CVE-2016-6259 in Xen
Summary
by MITRE
Xen 4.5.x through 4.7.x do not implement Supervisor Mode Access Prevention (SMAP) whitelisting in 32-bit exception and event delivery, which allows local 32-bit PV guest OS kernels to cause a denial of service (hypervisor and VM crash) by triggering a safety check.
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Analysis
by VulDB Data Team • 09/09/2022
The vulnerability identified as CVE-2016-6259 represents a critical flaw in the Xen hypervisor affecting versions 4.5.x through 4.7.x. This issue specifically targets the implementation of Supervisor Mode Access Prevention mechanisms within the hypervisor's 32-bit privilege level handling. The flaw stems from an incomplete implementation of SMAP whitelisting during exception and event delivery processes, creating a pathway for exploitation that can result in severe system instability. The vulnerability operates at the intersection of hypervisor security and guest operating system privilege management, where the hypervisor fails to properly enforce access restrictions that should prevent guest kernels from triggering critical safety checks.
The technical nature of this vulnerability lies in the hypervisor's failure to properly implement Supervisor Mode Access Prevention, a security feature designed to prevent supervisor-mode code from accessing user-mode memory regions. In 32-bit privilege-escalation scenarios, this deficiency allows malicious guest operating systems to manipulate the hypervisor's execution flow through carefully crafted exception handling sequences. The flaw specifically affects Privileged Virtual (PV) guest operating systems running in 32-bit mode, where the hypervisor's SMAP implementation does not adequately account for the whitelisting requirements during exception delivery. This creates a condition where guest kernels can trigger hypervisor safety mechanisms that were designed to prevent system crashes, but instead cause the hypervisor itself to become unstable.
The operational impact of CVE-2016-6259 manifests as a potential denial of service condition that can bring down both the hypervisor and all virtual machines running on that hypervisor. When exploited, the vulnerability allows a local 32-bit PV guest kernel to cause system-wide crashes, effectively rendering the entire virtualized environment unavailable. This represents a significant threat to cloud computing environments and virtualized infrastructure where multiple tenants rely on shared hypervisor resources. The vulnerability's exploitation requires local access within a 32-bit guest environment, making it less accessible than remote exploits but still highly concerning for multi-tenant deployments where guest isolation is critical. The impact extends beyond simple service interruption to potentially compromising the availability of critical services that depend on virtualized infrastructure.
Mitigation strategies for CVE-2016-6259 focus primarily on updating the Xen hypervisor to versions that properly implement SMAP whitelisting for 32-bit exception handling. System administrators should prioritize upgrading to Xen versions 4.8.0 and later, which contain the necessary security patches addressing this specific flaw. Additionally, implementing monitoring solutions that can detect abnormal exception handling patterns or memory access violations may provide early warning of potential exploitation attempts. The vulnerability aligns with CWE-119, which addresses "Improper Access to Memory Location," and demonstrates characteristics consistent with ATT&CK technique T1068, "Exploitation for Privilege Escalation," as the attack vector involves leveraging hypervisor privilege levels to cause system instability. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of a successful exploitation, while ensuring that 32-bit PV guest operating systems are properly monitored for suspicious activity patterns that could indicate attempts to exploit this vulnerability.