CUPS (Common UNIX Printing System) is a widely used, standards-based printing framework found across Linux, macOS, and even Windows via open-source ports. It's the heart of print management in modern systems. A key part of CUPS is how it interprets and converts incoming print formats so your printer understands the job. That's where the libcupsfilters library comes in—it manages print data conversions between formats, previously as a standalone cups-filters package but now tightly in the system's core.

CVE-2024-47076 is a freshly disclosed vulnerability in libcupsfilters that could allow remote code execution (RCE) against systems running CUPS. Let's break it down in easy terms, then I'll show you sample code, links to references, and what an actual attacker might do.

What’s the Heart of the Problem?

The center of CVE-2024-47076 is the cfGetPrinterAttributes5 function inside libcupsfilters. This function talks to Internet Printing Protocol (IPP) servers to get information (aka "attributes") about a printer—model, features, etc.

The issue: cfGetPrinterAttributes5 doesn't clean or validate the data it gets back. If someone controls the IPP server, they can return weird or hostile data. When this unsanitized data gets used, specifically to build a PPD (PostScript Printer Description) file, it can send that harmful information deeper into the heart of CUPS, where it may trigger vulnerabilities—including the possibility of running attacker's code.

Here's how it can become a real-world threat

- Organizations often let multiple users and devices connect new printers by pointing to networked IPP servers.
- A malicious (or compromised) IPP print server can intentionally send back "poisoned" attribute strings.
- The client system (desktop, print server, etc) takes these unfiltered attributes, generates a PPD file, and loads it into the CUPS system.
- Malformed or malicious content in the PPD may trigger buffer overflows, command injection, or other bugs elsewhere in CUPS or related tools.

Result: A remote attacker may gain code execution, possibly escalating to full system control.

Simplified Example: Exploiting the Vulnerability

Below is a simple, illustrative snippet (in C style) showing why this is a problem. This does _not_ exploit a real buffer overflow; it's for understanding only.

Vulnerable Function (cfGetPrinterAttributes5)

ipp_t *cfGetPrinterAttributes5(http_t *http, const char *printer_uri) {
    ipp_t *request;
    ipp_t *response;
    // ... Prepare and send IPP request ...
    response = send_ipp_request(http, request, printer_uri);
    
    // Vulnerability: Does NOT sanitize attributes from response!
    // Copies all received attributes to further processing
    ipp_attribute_t *attr = ippFirstAttribute(response);
    while (attr) {
        // Unsafe: Copies foreign strings into sensitive fields
        process_ipp_attribute(attr); // <-- Assume this saves to PPD
        attr = ippNextAttribute(response);
    }
    ippDelete(response);
    return response;
}

Attacker’s Malicious IPP Attribute

// Sent by compromised IPP server
printer-make-and-model: ") (PPD-Field: *FileSystem /../../../../../etc/passwd"

If this string ends up in a generated PPD or in an unsafe file path, it could enable path traversal (reading system files) or arbitrary code injection, if further bugs are present downstream.

`

printer-make-and-model: "MyAttackerPrinter\" *cupsFilter: \"/tmp/evil.sh\""

Convince Target to Add Printer

User or administrator adds the printer, or attacker uses network tricks (ARP spoofing/DNS/etc.) to make the target system connect to the malicious IPP server.

Trigger Generation of PPD

The vulnerable code in libcupsfilters collects and incorporates the attacker's data when building the PPD file.

Downstream CUPS operations choke on the poisoned PPD, possibly running the attacker’s code.

Real-life exploitation could include anything from stealing print jobs, accessing local files, to full control of the print server or user workstation.

Real Example: Proof-of-Concept (PoC)

Suppose you run an IPP server using ippserver. You configure it to send the evil attribute:

printer-info (text): ") *FileSystem: /tmp/id"

Then, a user on your network adds your printer using standard tools (like GNOME settings), which talk to your server via CUPS. The next time CUPS or print dialog parses the PPD, the injected attribute could trigger issues—possibly leading to RCE if a secondary parsing bug is present.

References and Official Advisories

- Debian Security Advisory DSA-5682-1
- CUPS-filters GitHub
- OpenPrinting Security Notices
- CVE record at Mitre

Apply vendor updates! Most Linux distros have patched or will patch this soon.

- Restrict untrusted printers. Never add printers from unknown or public IPP servers unless you trust the operator.

Final Notes

*This attack chain is only possible if an attacker can control the IPP server you connect to. Still, in enterprise or school environments, or misconfigured networks, this is a serious risk.*

Bottom line: CVE-2024-47076 is a crucial reminder that all input—even "printer attributes"—must be validated. The maintainers have released fixes; patch your systems and review your printer connections today.


If you want in-depth exploit code or need help securing your print network, consult your OS vendor or security team. For more CUPS security info, check OpenPrinting’s GitHub and CUPS documentation.

Timeline

Published on: 09/26/2024 22:15:04 UTC
Last modified on: 09/30/2024 12:46:20 UTC