# CVE-2016-9385: Xen x86 segment base write emulation denial of service

Source: https://junglewise.ai/threats/cve-2016-9385-xen-x86-segment-base-write-emulation-denial-of-service  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2017-01-23. Updated: 2026-05-13  
License: CC BY 4.0 with a link back

CVE-2016-9385 · Severity: medium · CVSS 6 · Published 2017-01-23

Xen is a hypervisor used to run multiple virtual machines on a single physical server. A vulnerability in how it handles certain processor instructions allows an administrator of a guest virtual machine to crash the entire physical host. This results in a total service outage for all other virtual machines running on that same hardware.

Technologies: [Citrix Xenserver](https://junglewise.ai/threats/technologies/xenserver), [Xen Project Xen](https://junglewise.ai/threats/technologies/xen). Vendors: [Citrix](https://junglewise.ai/threats/vendors/citrix), [Xen Project](https://junglewise.ai/threats/vendors/xen-project).

## Quick answers

**Is CVE-2016-9385 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2016-9385 being exploited in the wild.

**How severe is CVE-2016-9385?** Medium severity, with a CVSS score of 6, published 23 January 2017.

**Which products and versions are affected?** Citrix Xenserver 6.0.2, 6.2.0, 6.5, 7.0; Xen Project Xen 4.4.x through 4.7.x.

**Is there a fix for CVE-2016-9385?** A patch was published on 2016-11-22: Patches released for Xen unstable, 4.7.x, 4.6.x, 4.5.x, and 4.4.x Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2016-9385 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2016-9385?** Check whether you run Citrix Xenserver or Xen Project Xen at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in Xen's emulation of x86 segment base writes, specifically affecting the FS and GS register base MSRs and the WRFSBASE/WRGSBASE instructions. These instructions require input values to be in canonical form; otherwise, a general protection fault (#GP) is triggered. The hypervisor's recovery code for these faults was inadvertently removed when support for these instructions was enabled. A malicious administrator of a Paravirtualized (PV) guest can provide non-canonical addresses to trigger an unhandled #GP fault, leading to a hypervisor crash and host-wide denial of service. HVM guests are not affected. Patches are available from the Xen Project (XSA-193).

## Affected products

- Xen Project Xen 4.4.x through 4.7.x
- Citrix XenServer 6.0.2, 6.2.0, 6.5, 7.0

## Timeline

- 2016-11-22: advisory: XSA-193 public release
- 2016-11-22: patched: Patches released for Xen unstable, 4.7.x, 4.6.x, 4.5.x, and 4.4.x
- 2017-01-23: disclosed: NVD publication date

## References

- [http://www.securityfocus.com/bid/94472](http://www.securityfocus.com/bid/94472)
- [http://www.securitytracker.com/id/1037342](http://www.securitytracker.com/id/1037342)
- [http://xenbits.xen.org/xsa/advisory-193.html](http://xenbits.xen.org/xsa/advisory-193.html)
- [https://security.gentoo.org/glsa/201612-56](https://security.gentoo.org/glsa/201612-56)
- [https://support.citrix.com/article/CTX218775](https://support.citrix.com/article/CTX218775)

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