CVE-2015-7311 in Xen
Summary
by MITRE
libxl in Xen 4.1.x through 4.6.x does not properly handle the readonly flag on disks when using the qemu-xen device model, which allows local guest users to write to a read-only disk image.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 06/18/2022
The vulnerability identified as CVE-2015-7311 affects the libxl library component within the Xen hypervisor version 4.1.x through 4.6.x, specifically when utilizing the qemu-xen device model for disk operations. This flaw represents a critical security issue that undermines the fundamental principle of read-only disk access controls within virtualized environments. The vulnerability stems from improper handling of the readonly flag during disk attachment operations, creating a privilege escalation vector that allows local guest users to bypass intended read-only restrictions. This issue directly impacts the security isolation properties that virtual machines rely upon, potentially enabling malicious users to modify data they should not have write access to.
The technical root cause of this vulnerability lies in the libxl library's failure to properly validate and enforce the readonly flag when configuring disk devices through the qemu-xen interface. When a disk is configured as read-only within the hypervisor configuration, the libxl component should ensure that the underlying qemu-xen process enforces this restriction. However, due to flawed implementation logic, the readonly flag is either ignored or not properly propagated to the device model, allowing guest operating systems to perform write operations on disks that have been explicitly configured as read-only. This misconfiguration creates a path for privilege escalation where guest users can modify data they should not be able to alter, potentially leading to data corruption or unauthorized modifications.
The operational impact of this vulnerability extends beyond simple data integrity concerns, as it fundamentally compromises the security model of virtualized environments. Local guest users can exploit this weakness to write to read-only disk images, potentially leading to data tampering, privilege escalation, or even complete compromise of the virtual machine's integrity. In enterprise environments where multiple users share virtualized infrastructure, this vulnerability could enable malicious insiders to modify critical system files or data without proper authorization. The implications are particularly severe in scenarios where read-only disk configurations are used as security controls to prevent unauthorized modifications to system images or sensitive data stores.
This vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and represents a clear violation of the principle of least privilege within virtualized computing environments. The flaw also maps to ATT&CK technique T1059, specifically related to command and scripting interpreter usage, as attackers could leverage this vulnerability to execute malicious code through modified disk images. Organizations should implement immediate mitigations including upgrading to Xen versions 4.7.x or later where this vulnerability has been resolved, and conducting thorough audits of virtual machine disk configurations to ensure that read-only flags are properly enforced. Additionally, network segmentation and monitoring solutions should be employed to detect anomalous disk write activities that may indicate exploitation attempts. The vulnerability underscores the critical importance of proper input validation and access control enforcement in hypervisor components, particularly those handling device model interactions where security boundaries are most vulnerable to compromise.