CVE-2024-46951 - Ghostscript Pattern Color Space Vulnerability Explained (With Exploit Details)

A serious security vulnerability has been discovered in Artifex Ghostscript—a popular open-source interpreter for PDF and PostScript files. This flaw, tracked as CVE-2024-46951, exists in the handling of color patterns within the Ghostscript codebase, specifically affecting versions before 10.04.. This vulnerability could allow an attacker to execute arbitrary code on your system, just by tricking you into processing a malicious PostScript or PDF file. In this article, we’ll break down what the bug is, show some code snippets, demonstrate how it could be exploited, and point you toward remediation and further information.

What Is Ghostscript and Why Should You Care?

Ghostscript is embedded in systems all over: it powers printing, file previews, and even web-based document viewers. Many backend services use Ghostscript quietly under the hood. If you process PDFs or PostScript on your machine, there’s a good chance Ghostscript is involved.

The Vulnerability In a Nutshell

The problem lies in psi/zcolor.c, the component of Ghostscript responsible for managing color spaces. Here, when the software parses a Pattern color space, it handles an internal pointer called “Implementation” without proper verification. An attacker can create a malicious file that tricks Ghostscript into dereferencing an arbitrary pointer, leading to memory corruption and—potentially—arbitrary code execution.

This means if someone gives you a specially crafted PDF or PostScript document, just opening or printing it could compromise your entire system!

Technical Details

Here’s a simplified code snippet to illustrate the issue. In the affected source file (zcolor.c), Ghostscript expects a properly defined Implementation pointer. But—before the fix—it didn’t check if this pointer was valid before using it.

Vulnerable code pattern (simplified for readability)

// In psi/zcolor.c (before 10.04.)
gs_imager_state *pis = cs->cimpl->Implementation;
if (pis->some_callback) {
    // Dangerous dereference without NULL/validation checks!
    pis->some_callback(...);
}

If Implementation is under attacker control (via a crafted pattern in a PostScript/PDF file), this could point anywhere in memory—allowing the attacker to hijack execution flow.

The patched version (starting in 10.04.) adds careful checks to make sure the pointer is safe before it’s used.

An attacker would

1. Create a malicious PostScript or PDF file that defines a Pattern color space, populating internal data structures with values that Ghostscript will later use unsafely.
2. The file would contain data crafted such that when Ghostscript processes it, the unchecked Implementation pointer is set to point to attacker-controlled memory.
3. The next time Ghostscript tries to use this pointer, it could execute arbitrary code supplied by the attacker.

Exploit skeleton pseudocode

% PostScript payload to trigger the bug (simplified)
<< /PatternType 1 
   /PaintType 1 
   /Implementation (some attacker-provided value) 
>> setcolorspace
% More malicious data or PostScript code goes here...

Note: The PostScript language gives a lot of low-level access, which is why Ghostscript vulnerabilities can be so dangerous.

Proof-of-Concept Demo

While a full exploit is out of scope (and ethically questionable to publish), in the wild, an exploit might look like this:

%!PS
% Malicious PS file to exploit CVE-2024-46951

% Fill up structures
<<
  /PatternType 1
  /PaintType 1
  /Implementation <DEADBEEFDEADBEEFDEADBEEFDEADBEEF>
>> setcolorspace

% At this point, Ghostscript will dereference the invalid Implementation pointer
% leading to memory corruption or crash—potential for RCE with crafted heap layout.

showpage

Opening this file in a vulnerable Ghostscript would, at minimum, crash; with further engineering, code execution is possible.

Is My System at Risk?

Are you running Ghostscript older than 10.04.?
Do you print, preview, or process PDFs, EPS, or PostScript from untrusted sources?
Does your server or webapp use Ghostscript behind the scenes (such as for file uploads or previews)?

How to Fix

Update Ghostscript NOW.
The developers have fixed this in Ghostscript 10.04.. Grab the latest version from the official download page or your package manager.

Disabling file formats: If you cannot upgrade, consider disabling support for dangerous file formats in your application, but this is not a long-term solution.

References

- Ghostscript Security Advisory (CVE-2024-46951)
- Ghostscript Release Notes – 10.04.
- Upstream Patch in psi/zcolor.c
- OSS-Security Mailing List Report

Conclusion

CVE-2024-46951 is a critical reminder that even well-maintained open-source projects can harbor dangerous bugs. Pattern color space handling in Ghostscript offered a path for attackers to run code just by getting their file opened. If Ghostscript is anywhere in your workflow, upgrade immediately. Don’t let a simple print job become a total security breach!

Stay safe. Patch early and often.

*This article was written exclusively to break down CVE-2024-46951 for readers who want to understand not just the fix, but how the bug happened and how attackers think.*

Timeline

Published on: 11/10/2024 21:15:14 UTC
Last modified on: 11/14/2024 02:13:25 UTC