Bash SIP Exploit: Prevention & Response

TL;DR

This guide explains how a Session Initiation Protocol (SIP) exploit can be used to run commands on your server via the Bash shell, and provides steps to prevent it and respond if you’ve been compromised. It focuses on identifying vulnerable systems and mitigating the risk.

Understanding the SIP Exploit

The vulnerability arises from how some SIP servers handle specially crafted requests containing commands that are then executed by the Bash shell. This is often due to improper input validation or sanitisation.

Step 1: Identify Potentially Vulnerable Systems

1. Check your SIP server software version: Determine which SIP server you’re running (e.g., Asterisk, Kamailio, FreeSWITCH).
2. Search for known vulnerabilities: Use a vulnerability database like the National Vulnerability Database (https://nvd.nist.gov) or CVE details (https://www.cvedetails.com/) to see if your version is affected by any SIP exploits related to Bash command injection.
3. Look for outdated software: Older versions of SIP servers are more likely to have unpatched vulnerabilities.

Step 2: Prevention – Secure Your SIP Server

1. Update your SIP server: This is the most important step! Install the latest security patches from the vendor.
2. Disable unnecessary features: If you don’t need certain SIP features, disable them to reduce the attack surface.
3. Input Validation and Sanitisation: Ensure that all input received by your SIP server is properly validated and sanitised before being used in any commands or scripts. This prevents malicious code from being injected.
4. Restrict access: Limit access to your SIP server only to trusted networks and users. Use firewalls and access control lists (ACLs).
5. Regular Security Audits: Conduct regular security audits of your SIP server configuration and logs.

Step 3: Detecting a Compromise

1. Monitor System Logs: Regularly review system logs for unusual activity, such as unexpected processes or commands being executed. Look specifically at logs related to the SIP server.
2. Check Process Activity: Use tools like top or ps to monitor running processes and identify any suspicious ones.

ps aux | grep bash

3. Review Network Traffic: Analyse network traffic for unusual patterns, such as unexpected connections or large data transfers.
4. File Integrity Monitoring (FIM): Implement FIM to detect changes to critical system files.

Step 4: Responding to a Compromise

1. Isolate the System: Immediately disconnect the compromised server from the network to prevent further damage.
2. Identify the Entry Point: Determine how the attacker gained access (in this case, likely through the SIP exploit). Analyse logs and traffic for clues.
3. Contain the Damage: Identify any files that were modified or created by the attacker and remove them.
4. Restore from Backup: If possible, restore the system from a known good backup. Ensure the backup is clean before restoring.
5. Rebuild the System (Recommended): For critical systems, it’s often best to rebuild the server from scratch to ensure complete removal of any malicious code.
6. Change Passwords: Change all passwords associated with the compromised system and any related accounts.
7. Post-Incident Analysis: Conduct a thorough post-incident analysis to identify the root cause of the compromise and prevent similar incidents in the future.

Step 5: Example Log Analysis (Asterisk)

Check Asterisk’s full log file for suspicious commands being executed. The location varies depending on your configuration, but it is often found at /var/log/asterisk/full.

grep -i 'bash' /var/log/asterisk/full

Look for lines containing commands like (command=...) or similar patterns that indicate command execution within SIP requests. Investigate any unexpected or unauthorized commands found in the logs.