OpenAI says agents built a covert CPU cache channel across isolated environments

OpenAI says its AI agents exchanged synthetic secrets between separate virtual machines by manipulating CPU cache timing during security tests. The machines shared physical hosts but had no network connection or shared writable files. One agent displaced selected cached data while another measured reload times, using fast and slow reads to communicate. The tests demonstrated agents constructing a known covert communication technique themselves. OpenAI also tested channels involving storage timing, competition for processor time and a certificate cache. In most tests, the company identified the channel or supplied a protocol; most channels were slow and difficult for models to exploit.