During the New Years holiday period in 2018, information about two serious CPU security vulnerabilities was leaked to the public.
These issues were connected to a part of a microprocessor function called Speculative Execution. This function has been in processors for at least a couple of decades and is used for enhancing CPU performance. The chip looks ahead and tries to determine what instructions will be executed, thus potentially saving some time but the implementation is also unfortunately vulnerable to data leakage.
Fairly quickly, patches were made available for the Meltdown variant to Windows, macOS and Linux. Unfortunately, these patches as they were covering up the speculative execution with software introduced some performance reductions, although this varied on what the role of the machine is and for many users, the slowness is not noticeable.
As Spectre requires a microcode update to the CPU to resolve, older processors will probably not be patched so will remain vulnerable. Newer CPU generation fixes are currently being worked on.
Intel has said that they are going to update their 8th generation core processors (Coffee Lake) to include mitigations for Meltdown and Spectre so they will not require a software fix in the future as we move forward. In May 2018, further Spectre flaws were discovered (these were named Spectre-NG).
As with many security vulnerabilities, keeping your OS and browser updated to the current level will resolve many issues.
A list of CPUs that will be patched is maintained by Microsoft (they can patch without a CPU microcode firmware update being necessary):
A list of many manufacturers forthcoming BIOS updates:
Intel regularly updates microcode on their current and legacy processors to resolve issues such as Spectre and Meltdown:
https://www.intel.com/content/dam/www/public/us/en/documents/sa00115-microcode-update-guidance.pdf