https://www.theregister.co.uk/2018/07/23/spectre_return_stack_buffer/

Spectre, a class of vulnerabilities in the speculative execution mechanism employed in modern processor chips, is living up to its name by proving to be unkillable.

Despite a series of mitigations proposed by Intel, Google and others, recent claims by Dartmouth computer scientists to have solved Spectre variant 1, and a proposed chip design fix called SafeSpec, new variants keep appearing.