OpenAI, Anthropic, Google API Flaw Let Weaker AI Models Decode Stronger Models' Reasoning

This is a significant disclosure that undermines the confidentiality guarantees of "hidden" chain-of-thought reasoning in major LLM APIs.

Technical Breakdown:

• The Flaw: A cryptographic nonce or session-binding weakness in the encrypted reasoning objects (likely serialized JSON blobs) used by OpenAI, Anthropic, and Google. The encryption protected the content but not the context of the block.
• Attack Vector: Session Replay / Cross-Session Injection. A reasoning block captured from one API session (e.g., via a compromised log or MITM) could be replayed into a different session belonging to the same or a different user.
• Impact: The weaker model (or the API endpoint processing the replayed block) would decrypt and process the foreign reasoning, leaking the original session's internal monologue, including API keys, passwords, and proprietary logic that the stronger model was instructed to keep hidden.
• Affected Providers: OpenAI (o-series reasoning), Anthropic (Claude extended thinking), Google (Gemini thinking).

Defense: This is a provider-side fix. As a consumer, you cannot patch this. The immediate mitigation is to never pass secrets (API keys, passwords) into the reasoning context of any model, even if the provider claims it is encrypted or hidden. Treat the entire reasoning chain as a potential log sink. Monitor for anomalous session behavior or unexpected token consumption that might indicate a replayed block.

Source: https://thehackernews.com/2026/08/openai-anthropic-google-api-flaw-let.html