Codeforces

Researchers Extract Proprietary Reasoning Traces from Anthropic, OpenAI and Google LLMs, Revealing Hidden Secrets

A team of eight researchers from institutions including ELLIS Institute Tübingen, the Max Planck Institute for Intelligent Systems, Tübingen AI Center, MATS and Snyk published a preprint detailing a practical attack that recovers full reasoning traces from closed LLM APIs. The method requires only two API calls and works by feeding encrypted reasoning blocks from strong models such as Claude Opus 4.8 into weaker models from the same provider, such as Haiku 4.5, which then reproduce the hidden chain-of-thought verbatim. Analysis of 6,708 publicly shared agent logs from GitHub and Hugging Face yielded 315,320 recovered traces containing 704 unique secrets, including 62 API keys, 33 passwords and 24 access tokens that never appeared in visible session output. The attack also enables extraction of internal safety policies, system prompts and detailed harmful planning that providers normally filter from final answers. In addition, the same mechanism can be used in reverse to inject malicious instructions into shared logs that later get replayed by unsuspecting users. The authors recommend treating encrypted reasoning blocks as sensitive secrets and propose cryptographic binding of traces to sessions, users and models.