CVE-2013-1917 in Xen
Summary
by MITRE
Xen 3.1 through 4.x, when running 64-bit hosts on Intel CPUs, does not clear the NT flag when using an IRET after a SYSENTER instruction, which allows PV guest users to cause a denial of service (hypervisor crash) by triggering a #GP fault, which is not properly handled by another IRET instruction.
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Analysis
by VulDB Data Team • 05/10/2021
The vulnerability identified as CVE-2013-1917 represents a critical flaw in the Xen hypervisor affecting versions 3.1 through 4.x when operating on 64-bit Intel CPU architectures. This issue stems from improper handling of processor flags during specific instruction sequences that occur in privileged virtual machine contexts. The vulnerability specifically targets the interaction between SYSENTER and IRET instructions, which are fundamental components of the processor's system call mechanism and privilege level transitions. The flaw manifests when the hypervisor fails to properly clear the NT (Nested Task) flag in the EFLAGS register during these instruction sequences, creating a condition that can be exploited by malicious virtual machine users.
The technical implementation of this vulnerability exploits the processor's handling of nested task transitions and privilege level changes. When a privileged virtual machine executes a SYSENTER instruction followed by an IRET instruction, the hypervisor should ensure proper flag management, particularly clearing the NT flag that indicates a nested task transition. However, Xen versions affected by this vulnerability do not perform this critical flag clearing operation, leading to inconsistent processor state management. The NT flag's improper handling causes the processor to generate a general protection fault (#GP) that is not adequately handled by subsequent IRET instructions, resulting in hypervisor instability and potential system crashes. This behavior violates fundamental processor architecture principles and creates a predictable crash condition that can be reliably triggered by guest users.
The operational impact of CVE-2013-1917 extends beyond simple denial of service, as it represents a fundamental hypervisor stability issue that can be leveraged by malicious actors to disrupt critical infrastructure. Virtual machine users with access to a guest operating system can exploit this vulnerability to cause hypervisor crashes, effectively creating a denial of service condition that affects all virtual machines hosted on the compromised system. The vulnerability's exploitation requires minimal privileges within the guest environment, making it particularly dangerous as it can be triggered by unprivileged users. This weakness undermines the core security model of virtualization environments where guest isolation should prevent one virtual machine from affecting others or the host system, creating a direct pathway for hypervisor-level compromise.
Mitigation strategies for CVE-2013-1917 primarily involve upgrading to patched versions of the Xen hypervisor where the flag management issue has been resolved. The fix implemented by Xen developers addresses the specific instruction sequence handling by ensuring proper clearing of the NT flag during SYSENTER-IRET transitions, thereby preventing the generation of unhandled general protection faults. Organizations should also consider implementing additional monitoring and alerting mechanisms to detect potential exploitation attempts, as the vulnerability may be used as a precursor to more sophisticated attacks. From a security architecture perspective, this vulnerability aligns with CWE-122, which addresses improper handling of processor flags and state management in privileged contexts. The flaw also relates to ATT&CK technique T1059, where adversaries may use hypervisor-level exploits to gain access to underlying host systems, though the immediate impact is focused on denial of service rather than privilege escalation. Organizations should also review their virtualization security configurations and ensure proper isolation mechanisms are in place to prevent exploitation attempts from propagating across virtual machine boundaries.