Speculative Store Bypass is Variant 4 to Spectre and Meltdown's first three.
Google's Project Zero disclosed another speculative execution flaw similar to Spectre and Meltdown. The vulnerability, Speculative Store Bypass, could be exploited to expose user data across a broad range of devices (ZDNet). Intel calls it "Variant 4," and classifies it as "medium risk." Microsoft characterizes the risk to users as "low," but Variant 4, which has been designated CVE-2018-3639, is nonetheless being taken seriously (CNET). The issue affects Intel and AMD x86 chipsets, POWER 8, POWER 9, System z, and some ARM processors as well (SecurityWeek).
Some vendors have already issued mitigations. Those who face a more challenging patching problem (including Intel) intend to make fixes available within the next few weeks. Analysts tell users to expect some performance decline after applying patches. Intel says tests of the coming fixes have shown a two to eight percent decline in performance (WIRED).
Avast has compiled a timeline for this family of chipset vulnerabilities (Security Boulevard).