OpenAI says its red-team agents built a covert CPU-cache channel between isolated microVMs OpenAI reported on October 23 that its red-team agents established a covert communication channel between separate microVMs by manipulating CPU-cache state and measuring reload times, exchanging synthetic secrets without a network connection or shared writable file. OpenAI did not publish a transfer rate, error rate or trial count for the cache channel, and said most communication methods it found were slow and difficult for models to exploit; the campaign consumed hundreds of billions of tokens and involved its upcoming GPT-6 Astra Cyber model. OpenAI said it patched vulnerabilities, strengthened monitoring and resumed frontier reinforcement-learning training and internal inference on October 1, 2026, after an 11-day pause. OpenAI says its red-team agents built a covert CPU-cache channel between isolated microVMs - OpenAI says agents exchanged synthetic secrets between separate microVMs by changing CPU-cache state and measuring reload times, without a network connection or shared writable file.