The Xen hypervisor is widely used by cloud computing providers and virtual private server hosting companies.
Xen 4.6.1, released Monday, is flagged as a maintenance release, the kind that are put out roughly every four months and are supposed to include all bug and security patches released in the meantime.
"Due to two oversights the fixes for both XSA-155 and XSA-162 have only been partially applied to this release," the Xen Project noted in a blog post. The same is true for Xen 4.4.4, the maintenance release for the 4.4 branch that was released on Jan. 28, the Project said.
Security conscious users are likely to apply Xen patches to existing installations as they are made available, and not wait for maintenance releases. However, new Xen deployments would likely be based on the latest available versions, which right now contain incomplete fixes for two publicly known and documented security vulnerabilities.
XSA-162 and XSA-155 refer to two vulnerabilities for which patches were released in November and December respectively.
XSA-162, also tracked as CVE-2015-7504, is a vulnerability in QEMU, an open-source virtualization software program that is used by Xen. Specifically, the flaw is a buffer overflow condition in QEMU's virtualization of AMD PCnet network devices. If exploited, it could allow a user of a guest operating system that has access to a virtualized PCnet adapter to elevate his privileges to that of the QEMU process.
XSA-155, or CVE-2015-8550, is a vulnerability in Xen's paravirtualized drivers. Guest OS administrators could exploit the flaw to crash the host or to arbitrary execute code with higher privileges.
"In summary, a simple switch statement operating on shared memory is compiled into a vulnerable double fetch that allows potentially arbitrary code execution on the Xen management domain," said Felix Wilhelm, the researcher who found the flaw, in a blog post back in December.