CVE-2015-2152 in Xen
Summary
by MITRE
Xen 4.5.x and earlier enables certain default backends when emulating a VGA device for an x86 HVM guest qemu even when the configuration disables them, which allows local guest users to obtain access to the VGA console by (1) setting the DISPLAY environment variable, when compiled with SDL support, or connecting to the VNC server on (2) ::1 or (3) 127.0.0.1, when not compiled with SDL support.
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Analysis
by VulDB Data Team • 04/15/2022
The vulnerability described in CVE-2015-2152 represents a critical privilege escalation and information disclosure issue within the Xen virtualization platform affecting versions 4.5.x and earlier. This flaw exists in the VGA device emulation functionality for x86 HVM (Hardware Virtual Machine) guests, where the hypervisor fails to properly respect configuration settings that should disable certain backend services. The vulnerability stems from the improper handling of default backend configurations during VGA device emulation, creating unexpected access pathways that bypass intended security controls.
The technical implementation of this vulnerability occurs through the interaction between Xen's hypervisor and QEMU's emulation layers when processing VGA console access requests. When a guest operating system attempts to access VGA console functionality, the hypervisor incorrectly enables certain backends that should remain disabled according to the configuration parameters. This misconfiguration allows local guest users to exploit the system through two primary vectors: first, by setting the DISPLAY environment variable when the system is compiled with SDL support, and second, by connecting to VNC servers listening on localhost addresses (::1 or 127.0.0.1) when SDL support is not available. The vulnerability specifically affects the default backend initialization process where configuration settings are not properly enforced during the VGA device emulation lifecycle.
The operational impact of this vulnerability is significant as it provides local guest users with unauthorized access to VGA console functionality that should be restricted to administrators or privileged users. This access can be leveraged to obtain sensitive information, potentially execute arbitrary code, or perform privilege escalation attacks within the virtualized environment. The vulnerability essentially creates a backdoor access mechanism that bypasses normal authentication and authorization controls, allowing malicious users to gain insights into the underlying system configuration and potentially escalate their privileges. From a security perspective, this represents a failure in the principle of least privilege, where guest users can access system resources that should remain isolated from guest environments.
The vulnerability aligns with CWE-284 (Improper Access Control) and CWE-352 (Cross-Site Request Forgery) categories, as it demonstrates inadequate access control mechanisms and potential for unauthorized system access. From the MITRE ATT&CK framework perspective, this vulnerability maps to T1068 (Local Privilege Escalation) and T1083 (File and Directory Discovery) techniques, as it enables unauthorized access to system resources and potentially exposes sensitive information. The attack surface is particularly concerning in multi-tenant virtualized environments where guest users might attempt to exploit this vulnerability to gain access to other virtual machines or host system resources. Organizations running affected Xen versions should immediately implement mitigations including updating to patched versions, disabling unnecessary VGA console features, and implementing proper network segmentation to prevent unauthorized access to localhost services.
This vulnerability highlights the critical importance of proper configuration management and access control enforcement in virtualized environments. The flaw demonstrates how seemingly minor configuration inconsistencies can create significant security risks, particularly in hypervisor implementations where guest users have direct access to hardware emulation layers. Security practitioners should consider this vulnerability as part of broader virtualization security assessments and implement comprehensive monitoring to detect unauthorized access attempts to VGA console services. The patch for this vulnerability typically involves ensuring that configuration parameters are properly enforced during VGA device initialization and that default backend services are disabled when explicitly configured as such.