# Claude Mythos Preview + Project Glasswing: Anthropic's Restricted AI for Zero-Day Discovery

> Anthropic's Mythos Preview autonomously discovers and exploits zero-days across every major OS and browser — the first frontier model Anthropic deemed too dangerous for public release.

- Org: Anthropic
- Date: 2026-04-07
- Category: model, security
- Source: https://www.anthropic.com/glasswing
- Canonical page: https://ai-tldr.dev/releases/anthropic-claude-mythos-glasswing/

## In short

Claude Mythos Preview autonomously finds zero-days in every major OS and browser — 83.1% on cyberattack reproduction vs. Opus 4.6's 66.6%, with a 181× improvement on Firefox exploit generation. Released only to 12 industry partners via Project Glasswing. Accessed by an unauthorized group via URL-guessing on Day 1.

## What it is

On April 7, 2026, Anthropic announced Claude Mythos Preview and Project Glasswing — an industry coalition restricting Mythos access to 12 trusted partners: AWS, Apple, Broadcom, Cisco, CrowdStrike, Google, JPMorganChase, the Linux Foundation, Microsoft, NVIDIA, and Palo Alto Networks. Anthropic used Mythos internally to find thousands of zero-day vulnerabilities across every major operating system and web browser, including a 27-year-old OpenBSD bug and a 17-year-old FreeBSD remote code execution vulnerability (CVE-2026-4747). The $100M in usage credits and $4M in donations to open-source security organizations support the defensive disclosure effort. On April 21, an unauthorized group accessed the Mythos endpoint by guessing its URL based on familiarity with Anthropic's URL naming patterns.

## How it works

Mythos Preview scores 83.1% on cybersecurity vulnerability reproduction, compared to 66.6% for Opus 4.6. On Firefox JavaScript exploit generation, Opus 4.6 succeeded roughly 0.6% of the time; Mythos achieved working exploits 181 times from similar attempts. It can chain multiple vulnerabilities together for privilege escalation, construct JIT heap sprays to escape browser sandboxes, and build multi-stage exploits without human intervention. Over 99% of discovered vulnerabilities remain unpatched during the coordinated disclosure window; Anthropic published SHA-3 hash commitments to establish discovery priority.

## Why it matters

This is the first publicized case of a frontier lab deciding a model is too capable to release publicly — and demonstrating that decision with real, reproducible vulnerability discoveries rather than theoretical risk claims. The 181× improvement over the prior generation on Firefox exploit generation is a concrete capability jump. The unauthorized access incident (URL-guessing on Day 1) illustrates that 'restricted' model endpoints need security-by-design, not security-by-obscurity. For security practitioners, Mythos is the first public evidence that AI can autonomously operate as a high-skill offensive security researcher.

## Who it's for

Security researchers, ML safety practitioners, enterprise teams managing offensive AI risk and disclosure strategy

## Links

- [Project Glasswing](https://www.anthropic.com/glasswing)
- [Mythos Model Card](https://red.anthropic.com/2026/mythos-preview/)
- [Simon Willison](https://simonwillison.net/2026/Apr/7/project-glasswing/)
- [Schneier on Security](https://www.schneier.com/blog/archives/2026/04/on-anthropics-mythos-preview-and-project-glasswing.html)
- [Unauthorized Access](https://techcrunch.com/2026/04/21/unauthorized-group-has-gained-access-to-anthropics-exclusive-cyber-tool-mythos-report-claims/)