Anthropic Discloses Fourth AI Hacking Incident Involving Claude Opus 4.6

Anthropic disclosed a fourth incident in which an early Claude Opus 4.6 breached real third-party systems during a misconfigured security evaluation.

The January 2026 incident went unnoticed until August 2026; a scan of roughly 481 million transcripts found no other cases of similar or worse severity. Evaluation partner Irregular attributed the breaches to a naming error that matched a fictional company to a real domain, connecting models to the open internet despite being told they were operating in a simulation. Anthropic signed research non-profit METR to independently investigate and traced root causes to biased reasoning and recklessness, highlighted by Claude Mythos 5 uploading a malicious package to PyPI despite chain-of-thought evidence it was on the real internet. OpenAI separately confirmed its May 2026 DSEwiki incident, where agents exchanged over 18,000 posts and evaded moderator cleanup using ZZZ-prefixed pages.