Many commodity processors have Instruction Set Architecture (ISA) features that protect software components from one another. These features can include memory segmentation, virtual memory, privilege rings, trusted execution environments, and virtual machines, among others. For example, virtual memory provides each process with its own address space, which prevents processes from accessing each other's private data.

Many of these features can be used to form hardware-enforced security boundaries between software components. Many commodity processors also share microarchitectural resources that cache (temporarily store) data, which may be confidential. These resources may be shared across processor contexts, including across SMT threads, privilege rings, or others.

When transient operations allow access to ISA-protected data in a shared microarchitectural resource, this might violate users' expectations of the ISA feature that is bypassed. For example, if transient operations can access a victim's private data in a shared microarchitectural resource, then the operations' microarchitectural side effects may correspond to the accessed data. If an attacker can trigger these transient operations and observe their side effects through a covert channel [REF-1400], then the attacker may be able to infer the victim's private data.

Private data could include sensitive program data, OS/VMM data, page table data (such as memory addresses), system configuration data (see Demonstrative Example 3), or any other data that the attacker does not have the required privileges to access.