Update: Claude Mythos cybersecurity concerns

April 22, 2026

The other day I posted about cybersecurity concerns around Anthropic’s Mythos.

Bloomberg, Anthropic’s Mythos Model Is Being Accessed by Unauthorized Users

A small group of unauthorized users have accessed Anthropic PBC’s new Mythos AI model, a technology that the company says is so powerful it can enable dangerous cyberattacks, according to a person familiar with the matter and documentation viewed by Bloomberg News.[…] The unauthorized access, which has not previously been reported, highlights the challenge Anthropic faces in fully preventing its most powerful — and potentially dangerous — technology from spreading beyond approved partners. It also raises questions about whether anyone else may be using Mythos without permission, and for what purpose.

Wired, Anthropic’s Mythos finds and fixes 271 bugs in Firefox

Amid a raging debate over the impact that new AI models will have on cybersecurity, Mozilla said on Tuesday that its Firefox 150 browser release this week includes protections for 271 vulnerabilities identified using early access to Anthropic's Mythos Preview. The Firefox team says that it has taken resources and discipline to adjust to the firehose of bugs that new AI tools can uncover, but that this big lift is necessary for the security of Mozilla’s users, given that the capabilities will inevitably be in attackers’ hands soon.[…] Companies like Anthropic and OpenAI seem to be trying to get as many major players as possible to go through this overhaul before the capabilities are more widely available.

The Firefox number highlights Mythos's effectiveness. 271 bugs are a lot to uncover at once. The other concern is how quickly Mythos will fall into the hands of attackers. According to Bloomberg, it has already happened.