CVE-2020-25637: Red Hat Libvirt DOS Vulnerability

CVE-2020-25637 Overview

A double free memory issue was found in the libvirt API in versions before 6.8.0. The vulnerability exists in the functionality responsible for requesting information about network interfaces of a running QEMU domain. This flaw specifically affects the polkit access control driver, where clients connecting to the read-write socket with limited ACL permissions could exploit this vulnerability to crash the libvirt daemon, resulting in a denial of service, or potentially escalate their privileges on the system.

Critical Impact

Successful exploitation could lead to denial of service through daemon crash or privilege escalation, threatening data confidentiality, integrity, and system availability.

Affected Products

• Red Hat libvirt (versions before 6.8.0)
• openSUSE Leap 15.1
• openSUSE Leap 15.2

Discovery Timeline

• October 6, 2020 - CVE-2020-25637 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25637

Vulnerability Analysis

This vulnerability is classified as CWE-415 (Double Free), a memory corruption flaw that occurs when a program attempts to free the same memory location twice. In the context of libvirt, the double free condition manifests within the API responsible for handling network interface information requests for QEMU virtual machine domains.

The vulnerability requires local access and high privileges to exploit, but once exploited, it can impact confidentiality, integrity, and availability of the affected system. The flaw is particularly concerning in virtualization environments where libvirt serves as a critical management layer for QEMU/KVM virtual machines.

Root Cause

The root cause of CVE-2020-25637 lies in improper memory management within the libvirt API's network interface information handling code. When processing requests through the polkit access control driver, the code path can lead to a scenario where the same memory block is freed multiple times. This double free condition occurs due to insufficient tracking of memory allocation states, allowing memory that has already been deallocated to be freed again.

Attack Vector

The attack requires local access to the system with the ability to connect to the libvirt read-write socket. An attacker with limited ACL permissions on the polkit access control system can craft malicious requests to the libvirt API that trigger the double free condition. The exploitation path involves:

1. Establishing a connection to the libvirt read-write socket with limited ACL permissions
2. Sending specially crafted requests for network interface information of a running QEMU domain
3. Triggering the double free condition in the polkit access control driver

The double free vulnerability can be exploited to corrupt memory management structures, potentially allowing an attacker to redirect execution flow or cause a denial of service by crashing the libvirt daemon. Detailed technical analysis is available in the Red Hat Bug Report #1881037.

Detection Methods for CVE-2020-25637

Indicators of Compromise

• Unexpected crashes or restarts of the libvirt daemon (libvirtd)
• Anomalous memory allocation patterns in libvirt processes
• Suspicious client connections to the libvirt read-write socket from users with limited permissions
• Core dumps from libvirtd indicating memory corruption or double free errors

Detection Strategies

• Monitor system logs for libvirtd crash events and restart patterns
• Implement audit logging for connections to the libvirt socket (/var/run/libvirt/libvirt-sock)
• Deploy memory corruption detection tools to identify double free conditions
• Review polkit authentication logs for unusual access patterns to libvirt resources

Monitoring Recommendations

• Enable verbose logging for libvirtd to capture detailed error information
• Configure system monitoring to alert on libvirtd process crashes or unexpected terminations
• Implement network monitoring for local socket connections to libvirt services
• Deploy endpoint detection solutions to identify privilege escalation attempts

How to Mitigate CVE-2020-25637

Immediate Actions Required

• Upgrade libvirt to version 6.8.0 or later immediately
• Restrict access to the libvirt read-write socket to only trusted users and services
• Review and tighten polkit ACL configurations to limit exposure
• Monitor systems for signs of exploitation until patches can be applied

Patch Information

Red Hat and other affected vendors have released security updates to address this vulnerability. The fix is included in libvirt version 6.8.0 and later. Organizations should apply the appropriate patches for their distribution:

• Red Hat: Refer to the Red Hat Bug Report #1881037 for patch information
• openSUSE: Security updates available via the openSUSE Security Announcement
• Debian: Updates available through the Debian LTS Security Announcement
• Gentoo: Refer to GLSA 202210-06

Workarounds

• Restrict socket access permissions to minimize exposure (chmod 600 /var/run/libvirt/libvirt-sock)
• Implement strict polkit rules to limit which users can access libvirt functionality
• Consider running libvirtd with reduced privileges where possible
• Use network segmentation to isolate virtualization management interfaces

# Restrict libvirt socket permissions
chmod 600 /var/run/libvirt/libvirt-sock
chown root:libvirt /var/run/libvirt/libvirt-sock

# Verify libvirt version
libvirtd --version

# Restart libvirtd after applying patches
systemctl restart libvirtd