In January 2022, a critical vulnerability known as CVE-2022-0137 was reported in HTMLDOC, a popular open-source program for converting HTML files and web pages to PDF or PostScript. This vulnerability allows attackers to write data *outside* the boundaries of a memory buffer—a serious issue called a *heap buffer overflow*. It is specifically found in the image_set_mask function before version 1.9.15.

In this post, I’ll break down what went wrong, show you some code, explain the exploitability, and point you to resources for protecting your systems.

What is a Heap Buffer Overflow?

A heap buffer overflow happens when a program stores more data in a memory area (buffer) than the buffer can handle. If exploited, this can let attackers overwrite important data, crash the program, or even run their own code on the system.

Where's the Problem? (The image_set_mask Function)

The issue lives in the source file image.cxx, inside the image_set_mask function, which is used to apply image masks when processing images.

In vulnerable versions (before 1.9.15), the code allocates a buffer for holding the mask, but fails to properly check if the size is big enough for the data written into it. This lets an attacker craft a malicious image or HTML file, which, when opened by HTMLDOC, overflows the buffer and writes into the heap.

Here's a simplified code snippet inspired by the bug

unsigned char *mask = (unsigned char *)malloc(mask_size);
//...
for (int i = ; i < bits; i++)
{
    int offset = /* computation based on image and mask properties */;
    mask[offset] = data[i]; // No proper bounds check on 'offset'
}

If offset becomes too large (due to crafted input), you get a buffer overflow problem.

Now, let’s see how this could be exploited.

Suppose an attacker crafts an image or HTML file with a mask table that, when processed, sets an offset value outside the memory range of mask. The attacker’s data can be inserted before or after the buffer, leading to:

Crashes (denial of service)

- Possible execution of arbitrary code (if heap layout is favorable and additional vulnerabilities exist)

Example Exploit Input (Conceptual)

Imagine an HTML file with a malformed image with a specially-created mask (payload omitted for safety):

<img src="evil.png" mask="crafted-bitmap-to-overflow" />

When HTMLDOC tries to handle this, it allocates the buffer, the offset jumps beyond the end, and boom—overflow.

Note: Actual reliable code execution might require ASLR bypass or chaining with other bugs. But crashing is easy.

Here's a minimal Python script to generate a problematic PNG file (not full exploit, just a crash)

from PIL import Image

img = Image.new('1', (1, 10000))  # Very tall image, triggers large mask
img.save('boom.png')

Saving this as boom.png and converting with a vulnerable HTMLDOC

htmldoc --webpage boom.png

Could trigger a crash, demonstrating the vulnerability.

Fix Status

The vulnerability was fixed in HTMLDOC version 1.9.15 by adding proper bounds checking in the mask-handling code.

Fix snippet (simplified)

if (offset < mask_size) {
    mask[offset] = data[i];
}
// Else: ignore, avoid overflow

References

- CVE-2022-0137 Detail
- HTMLDOC Official Website
- Fix Commit on GitHub
- Debian Security Advisory

Conclusion

CVE-2022-0137 is a prime example of how input validation can make or break the security of even widely-used open source tools. Timely patching and cautious handling of input files are your first lines of defense!

If you want to dig deeper, check out the original commit fixing the bug or read the NVD entry for the official documentation.

Timeline

Published on: 11/14/2022 18:15:00 UTC
Last modified on: 02/02/2023 18:31:00 UTC