Modern CPUs have speculative execution capabilities, which improves processor performance. Depending on the design and architecture of the CPU, speculative execution can introduce side-channel-attack vulnerabilities.
[Similar to previous post, I included this URL in an early Spectre/Meltdown posting, but was having a hard time finding it.]
https://vuls.cert.org/confluence/plugins/servlet/mobile?contentId=36503581#content/view/36503581