CVE-2012-3516 in XenServer
Summary
by MITRE
The GNTTABOP_swap_grant_ref sub-operation in the grant table hypercall in Xen 4.2 and Citrix XenServer 6.0.2 allows local guest kernels or administrators to cause a denial of service (host crash) and possibly gain privileges via a crafted grant reference that triggers a write to an arbitrary hypervisor memory location.
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Analysis
by VulDB Data Team • 04/21/2017
The vulnerability identified as CVE-2012-3516 represents a critical security flaw in the Xen virtualization platform affecting versions 4.2 and Citrix XenServer 6.0.2. This issue resides within the grant table hypercall mechanism, specifically in the GNTTABOP_swap_grant_ref sub-operation that governs how grant references are managed between guest operating systems and the hypervisor. The flaw allows malicious actors with access to guest kernels or administrator privileges to manipulate the grant reference system in ways that can lead to severe system instability and potential privilege escalation. The vulnerability operates at the intersection of hypervisor security and virtualization integrity, where guest-level code can directly influence hypervisor memory management through carefully crafted grant references.
The technical implementation of this vulnerability stems from inadequate input validation and memory access controls within the grant table hypercall handling code. When the GNTTABOP_swap_grant_ref operation processes a crafted grant reference, it fails to properly validate the memory addresses being referenced, allowing attackers to specify arbitrary hypervisor memory locations for write operations. This validation failure creates a path where malicious grant references can be used to overwrite critical hypervisor data structures, potentially leading to immediate system crashes or more insidious privilege escalation attacks. The vulnerability is classified under CWE-125 as an out-of-bounds write condition, where the hypervisor fails to properly bounds-check memory access operations during grant reference swapping. The flaw demonstrates a classic case of insufficient privilege separation between guest and hypervisor contexts, where guest-level code can directly influence hypervisor memory management without proper authorization checks.
The operational impact of CVE-2012-3516 extends beyond simple denial of service scenarios to encompass potential privilege escalation and system compromise. A successful exploitation can result in complete host system crashes, forcing administrators to restart virtualization environments and potentially leading to service disruption across multiple virtual machines. More concerning is the potential for privilege escalation, where local guest administrators could leverage this vulnerability to gain elevated privileges within the hypervisor context, effectively compromising the entire virtualization infrastructure. The attack vector requires either guest kernel access or administrative privileges within a guest operating system, making it particularly dangerous in multi-tenant virtualization environments where guest isolation is paramount. This vulnerability directly impacts the fundamental security model of Xen virtualization, undermining the isolation guarantees that separate virtual machines should maintain from each other and from the underlying hypervisor.
Mitigation strategies for CVE-2012-3516 primarily involve immediate patching of affected systems, with the release of Xen 4.3 and subsequent versions addressing the core validation issues in the grant table hypercall implementation. Organizations should implement comprehensive monitoring for unauthorized guest access or privilege escalation attempts, as the vulnerability may be used as a stepping stone for more sophisticated attacks. The remediation process should include updating all Xen hypervisor installations to versions that have addressed the specific grant reference validation flaws, particularly focusing on the GNTTABOP_swap_grant_ref operation. System administrators should also consider implementing additional security controls such as hypervisor hardening measures, restricted guest privileges, and enhanced monitoring of hypercall operations. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques and system compromise tactics, where attackers leverage hypervisor-level vulnerabilities to gain elevated access to the underlying infrastructure. Organizations should also review their virtualization security policies and ensure proper isolation mechanisms are in place to limit the potential impact of such vulnerabilities in multi-tenant environments.