CVE-2020-15708: Ubuntu Linux libvirt RCE Vulnerability

CVE-2020-15708 Overview

Ubuntu's packaging of libvirt in Ubuntu 20.04 LTS created a control socket with world read and write permissions. An attacker could use this to overwrite arbitrary files or execute arbitrary code. This vulnerability stems from improper permission assignment during the packaging process, creating a significant local privilege escalation vector on affected systems.

Critical Impact

Local attackers with access to the system can exploit world-readable and world-writable control socket permissions to achieve arbitrary file overwrites or execute arbitrary code, potentially gaining elevated privileges.

Affected Products

• Canonical Ubuntu Linux 20.04 LTS

Discovery Timeline

• 2020-11-06 - CVE CVE-2020-15708 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15708

Vulnerability Analysis

This vulnerability (CWE-732: Incorrect Permission Assignment for Critical Resource) is a classic case of improper access control on a Unix socket. The libvirt daemon creates a control socket that facilitates communication between clients and the virtualization management service. In Ubuntu 20.04 LTS, the packaging incorrectly set the socket permissions to allow read and write access for all users (world permissions), rather than restricting access to authorized users or groups only.

The local attack vector means an attacker must already have some form of access to the affected system. Once on the system, the exploitation does not require special privileges—any local user can interact with the improperly permissioned socket. This makes the vulnerability particularly dangerous in multi-user environments or systems where unprivileged user accounts may be compromised.

Root Cause

The root cause lies in the Ubuntu-specific packaging of libvirt. During the package build or installation process, the control socket was created with overly permissive file system permissions (mode 0666 or similar world-readable/writable permissions). This violates the principle of least privilege, as the socket should only be accessible to the libvirt group or specific administrative users. The packaging scripts or systemd unit files responsible for socket activation failed to properly restrict access.

Attack Vector

An attacker with local access to an Ubuntu 20.04 LTS system running the vulnerable libvirt package can connect to the improperly secured control socket. Through this connection, the attacker can send commands to the libvirt daemon that allow arbitrary file overwrites. By overwriting critical system files or injecting malicious code into executable locations, the attacker can escalate privileges to root or achieve persistent code execution.

The attack flow typically involves:

1. Identifying the world-accessible libvirt control socket on the file system
2. Connecting to the socket using standard Unix socket communication
3. Issuing commands through the socket to manipulate files or trigger code execution
4. Leveraging the overwritten files or executed code to gain elevated privileges

Detection Methods for CVE-2020-15708

Indicators of Compromise

• Unexpected connections to the libvirt control socket from unauthorized user accounts
• File permission changes on /var/run/libvirt/ directory or socket files showing world-readable/writable permissions
• Suspicious file overwrites or modifications in system directories that correlate with libvirt socket activity
• Anomalous process spawning from libvirt-related services by non-administrative users

Detection Strategies

• Audit file permissions on libvirt sockets using ls -la /var/run/libvirt/ and flag any sockets with world read/write permissions
• Monitor auditd logs for unauthorized access attempts to libvirt socket files
• Implement file integrity monitoring (FIM) on critical system files to detect unauthorized overwrites
• Review system authentication logs for unusual local user activity preceding privilege escalation

Monitoring Recommendations

• Enable and monitor libvirt daemon logging to track socket connections and command execution
• Configure auditd rules to watch the /var/run/libvirt/ directory for access events
• Deploy endpoint detection solutions to monitor for suspicious process chains involving libvirt

How to Mitigate CVE-2020-15708

Immediate Actions Required

• Update libvirt packages to the patched version immediately using sudo apt update && sudo apt upgrade libvirt-daemon libvirt-daemon-system
• Verify socket permissions are correctly set after patching by checking /var/run/libvirt/ socket files
• Audit systems for signs of prior exploitation, including unauthorized file modifications
• Restrict local user access on affected systems until patches are applied

Patch Information

Canonical has released a security update to address this vulnerability. The fix is documented in Ubuntu Security Notice USN-4452-1. Users should apply the security update through the standard Ubuntu package management system. The patched packages correct the socket permissions to ensure only authorized users and groups can access the libvirt control socket.

Workarounds

• Manually correct socket permissions using chmod 660 on the affected libvirt socket files as a temporary measure
• Restrict access by ensuring only the libvirt group can access the socket directory
• Limit local user accounts on affected systems to reduce the attack surface until patching is complete
• Consider disabling libvirt services temporarily if virtualization functionality is not required

# Temporary permission fix (apply patch as permanent solution)
sudo chmod 660 /var/run/libvirt/libvirt-sock
sudo chown root:libvirt /var/run/libvirt/libvirt-sock

# Verify permissions are correct
ls -la /var/run/libvirt/libvirt-sock
# Expected output should show: srw-rw---- root libvirt