Various hypervisors and operating systems are scrambling to patch around an x86 bug that lets an admin-level guest crash the underlying CPU, causing a denial-of-service to anyone else on the same machine.
The issue, described here, is that with some x86 CPUs, an attacker with kernel-mode code execution privileges on a guest operating system can hang the CPU.
The two CVEs Redmond cites are described by Xen (which has been updated to address the same issues) here. There's an alignment check exception (CVE-2015-5307) that can trigger an infinite loop in some x86 CPUs, and a debug exception (CVE-2015-8104) that can result in an infinite loop or a stack fault, depending on how it's triggered.
Patches have been issued for various 2008 and 2012 versions of Windows Server, and Windows 8.0, 8.1 and 10.
Redmond's not the only vendor to cover this one off. As well as the Xen note, there are advisories from:
Users of kvm or Xen on other Linux variants should keep an eye out for their exposure and patch information. ®