CVE-2021-3468: Avahi Denial of Service (DoS) Vulnerability

CVE-2021-3468 Overview

A flaw was found in Avahi versions 0.6 up to 0.8 that allows a local attacker to trigger a denial of service condition. The event used to signal the termination of the client connection on the Avahi Unix socket is not correctly handled in the client_work function, allowing a local attacker to trigger an infinite loop. The highest threat from this vulnerability is to the availability of the Avahi service, which becomes unresponsive after this flaw is triggered.

Critical Impact

Local attackers can cause the Avahi mDNS/DNS-SD daemon to become completely unresponsive through an infinite loop, disrupting local network service discovery functionality.

Affected Products

• Avahi versions 0.6 through 0.8
• Debian Linux 9.0
• Linux distributions using vulnerable Avahi packages

Discovery Timeline

• 2021-06-02 - CVE-2021-3468 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3468

Vulnerability Analysis

This vulnerability is classified as CWE-835 (Loop with Unreachable Exit Condition), commonly known as an infinite loop vulnerability. The flaw exists in the way Avahi handles client connection termination events on its Unix socket interface.

When a client connection terminates, the client_work function is responsible for processing this event and cleaning up resources. However, due to improper event handling logic, certain termination scenarios cause the function to enter a loop that cannot exit under normal conditions. This results in the Avahi daemon consuming CPU resources indefinitely while becoming unable to process legitimate service discovery requests.

The local attack vector means an attacker must have local access to the system to exploit this vulnerability. The attack requires low privileges and no user interaction, making it straightforward to execute once local access is obtained.

Root Cause

The root cause lies in the improper handling of the Unix socket connection termination event within the client_work function. The code fails to correctly detect or respond to the client disconnection signal, leading to a condition where the loop continues executing without a valid exit path. This is a classic case of missing or incorrect loop termination logic when processing socket events.

Attack Vector

The attack is executed locally by a low-privileged user who can interact with the Avahi Unix socket. The attacker initiates a connection to the Avahi daemon and then terminates it in a specific manner that triggers the faulty event handling path. Once triggered, the Avahi service enters an infinite loop and becomes unresponsive to all subsequent requests.

The attack mechanism involves:

1. The attacker establishes a connection to the Avahi Unix socket
2. The connection is terminated in a way that triggers the vulnerable code path in client_work
3. The event signaling connection termination is mishandled
4. The daemon enters an infinite loop, consuming CPU and becoming unresponsive
5. All mDNS/DNS-SD service discovery functionality on the system is disrupted

Detection Methods for CVE-2021-3468

Indicators of Compromise

• Avahi daemon process (avahi-daemon) consuming unusually high CPU resources (near 100% of a single core)
• Avahi service becoming unresponsive to legitimate service discovery queries
• System logs showing Avahi service timeouts or connection failures
• Network service discovery (mDNS/Bonjour) functionality ceasing to work on the affected host

Detection Strategies

• Monitor Avahi daemon CPU usage and alert on sustained high utilization patterns
• Implement health checks that periodically query the Avahi service for responsiveness
• Use process monitoring tools to detect when Avahi enters an unresponsive state
• Check for abnormal connection patterns to the Avahi Unix socket

Monitoring Recommendations

• Configure system monitoring to track avahi-daemon process metrics including CPU, memory, and thread states
• Set up automated alerts for Avahi service health degradation or unresponsiveness
• Monitor system logs for Avahi-related error messages or service restart events
• Implement network-level monitoring for mDNS traffic anomalies that may indicate service disruption

How to Mitigate CVE-2021-3468

Immediate Actions Required

• Update Avahi to a patched version that addresses CVE-2021-3468
• Review and restrict local user access to systems running Avahi services
• Consider disabling Avahi if mDNS/DNS-SD functionality is not required
• Implement process monitoring to automatically restart Avahi if it becomes unresponsive

Patch Information

Security patches for this vulnerability are available through distribution package managers. Refer to the Red Hat Bug Report #1939614 for detailed tracking information. Debian users should consult the Debian LTS Advisory June 2022 and Debian LTS Advisory June 2023 for updated package information.

Workarounds

• Disable the Avahi daemon if mDNS/DNS-SD service discovery is not essential for system operation
• Restrict local user access to minimize the attack surface for this local-only vulnerability
• Implement systemd service watchdog functionality to automatically restart Avahi if it becomes unresponsive
• Use SELinux or AppArmor policies to further restrict Avahi daemon interactions

# Disable Avahi service if not required
sudo systemctl stop avahi-daemon
sudo systemctl disable avahi-daemon

# Or configure automatic restart on failure (systemd)
# Add to /etc/systemd/system/avahi-daemon.service.d/restart.conf
# [Service]
# Restart=on-failure
# RestartSec=5