Vulnerabilities Discovered in Common UNIX Printing System (CUPS) Can Enable DDoS Attacks

Recent vulnerabilities in the Common UNIX Printing System (CUPS) could allow attackers to access sensitive data or execute malicious print jobs. More alarmingly, over 198,000 CUPS servers were found exposed online, 34% of which (around 58,000 systems) are exploitable for Distributed Denial of Service (DDoS) attacks. These vulnerabilities also pose a risk of remote code execution (RCE) attacks.

What is especially concerning about these vulnerabilities is that minimal resources are required to initiate a successful attack. It is estimated that within mere seconds, every vulnerable CUPS service currently exposed on the internet could be co-opted by an attacker, with costs amounting to less than a single U.S. cent on modern hyperscale platforms. Major vendors, including Canonical and Red Hat, have promptly issued security patches to address the vulnerabilities. However, many systems are still unpatched or running outdated versions of CUPS, making them easy targets.

Dangers of the vulnerabilities

Successful attacks could allow unauthorized users access to sensitive data, disrupt printing services through denial-of-service, or turn the compromised system into a platform for launching further attacks across the network:

• Malicious print jobs: Attackers can craft print jobs containing malicious code that, when processed, may enable a full system compromise.
• Untrusted printer discovery: An attacker can exploit the CUPS-browsed service to send malicious packets from a fake print server, leading to harmful code execution.
• Insecure temporary files: Attackers can inject malicious code into temporary files created during printer description handling, potentially gaining unauthorized system access.

Our guidance

Organizations should take the following actions in response to the DDoS amplification risk and other vulnerabilities:

• Patch immediately: Apply the latest CUPS updates as soon as they are released by your Linux distribution vendor. Check repositories regularly for updates.
• Limit exposure: Disable the cups-browsed service if not required for automatic printer discovery, which reduces unnecessary attack surfaces.
• Firewall protection: Ensure that your firewall only permits trusted devices to communicate with the CUPS server (UDP port 631). Restricting access can help prevent unauthorized traffic from exploiting the vulnerabilities.
• Monitor for inconsistency: Regularly review CUPS logs for unusual or suspicious activity, especially unexpected traffic patterns that could indicate DDoS attempts or other attacks.
• Strengthen network defenses: Enhance your network’s ability to detect and mitigate DDoS attacks. Pay particular attention to traffic amplification patterns that could indicate a misconfigured or vulnerable CUPS server being leveraged.

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