Speculative Execution Side Channel.

A microarchitectural vulnerability class where CPUs leak data through caches and predictors after executing instructions speculatively along paths that should not have run. It belongs to the Application Security category of cybersecurity.

A microarchitectural vulnerability class where CPUs leak data through caches and predictors after executing instructions speculatively along paths that should not have run.

Speculative-execution side channels exploit a fundamental performance feature of modern CPUs: branch prediction, out-of-order execution, and speculation execute instructions before the architectural state confirms they should run. While transient results are discarded on misprediction, they leave measurable traces in caches, store buffers, and predictors that an attacker can observe via timing oracles. The Spectre family (variant 1, 2, BHI, RSB, Spectre-NG), Meltdown, MDS, L1TF, Foreshadow, and Downfall use this primitive to read protected memory across processes, VMs, browser sandboxes, or enclaves. Mitigations combine microcode updates, retpolines/IBRS, kernel page-table isolation (KPTI), site isolation in browsers, and defensive code patterns.

Defences for Speculative Execution Side Channel typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: Spectre-class vulnerability, Transient execution attack.