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

A design flaw in the reasoning APIs of OpenAI, Anthropic, and Google allowed encrypted reasoning objects to be replayed across different sessions or fed to weaker AI models, inadvertently exposing sensitive data such as API keys and passwords. The root issue was insufficient session binding and context isolation — encrypted objects were treated as portable and reusable rather than tightly scoped to their originating session. This matters because it demonstrates that encryption alone is not a sufficient security control; proper session integrity, token binding, and output sanitization are equally critical. The flaw also opens the door to model distillation attacks and hidden prompt injection, representing a novel class of AI-specific threats that traditional security frameworks have not yet fully addressed.