Assessing Claude Mythos Preview’s cybersecurity capabilities

This post details the cybersecurity capabilities of Claude Mythos Preview, highlighting its ability to autonomously find and exploit zero-day vulnerabilities in operating systems and web browsers. It describes specific examples of complex exploit generation, including chaining vulnerabilities, JIT heap spraying, privilege escalation, and ROP chain construction. The post also contrasts Mythos Preview's exploit development success rate with previous models like Opus 4.6, showing a dramatic improvement. It explains the evaluation methodology using OSS-Fuzz corpus and real-world security tasks, focusing on memory safety vulnerabilities due to their prevalence in critical systems and ease of verification. The agentic scaffold used involves prompting Claude to find vulnerabilities, experiment with the code, and output a bug report with a proof-of-concept exploit. The post emphasizes that these capabilities emerged as a downstream consequence of general improvements in code, reasoning, and autonomy, rather than explicit training for exploitation.