CVE-2007-5907 in Xen
Summary
by MITRE
Xen 3.1.1 does not prevent modification of the CR4 TSC from applications, which allows pv guests to cause a denial of service (crash).
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Analysis
by VulDB Data Team • 08/02/2019
The vulnerability identified as CVE-2007-5907 affects Xen hypervisor version 3.1.1 and represents a critical flaw in privilege management within virtualized environments. This issue stems from the hypervisor's failure to properly enforce restrictions on Control Register 4 modifications, specifically concerning the Time Stamp Counter (TSC) functionality. The vulnerability allows paravirtualized (pv) guest operating systems to manipulate the CR4 register in ways that were not intended by the hypervisor architecture, creating a potential vector for malicious activity.
The technical flaw manifests through the improper handling of CR4 register modifications within the hypervisor's privilege separation mechanisms. Control Register 4 contains several important flags including the Time Stamp Counter enable bit (TSC), which when modified can affect the behavior of time-related operations across the virtual machine environment. In a properly secured hypervisor, direct modification of CR4 by guest operating systems should be restricted or properly mediated to prevent unauthorized access to system resources. However, Xen 3.1.1 fails to enforce these restrictions adequately, allowing pv guests to modify the TSC-related bits in CR4 without proper authorization.
This vulnerability creates significant operational impact by enabling a denial of service condition that can result in complete system crashes. When pv guests manipulate the TSC through unauthorized CR4 modifications, they can trigger cascading failures within the hypervisor's time management subsystems. The instability introduced by these modifications can cause the hypervisor to become unresponsive or crash entirely, effectively rendering the virtualized environment unusable. This type of vulnerability is particularly dangerous in production environments where multiple virtual machines share the same physical host, as a single compromised guest could potentially affect all other virtual machines running on that host.
The vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and represents a failure in the hypervisor's privilege separation model. From an ATT&CK perspective, this issue maps to techniques involving privilege escalation and denial of service, specifically targeting the hypervisor layer through guest operating system manipulation. The flaw demonstrates a fundamental breakdown in the security model where guest operating systems should be isolated from critical hypervisor functions. Organizations implementing virtualization technologies must understand that such vulnerabilities can undermine the entire security posture of their virtualized infrastructure.
Mitigation strategies for this vulnerability primarily involve upgrading to a patched version of the Xen hypervisor where proper CR4 register restrictions have been implemented. System administrators should also consider implementing additional monitoring and logging mechanisms to detect unauthorized CR4 modifications. The recommended approach includes maintaining up-to-date virtualization software, implementing proper access controls for guest operating systems, and establishing robust monitoring procedures to detect anomalous behavior patterns that might indicate exploitation attempts. Organizations should also review their virtualization security configurations and ensure that appropriate isolation measures are in place between different virtual machines sharing the same physical host resources.