Executive brief
Xen is a virtualization platform that allows multiple operating systems to run on a single physical server. A flaw in how it handles certain processor instructions allows a user inside a virtual machine to break out and access the underlying host system. This could result in the attacker stealing data from other virtual machines, crashing the entire server, or taking full control of the host hardware.
Technical details
A vulnerability exists in Xen's emulation of x86 bit test instructions (BT, BTC, BTR, and BTS) when used with a destination memory operand and a source register. During emulation, Xen incorrectly recalculates the memory address and register operand, discarding high bits of an intermediate expression. This logic error allows a malicious 64-bit guest to cause the hypervisor to perform unintended memory accesses or register updates. An attacker can leverage this to achieve arbitrary memory modification on the host, leading to host crashes (DoS), information disclosure, or full host-level privilege escalation. The issue is primarily exposed to 64-bit x86 guests; on Xen 4.7 and later, it typically requires specific guest privileges or non-default CPU configurations to exploit.
Affected products
- Xen Project Xen All versions (exposed to 64-bit x86 guests)
- Citrix XenServer 6.0.2, 6.2.0, 6.5, 7.0
Timeline
- 2016-11-22: disclosed: Initial public release of XSA-195
- 2016-11-22: patched: Patch released by Xen Project
- 2017-01-23: advisory: NVD publication date