That warning arrived Tuesday in the form of a security advisory released by the U.S. Computer Emergency Readiness Team (US-CERT). "Some 64-bit operating systems and virtualization software running on Intel CPU hardware are vulnerable to a local privilege escalation attack," it read. "The vulnerability may be exploited for local privilege escalation or a guest-to-host virtual machine escape."
"All systems running 64-bit Xen hypervisor running 64-bit PV [para-virtualized] guests on Intel CPUs are vulnerable to this issue," read a security advisory released by the open source Xen project.
[ A flaw in the popular--and free--MySQL and MariaDB databases leaves them open to brute-force attacks. Read more at MySQL Database Flaw Leaves Passwords Vulnerable. ]
Metasploit penetration testing framework founder, developer, and researcher H.D. Moore characterized the bug as a "serious guest-to-host escape vulnerability," noting that while it affects the Xen platform, it doesn't affect VMware.
The Xen project said it's updated the Xen code base to eliminate the vulnerability. Likewise, multiple vendors and providers of virtualization products, including FreeBSD, Microsoft, NetBSD, Oracle, Red Hat, and SUSE Linux, have released updated software to patch the Xen hypervisor flaw. Apple, Intel, and VMware have confirmed that the vulnerability doesn't exist in their products. Meanwhile, Debian GNU/Linux, Fedora Project, Gentoo Linux, HP, and IBM have yet to confirm whether their software is vulnerable to the escape-to-hypervisor vulnerability.
IBM has warned about the information security threat that "escape-to-hypervisor" attacks can pose, especially as roughly one-third of all virtualization bugs discovered by 2010 were in the hypervisor. Since virtualized environments run multiple instances of operating systems, an attacker that escaped from any one of those instances and gained administrative-level rights could then access any other virtualized environment running on the same server.
The security bulletins released in the wake of the Xen flaw announcements read along similar lines. "An unprivileged user in a 64-bit para-virtualized guest, that is running on a 64-bit host that has an Intel CPU, could use this flaw to crash the host or potentially escalate their privileges, allowing them to execute arbitrary code at the hypervisor level," read the related Red Hat security bulletin, which noted that the flaw affects the Xen hypervisor implementation as shipped with Red Hat Enterprise Linux 5.
According to Microsoft, the Xen flaw is present in its Windows User Mode Scheduler. "An attacker who successfully exploited this vulnerability could run arbitrary code in kernel mode. An attacker could then install programs; view, change, or delete data; or create new accounts with full administrative rights," read Microsoft's related security bulletin. It said that Intel x64-based versions of Windows 7 and Windows Server 2008 R2 are affected by the flaw, as are Windows XP SP3 and Server 2003 SP2.
But there are mitigating factors. "An attacker must have valid logon credentials and be able to log on locally to exploit this vulnerability," said Microsoft. "The vulnerability could not be exploited remotely or by anonymous users." This means that--as noted by US-CERT--this is a "local privilege escalation" vulnerability.
According to Xen, another mitigation technique is to run the hypervisor in hardware-assisted virtualization mode, or to use 32-bit para-virtualization.
Both Microsoft and Red Hat rate the related hypervisor vulnerability in their products not "serious" but just "important," owing to the fact that an attacker must possess valid login credentials before executing the attack, or, in the case of Red Hat Linux guests, must possess at least "privileged guest user" status.
Credit for spotting the vulnerability goes to kernel and virtualization security researcher Rafal Wojtczuk of Bromium, who alerted the Xen project.
On a related note, Wojtczuk is scheduled to present a session at next month's Black Hat (BH) conference in Las Vegas on the topic of exploiting "user-to-kernel privilege escalation" bugs in four Intel-CPU-based operating systems--not including Linux. He's described the vulnerabilities involved as being "widespread and reliably exploitable."
"The BH talk will be precisely about the vulnerability that vendors released patches for yesterday," said Wojtczuk via email. "So naturally by BH time, this will not be a zero-day issue."
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