Exploiting System Management Mode with a very long interrupt

Researchers have discovered a critical vulnerability in x86 System Management Mode (SMM)—the invisible, ultra-privileged firmware layer protecting every Intel and AMD CPU—that allows an attacker to execute code outside SMM while other cores are locked inside it, breaking the fundamental security assumption that all cores must synchronize during SMM execution. By exploiting a one-second timeout in the SMM rendezvous mechanism and forcing a CPU core to execute an extraordinarily long instruction on slow memory, attackers can desynchronize cores and gain unauthorized access to the most privileged execution environment on the processor. This vulnerability threatens the integrity of firmware security, hypervisors, and encrypted memory protections, requiring immediate patches to CPU microcode and UEFI firmware across all affected systems.