CUPS: Unraveling a Critical Vulnerability Chain in Unix Printing Systems

A series of critical vulnerabilities has been uncovered in the Common Unix Printing System (CUPS), specifically in the cups-browsed component and related libraries. This vulnerability chain allows remote, unauthenticated attackers to potentially execute arbitrary code with root privileges on affected systems. The discovery highlights significant security risks in a widely-used open-source component and raises crucial questions about legacy system support and security in modern IT environments.

What is CUPS?

Common UNIX Printing System (CUPS) is an open source printing system that allows a system to act as a print server. It manages print jobs and queues and provides support for printing via the Internet Printing Protocol (IPP), making it a widely used printing service on UNIX-like operating systems, including Linux and macOS

TL;DR

On September 26 Simone Margaritelli, a security researcher, published a blog post that sums up the vulnerabilities. According to Margaritelli, a remote unauthenticated attacker can exploit vulnerabilities in printer systems to replace or install new IPP (Internet Printing Protocol) URLs. This allows the attacker to execute arbitrary commands on the target computer when a print job is initiated, without the user’s knowledge. Margaritelli noted that there are some conditions for this vulnerability to be exploited.

• The cups-browsed service has manually been enabled or started.
• An attacker has access to a vulnerable server, which:
  • Allows unrestricted access, such as the public internet, or
  • Gains access to an internal network where local connections are trusted
• Attacker advertises a malicious IPP server, thereby provisioning a malicious printer
• A potential victim attempts to print from the malicious device
• Attacker executes arbitrary code on victim’s machine

Key Vulnerabilities

• CVE-2024-47176 (CVSS 9.9):
  • Affects: cups-browsed <= 2.0.1
  • Issue: Binds to UDP INADDR_ANY:631, accepting packets from any source
  • Impact: Allows initial access and connection to attacker-controlled IPP servers
• CVE-2024-47076:
  • Affects: libcupsfilters <= 2.1b1
  • Issue: Lack of validation for IPP attributes from external servers
  • Impact: Enables injection of attacker-controlled data into the CUPS system
• CVE-2024-47175:
  • Affects: libppd <= 2.1b1
  • Issue: Insufficient sanitization when writing IPP attributes to PPD files
  • Impact: Allows injection of malicious content into PPD files
• CVE-2024-47177:
  • Affects: cups-filters <= 2.0.1
  • Issue: Arbitrary command execution via FoomaticRIPCommandLine PPD parameter
  • Impact: Enables remote code execution with root privileges

Attack Chain Breakdown

1. Initial Access:
   • Attacker sends a crafted UDP packet to port 631
   • CUPS-browsed processes the packet and connects to a malicious IPP server
2. Data Injection:
   • Malicious server returns crafted printer attributes
   • libcupsfilters processes these attributes without proper sanitization
3. PPD File Manipulation:
   • libppd generates a PPD file with injected malicious content
   • Includes dangerous FoomaticRIPCommandLine directive
4. Code Execution:
   • When a print job is processed, foomatic-rip executes the injected commands
   • Results in arbitrary code execution with root privileges

Impact Assessment

• Affected Systems: Most Linux distributions, some BSDs, potentially ChromeOS and Oracle Solaris
• Exposure: Hundreds of thousands of internet-exposed devices potentially vulnerable
• Severity: Unauthenticated remote code execution with root privileges

Mitigation Strategies

• Disable and remove cups-browsed if not essential
• Update all CUPS-related packages to latest versions
• Implement strict UDP filtering for port 631
• Restrict DNS-SD (Service Discovery) traffic

Container Environments and Cloud-Native Applications

It’s important to note that while this vulnerability chain is severe for affected systems, its impact may be limited in modern containerized and cloud-native environments. Most container images, especially those built on minimal base images or following security best practices, typically do not include the CUPS printing system or the vulnerable CUPS-browsed service. Similarly, many cloud-native applications and microservices architectures don’t rely on traditional printing services. This means that a significant portion of modern, containerized workloads are likely not directly affected by this vulnerability chain. However, CISOs and security teams should still conduct thorough assessments, as legacy applications, development environments, or specific use-cases might still incorporate CUPS, potentially introducing risk into otherwise secure container ecosystems.

How ARMO can help

In light of vulnerabilities like the CUPS exploit chain, ARMO Platform offers critical advantages for Cloud and Kubernetes environments. Our Vulnerability “in use” feature alerts on vulnerabilities which are in components actually loaded into memory, filtering out issues in dormant CUPS libraries. This approach allows security teams to prioritize real, active threats efficiently. Furthermore, ARMO Platform’s threat detection and response provide an additional layer of defense. By continuously monitoring for anomalies such as unexpected process launches or unusual network activity, ARMO  identifies potential exploitation attempts of vulnerabilities like those in CUPS, even in scenarios where the vulnerable component unexpectedly exists in the environment. This multi-faceted approach – combining smart vulnerability filtering with advanced behavioral analysis – ensures that organizations can maintain a robust security posture against both known vulnerabilities and novel attack vectors in their Kubernetes ecosystems.

Using the ARMO’s SBOM view one can search for the CUPS component to see if it even exists in the environment.