# Anthropic Mythos Breach (April 2026)

**Type:** Security incident  
**Date:** April 7, 2026 (breach); April 21, 2026 (public disclosure)  
**Affected System:** Claude Mythos Preview model  
**Attack Vector:** Third-party contractor access via Discord intelligence group  
**Status:** Under investigation  

## Overview

Unauthorized access to Anthropic's Mythos Preview model occurred on April 7, 2026 — the same day the model was publicly announced. The breach was facilitated through a third-party vendor environment by an individual employed at a contractor working with Anthropic.

## Technical Details

**Model Capability:** Mythos Preview was capable of 83.1% first-attempt exploit generation for zero-day vulnerabilities — deemed too dangerous for public release by Anthropic.

**Deployment Model:** Limited to 40 partner organizations including Amazon, Apple, Broadcom, Cisco, CrowdStrike, Linux Foundation, Microsoft, and Palo Alto Networks.

**Breach Method:** Contractor shared URL naming conventions consistent with Anthropic's other model deployments through a private Discord channel dedicated to gathering intelligence on unreleased AI models.

## Official Response

Anthropic statement (April 21, 2026): "We're investigating a report claiming unauthorized access to Claude Mythos Preview through one of our third-party vendor environments. There is no evidence that the unauthorized access has impacted Anthropic's core systems or extended beyond the vendor environment."

## Governance Implications

The breach represents a structural test of the limited-partner deployment model for ASL-4 equivalent capabilities. The 40-partner deployment created 40 supply chains, each with independent contractor ecosystems and varying security postures. The breach occurred at this boundary on day 1, before any operational security learning could occur.

## Timeline

- **2026-04-07** — Mythos Preview announced; unauthorized access occurs same day via contractor
- **2026-04-19-21** — Parallel reporting: NSA has access, CISA does not have access
- **2026-04-21** — Bloomberg confirms unauthorized access; Anthropic announces investigation

## Sources

- TechCrunch, April 21, 2026
- Bloomberg, April 21, 2026
- Engadget, April 21, 2026
- CyberNews reporting on Discord community involvement