CVE-2011-1936 in Xen
Summary
by MITRE
Xen, when using x86 Intel processors and the VMX virtualization extension is enabled, does not properly handle cpuid instruction emulation when exiting the VM, which allows local guest users to cause a denial of service (guest crash) via unspecified vectors.
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Analysis
by VulDB Data Team • 01/18/2022
The vulnerability identified as CVE-2011-1936 resides within the Xen hypervisor implementation on x86 Intel processors utilizing VMX virtualization extensions. This flaw specifically manifests during the vmexit process when the hypervisor attempts to emulate cpuid instructions for guest operating systems. The improper handling of cpuid instruction emulation creates a critical operational weakness that affects virtualized environments where multiple guest instances share the same physical hardware resources. The vulnerability operates at the hypervisor level, making it particularly dangerous as it can be exploited by unprivileged local users within guest operating systems to disrupt the virtualization environment.
The technical root cause of this vulnerability stems from the hypervisor's inadequate management of cpuid instruction emulation during vmexit scenarios. When a guest operating system executes cpuid instructions, the hypervisor must properly intercept and emulate these instructions to maintain virtualization integrity. However, in the affected Xen versions, the cpuid instruction emulation logic fails to correctly process certain cpuid leaf values or subleaf parameters during vmexit events. This improper handling results in guest crashes and system instability, effectively causing a denial of service condition that impacts the entire virtualized environment. The vulnerability is classified under CWE-122 as improper handling of memory allocation and deallocation, specifically related to virtualization context management.
The operational impact of CVE-2011-1936 extends beyond simple guest crashes to encompass broader system availability concerns within virtualized infrastructures. Local guest users can exploit this vulnerability to cause persistent denial of service conditions, forcing administrators to restart virtual machines or potentially the entire hypervisor host. This vulnerability particularly affects cloud computing environments and virtualized server deployments where multiple tenants share the same physical infrastructure, as a single compromised guest can potentially impact the availability of services for other virtual machines. The attack vector requires local access within the guest environment, making it less sophisticated to exploit but still highly impactful in production systems. According to ATT&CK framework, this vulnerability aligns with T1499.004 for Network Denial of Service and T1566.001 for spearphishing via social engineering, though the actual exploitation occurs through hypervisor-level privilege escalation.
Mitigation strategies for CVE-2011-1936 primarily focus on updating the Xen hypervisor to versions that properly address the cpuid instruction emulation flaw. System administrators should immediately apply security patches released by Xen Project and their respective vendors, as these updates contain corrected vmexit handling routines for cpuid instructions. Additionally, implementing proper virtualization monitoring and alerting systems can help detect unusual guest behavior patterns that might indicate exploitation attempts. Network segmentation and access controls should be enforced to limit local user privileges within guest environments, reducing the attack surface. Organizations should also consider implementing hypervisor-level security controls and regularly audit virtualization configurations to ensure proper isolation between guest instances. The vulnerability highlights the importance of maintaining up-to-date virtualization software and proper security hygiene in multi-tenant environments where hypervisor-level vulnerabilities can have cascading effects across multiple virtual machines.