New variants of the side-channel central processing unit (CPU) hardware vulnerabilities (aka Meltdown/Spectre) have been discovered. The new variants dubbed 3A and 4 have been found by Microsoft and Google researchers.
The two new variants are further described in a recent Intel security update and summarized below.
Variant 3a is a Rogue System Register Read (RSRE) vulnerability (CVE-2018-3640): “Systems with microprocessors utilizing speculative execution and that perform speculative reads of system registers may allow unauthorized disclosure of system parameters to an attacker with local user access via a side-channel analysis.”
Variant 4 is a Speculative Store Bypass (SSB) vulnerability (CVE-2018-3639): “Systems with microprocessors utilizing speculative execution and speculative execution of memory reads before the addresses of all prior memory writes are known may allow unauthorized disclosure of information to an attacker with local user access via a side-channel analysis.”
To review more details, you may also check out the US-CERT advisory that includes a brief history of the side-channel vulnerabilities and latest developments on the threat.