CVE-2020-1712: Systemd Use-After-Free Vulnerability

CVE-2020-1712 Overview

A heap use-after-free vulnerability was discovered in systemd before version v245-rc1, occurring when asynchronous Polkit queries are performed while handling D-Bus messages. This memory corruption flaw exists in the way systemd processes authentication requests through its D-Bus interface, where freed memory can be accessed during the handling of specially crafted messages.

Critical Impact
A local unprivileged attacker can exploit this vulnerability to crash systemd services or potentially execute arbitrary code with elevated privileges, leading to complete system compromise.

Affected Products

systemd_project systemd (versions before v245-rc1)
redhat enterprise_linux 8.0
redhat openshift_container_platform 4.0
redhat ceph_storage 4.0
redhat discovery
redhat migration_toolkit 1.0
debian debian_linux 9.0

Discovery Timeline

2020-02-05 - Vulnerability disclosed on Openwall OSS Security mailing list
2020-03-31 - CVE CVE-2020-1712 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-1712

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) affects the systemd init system, which is responsible for bootstrapping the user space and managing system processes on most modern Linux distributions. The flaw resides in how systemd handles asynchronous Polkit authorization queries during D-Bus message processing.

When systemd services receive D-Bus messages that require PolicyKit authorization, they perform asynchronous queries to the Polkit daemon. During this asynchronous operation, if the original D-Bus message or its associated data structures are freed prematurely while the Polkit query is still pending, a use-after-free condition occurs when the callback attempts to access the already-freed memory.

The vulnerability requires local access to exploit, meaning an attacker must have some level of access to the target system. However, no elevated privileges are required to trigger the flaw, making it exploitable by any local user account. Successful exploitation can result in denial of service through crashing systemd services, or potentially arbitrary code execution if the attacker can control the contents of the freed memory region.

Root Cause

The root cause lies in improper memory lifecycle management within systemd's D-Bus message handling code when interacting with Polkit for authorization decisions. The asynchronous nature of Polkit queries creates a race condition where the memory backing a D-Bus message can be freed before the Polkit callback completes. The callback then operates on a dangling pointer, leading to heap corruption and potential code execution.

Attack Vector

The attack vector is local, requiring an authenticated user to send specially crafted D-Bus messages to vulnerable systemd services. The attacker constructs D-Bus messages that trigger Polkit authorization queries and times the attack to cause the use-after-free condition. By carefully manipulating heap memory allocation patterns, an attacker may be able to replace the freed memory with attacker-controlled data, potentially achieving code execution with the privileges of the targeted systemd service.

The exploitation mechanism involves:

Identifying a systemd service that performs asynchronous Polkit queries
Sending crafted D-Bus messages to trigger the vulnerable code path
Timing message handling to cause premature memory deallocation
Optionally performing heap manipulation to control freed memory contents
Triggering the callback to access attacker-controlled data

Detection Methods for CVE-2020-1712

Indicators of Compromise

Unexpected crashes of systemd services, particularly those interfacing with D-Bus and Polkit
Unusual D-Bus message activity from non-privileged user accounts targeting systemd services
Core dumps from systemd-related processes showing heap corruption signatures
Abnormal system behavior following D-Bus communication anomalies

Detection Strategies

Monitor system logs for segmentation faults or SIGABRT signals from systemd services
Implement D-Bus message auditing to detect unusual patterns of authorization requests
Deploy memory corruption detection tools such as AddressSanitizer in development/testing environments
Use kernel-level monitoring to detect suspicious privilege escalation attempts following systemd service crashes

Monitoring Recommendations

Configure systemd journal to capture detailed crash information for forensic analysis
Implement centralized log collection for all systemd service events across the infrastructure
Set up alerts for repeated systemd service restarts that may indicate exploitation attempts
Monitor for unexpected privilege changes following systemd-related system events

How to Mitigate CVE-2020-1712

Immediate Actions Required

Update systemd to version v245-rc1 or later, which contains the security fixes
Apply vendor-specific patches from Red Hat, Debian, or other distribution maintainers
Restrict local user access where possible to reduce the attack surface
Monitor systemd services for crashes or unusual behavior as an interim measure

Patch Information

The systemd project has released multiple commits to address this vulnerability:

Primary security fix commit - Core fix for the use-after-free issue
Security enhancement commit - Additional hardening measures
Caching issue fix - Related caching improvements
Fallback issue fix - Fallback mechanism corrections

For Red Hat Enterprise Linux and related distributions, consult the Red Hat Bugzilla entry for CVE-2020-1712 for specific patch information. Debian users should reference the Debian LTS Security Announcement for applicable updates.

Workarounds

Limit local user access to systems running vulnerable systemd versions
Implement mandatory access control (SELinux/AppArmor) policies to restrict D-Bus communications
Monitor and audit D-Bus traffic for suspicious authorization request patterns
Consider isolating critical workloads until patches can be applied

bash

# Check current systemd version
systemctl --version

# For Red Hat-based systems, update systemd
sudo yum update systemd

# For Debian-based systems, update systemd
sudo apt-get update && sudo apt-get upgrade systemd

# Verify the update was applied
systemctl --version