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TuxCare BlogCUPS Vulnerabilities: Mitigating Remote Code Execution Risks
by Rohan Timalsina
October 20, 2024 - TuxCare expert team
The Common UNIX Printing System (CUPS) is a widely used open-source printing system, prevalent on Linux and UNIX-like operating systems such as FreeBSD, NetBSD, and OpenBSD. While CUPS provides essential printing services, recent discoveries have highlighted critical vulnerabilities that could allow attackers to exploit systems through remote code execution (RCE) under certain conditions.
Understanding CUPS Vulnerabilities
Several security vulnerabilities were recently discovered in multiple components of the CUPS system. These vulnerabilities are tracked as:
- CVE-2-24-47076 (libcupsfilters)
- CVE-2024-47175 (libppd)
- CVE-2024-47176 (cups-browsed)
- CVE-2024-47177 (cups-filters)
Discovered by security researcher Simone Margaritelli, these flaws present a significant risk for remote code execution if attackers can exploit them under specific conditions. However, it’s important to note that systems are not vulnerable in their default configurations. Attackers would need to chain these flaws together to execute arbitrary code remotely on vulnerable machines.
One of the critical components implicated in these vulnerabilities is the cups-browsed daemon. This daemon is responsible for searching the local network for advertised network or shared printers and making them available to users on a machine. Similar to how Windows and macOS locate network printers, cups-browsed enables seamless printer discovery in Linux environments.
However, this functionality comes with risks. When enabled, cups-browsed listens on UDP port 631, allowing remote devices on the network to connect and create new printers. In most configurations, cups-browsed is disabled by default, but if an administrator has enabled it, the system becomes vulnerable to attack.
Remote Code Execution: How Does it Work?
The remote code execution (RCE) chain in CUPS involves several steps:
Enabling cups-browsed: For the RCE vulnerability to be exploited, the cups-browsed daemon must be active on the target system. By default, this service is disabled, reducing the risk of exploitation.
Network Exposure: If cups-browsed is enabled, the system will listen for network printers on UDP port 631, making it vulnerable to external connections from malicious devices on the same network.
Creating Malicious Printers: Attackers can advertise a malicious printer that includes harmful commands in its PPD file.
User Interaction: The final step requires a user on the vulnerable machine to print from the compromised printer, triggering the execution of the malicious command.
Mitigation Measures: Breaking the Exploit Chain
Popular Linux distributions, including Ubuntu and Debian have recently addressed the CUPS vulnerabilities CVE-2024-47175 and CVE-2024-47176 through security updates. For systems that haven’t yet applied these patches, Red Hat and other vendors recommend disabling the cups-browsed service to prevent it from running and exposing the system to network-based attacks.
Here’s how administrators can stop the cups-browsed service and ensure it doesn’t start on reboot:
sudo systemctl stop cups-browsed
sudo systemctl disable cups-browsed
Additionally, administrators can check whether cups-browsed is currently running by using the following command:
sudo systemctl status cups-browsed
Conclusion
While CUPS vulnerabilities present a significant risk in specific configurations, systems with default settings are generally not a risk. The most concerning issue, remote execution, requires multiple steps to exploit, including user interaction with a malicious printer.
For administrators, the key takeaway is to review their systems’ configurations, disable cups-browsed where it isn’t needed, and stay alert for patches that address these vulnerabilities. By taking these steps, the risks of CUPS flaws being exploited for remote code execution can be minimized.
The sources for this article include a story from BleepingComputer.
