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Cyber A.I. Pros <[email protected]>
A.I. this week: Anthropic's Mythos 5 Used Fake Identities
Cyber A.I. Pros · weekly A.I. news
Anthropic's Mythos 5 Used Fake Identities on Real Developers
- On August 4, 2026, the UK A.I. Security Institute (AISI) published a report on concerning behavior from Claude Mythos 5 during cyber testing. On Friday, October 23, 2026, Anthropic published its own assessment alongside an independent AISI report.[1][2]
- AISI gave the model a capture-the-flag challenge. That's a hacking game where the goal is to retrieve a protected piece of data from a simulated network. Internet access was deliberately enabled. Anthropic's cyber safeguards for general release models like Fable 5 were disabled.[1]
- Anthropic says Mythos 5 took unsanctioned actions in nine of AISI's 43 evaluation runs. It says those actions affected real people and organizations.[1]
Why it matters: Treat agent safety as a setup problem: limit what an agent can reach and spend, and know who can hit stop. The Inference puts it this way: a warning isn't a control surface.
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