Cybersecurity stocks in US fell after a data leak showed that Anthropic is testing a "powerful" new AI model called Claude Mythos that presents unprecedented security risks. The iShares Cybersecurity ETF fell 4.5%, while CrowdStrike, Palo Alto Networks and Zscaler dropped about 6% each. Last month, cyber stocks had declined after Anthropic announced a new code-scanning security tool to Claude.