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Xen Patches Serious Privilege Escalation Flaw

The Xen Project has released patches for two vulnerabilities, including a serious issue that could allow an attacker to escape the guest virtual machine.

The Xen Project has released patches for two vulnerabilities, including a serious issue that could allow an attacker to escape the guest virtual machine.

According to an advisory made public on Tuesday, a malicious paravirtual (PV) guest administrator can escalate their privileges to the ones of the host. The Xen Project pointed out that while all Xen versions are vulnerable, only PV guests running on x86 hardware are exposed. The vulnerability does not affect hardware virtual machine (HVM) guests.

The vulnerability, discovered by Jérémie Boutoille of Quarkslab and tracked as CVE-2016-6258, has also been analyzed by the developers of the security-oriented, open-source operating system Qubes, which uses Xen hypervisor for security isolation between domains.

“An attacker who exploits this bug can break Qubes-provided isolation,” Qubes developers explained. “This means that if an attacker has already exploited another vulnerability, e.g. in a Web Browser or Networking or USB stack, then the attacker would be able to compromise the whole Qubes system.”

Qubes has classified the vulnerability as “critical,” but it has not been able to precisely determine how easy and reliable it is to exploit. This and other serious security bugs in Xen have made Qubes developers consider switching to a different hypervisor, but they have yet to find a good alternative.

Xen is used in Linux distributions and cloud services provided by Amazon, IBM, Linode, Rackspace and others. Red Hat indicated that the bug will not be fixed in Red Hat Enterprise Linux 5 and Amazon assured AWS customers that their data and instances are not impacted.

Citrix, which rates the flaw as “high severity,” has informed customers of its XenServer product that all currently supported versions up to and including XenServer 7.0 are affected. The company has released hotfixes.

The second vulnerability patched by the Xen Project, CVE-2016-6259, allows a malicious 32-bit PV guest kernel to crash the hypervisor and cause other VMs on the host to enter a denial-of-service (DoS) condition.

Vendors and open source projects that are on the Xen Project’s pre-disclosure list received the patches before the vulnerabilities were disclosed.

“Any Xen-based public cloud is eligible to be on our ‘pre-disclosure’ list. Cloud providers on the list were notified of the vulnerability and provided a patch two weeks before the public announcement in order to make sure they all had time to apply the patch to their servers,” Xen Project Chairperson Lars Kurth told SecurityWeek.

“Xen Project follows industry-accepted best practices regarding software security,” Kurth added. “This includes not discussing any details with security implications during our embargo period. This is to encourage anyone to report bugs they find to the Xen Project Security team. This also allows Xen Project security team to assess, respond and prepare updated software packages before public disclosure and broad compromise occurs.”

Related: Xen Hypervisor Flaws Force Amazon, Rackspace to Reboot Servers

Related: Several Flaws Patched in Xen Hypervisor

Written By

Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.

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