-of-concept is involved, validate the details against the vendor advisory before making production changes. At the time this draft was generated, the RSS feed was the primary source used for this queue entry.

What You Should Do Now

1. Read the source report and confirm whether your organization uses the affected product, service, or dependency.
2. Check vendor advisories for patched versions, mitigations, indicators of compromise, or detection logic.
3. Prioritize exposed internet-facing systems, privileged services, and high-value environments first.
4. Add the story to the next security review or incident triage cycle if it matches your technology stack.
5. Keep this draft updated with confirmed version numbers and direct advisory links before publishing.

Sources: Schneier on Security