CVE-2015-5166 in Xen
Summary
by MITRE
Use-after-free vulnerability in QEMU in Xen 4.5.x and earlier does not completely unplug emulated block devices, which allows local HVM guest users to gain privileges by unplugging a block device twice.
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Analysis
by VulDB Data Team • 06/07/2022
The CVE-2015-5166 vulnerability represents a critical use-after-free flaw in the QEMU emulator component of Xen hypervisor versions 4.5.x and earlier. This vulnerability specifically affects the handling of emulated block devices within HVM (Hardware Virtual Machine) guest environments, creating a privilege escalation vector that can be exploited by local users within compromised virtual machines. The flaw stems from incomplete device unplug operations that leave memory references in an invalid state, allowing malicious code to manipulate freed memory structures.
The technical implementation of this vulnerability involves the improper management of block device lifecycle operations within the QEMU virtualization layer. When a block device is unplugged from a virtual machine, the system should properly release all associated memory structures and invalidate any references to prevent further access. However, in affected Xen versions, the unplug process fails to completely terminate all device references, leaving behind dangling pointers that can be manipulated by an attacker. This use-after-free condition occurs because the memory allocated for block device structures is freed but not properly sanitized, allowing subsequent operations to access or reuse the same memory locations.
From an operational perspective, this vulnerability enables local privilege escalation within HVM guest environments, allowing attackers to elevate their privileges from standard user level to root or administrative access. The attack requires a local user within the guest operating system to perform a specific sequence of device unplugging operations twice, which triggers the memory corruption. This type of attack is particularly concerning in multi-tenant virtualization environments where guest isolation is critical for security. The vulnerability can be exploited in scenarios where virtual machines are not properly isolated, potentially allowing an attacker to compromise the entire host system or other guest instances.
The impact of CVE-2015-5166 extends beyond simple privilege escalation, as it can be leveraged as a stepping stone for broader system compromise. According to the CWE taxonomy, this vulnerability maps to CWE-416, which describes the use of freed memory condition, while the ATT&CK framework categorizes this under privilege escalation techniques using software vulnerabilities. The vulnerability's exploitation pathway aligns with the ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation" and demonstrates how virtualization-specific flaws can be weaponized to achieve unauthorized system access. Organizations running affected Xen hypervisor versions face significant risk, particularly in cloud environments where multiple tenants share the same physical infrastructure, as this vulnerability could enable attackers to escape guest boundaries and compromise the underlying host system.
Mitigation strategies for CVE-2015-5166 focus on immediate patching of affected Xen hypervisor versions to 4.6.0 or later, which contain the necessary fixes for proper block device unplug operations. Additionally, administrators should implement strict virtual machine isolation policies and consider disabling unnecessary block device operations within guest environments. Network segmentation and monitoring for suspicious device unplugging activities can serve as additional defensive measures. The vulnerability also highlights the importance of regular security assessments of virtualization platforms and maintaining up-to-date hypervisor versions to protect against known exploitation vectors. Organizations should conduct comprehensive vulnerability scans to identify systems running affected Xen versions and prioritize patch deployment to prevent exploitation of this privilege escalation flaw.