Researchers from University of California, Riverside (UCR) discovered another Spectre-class attack. Called SpectreRSB, this attack abuses the speculative execution feature that's used by all modern processors. In particular, the new strain recovers data belonging to other processes from the Return Stack Buffer (RSB), and can even be used to spill SGX secrets.
SpectreRSB not only affects Intel, AMD, and ARM processors, but also bypasses all mitigations that were rolled out for Spectre and Meltdown.
The difference from previous Spectre-like attacks is that SpectreRSB recovers data from the speculative execution process by attacking a different CPU component involved in this "speculation" routine, namely the Return Stack Buffer (RSB). Previous Spectre attacks have targeted the branch predictor unit or parts of the CPU cache.
In the grand architecture of a CPU, the RSB is a component that is involved in the speculative execution routine and works by predicting the return address of an operation the CPU is trying to compute in advance, part of its "speculation."