Transient execution CPU vulnerabilities are vulnerabilities in a computer system in which a speculative execution optimization implemented in a microprocessor is exploited to leak secret data to an unauthorized party. The classic example is Spectre that gave its name to this kind of side-channel attack, but since January 2018 many different vulnerabilities have been identified. Overview Modern computers are highly parallel devices, composed of components with very different performance characteristics. If an operation (such as a branch) cannot yet be performed because some earlier slow operation (such as a memory read) has not yet completed, a microprocessor may attempt to predict the result of the earlier operation and execute the later operation speculatively, acting as if the prediction was correct. The prediction may be based on recent behavior of the system. When the earlier, slower operation completes, the microprocessor determines whether prediction was correct or incorrect. If it was correct then execution proceeds uninterrupted; if it was incorrect then the microprocessor rolls back the speculatively executed operations and repeats the original instruction with the real result of the slow operation. Specifically, a transient instruction refers to an instruction processed by error by the processor (incriminating the branch predictor in the case of Spectre) which can affect the micro-architectural state of the processor, leaving the architectural state without any trace of its execution. In terms of the directly visible behavior of the computer it is as if the speculatively executed code "never happened". However, this speculative execution may affect the state of certain components of the microprocessor, such as the cache, and this effect may be discovered by careful monitoring of the timing of subsequent operations. If an attacker can arrange that the speculatively executed code (which may be directly written by the attacker, or may be a suitable gadget that they have found in the targeted system) operates on secret data that they are unauthorized to access, and has a different effect on the cache for different values of the secret data, they may be able to discover the value of the secret data. Starting in 2017, multiple examples of such vulnerabilities were identified, with publication starting in early 2018. In March 2021 AMD security researchers discovered that the Predictive Store Forwarding algorithm in Zen 3 CPUs could be used by malicious applications to access data it shouldn't be accessing. According to Phoronix there's little impact in disabling the feature.