Lovely economical summary by the UK ICO’s Head of Technology Policy of the two vulnerabilities currently obsessing the CPU-design industry:
In essence, the vulnerabilities provide ways that an attacker could extract information from privileged memory locations that should be inaccessible and secure. The potential attacks are only limited by what is being stored in the privileged memory locations – depending on the specific circumstances an attacker could gain access to encryption keys, passwords for any service being run on the machine, or session cookies for active sessions within a browser. One variant of the attacks could allow for an administrative user in a guest virtual machine to read the host server’s kernel memory. This could include the memory assigned to other guest virtual machines.