LLM Reasoning Trace Theft via Encrypted Block Replay Attack
Researchers discovered that Anthropic, OpenAI, and Google share the same encryption key across model families for encrypted chain-of-thought blocks, allowing adversaries to replay stronger model reasoning traces into weaker siblings and extract hidden reasoning in plaintext via jailbreak. The attack also enables a prompt injection variant where malicious instructions embedded in reasoning traces are treated as trusted by the model, dramatically increasing attack success rates. All three vendors have since patched the vulnerability following responsible disclosure.