Security vulnerabilities in widely used frameworks like Qt can have far-reaching impact. In this post, we’ll break down CVE-2023-32763—a real buffer overflow flaw affecting Qt’s handling of SVG files, leading to potential memory corruption and code execution.

Qt is a popular C++ framework used for building cross-platform apps with rich UIs. Many applications use its SVG support for scalable graphics. If you use or develop Qt-based apps, especially if they load SVG files from untrusted sources, you need to know about this bug.

> Versions Affected

> - All Qt before 5.15.15
> - Qt 6.x before 6.2.9
> - Qt 6.3.x through 6.5.x before 6.5.1

What is CVE-2023-32763?

This vulnerability lets attackers cause a buffer overflow by crafting SVG files that use embedded images. When such an SVG is rendered, a bug in the QTextLayout class can cause Qt to write data past the end of a memory buffer.

A successful exploit can crash applications, corrupt data, or even run attacker code—potentially leading to a full system compromise.

Where’s the Bug?

The bug lies deep in Qt’s SVG rendering pipeline. Specifically, when Qt parses an SVG and encounters certain nodes, such as those containing <image> elements, it uses the QTextLayout class to prepare and render text. If the SVG is crafted just right, the code mishandles a buffer size calculation—causing a classic buffer overflow.

Here’s a snippet (not the actual vulnerable code, but to illustrate)

// Pseudocode example showing potential mishandling
QTextLayout layout(someInputString);
layout.beginLayout();
for (int i = ; i < userDefinedCount; ++i) {
    layout.createLine();
    // If userDefinedCount is much larger than expected,
    // underlying buffer may be overrun!
}
layout.endLayout();

When loading SVG files, the SVG image element can be abused to influence internal font metrics, causing an unsafe state.

Potentially run unchecked code, depending on memory layout and exploit ingenuity

If you use a Qt-based app that loads SVGs from the web or from files (think image viewers, chat apps, design tools), you’re at risk.

Let’s walk through a hypothetical attack

1. Attacker crafts an SVG file with an embedded <image> tag and malicious font/text properties, causing Qt’s buffer allocation to underrun.

The target app’s SVG handler parses and attempts to render the image.

4. The app crashes or, with deeper knowledge of memory layout, the attacker can further manipulate the program.

Here’s a totally simplified SVG snippet that tries to hit the bug (adapt to your app for research only):

<svg width="400" height="110">
  <image href="data:image/png;base64,iVBORwKGgoAAAANSUhEUg..." x="" y="" height="110" width="400"/>
  <text x="" y="15" font-size="9999999999999999999">Hello overflow!</text>
</svg>

Note: The actual exploit may require tuning font-size, layout elements, and so on to trigger the exact buffer mismanagement.

References and Patch

Official advisory:  
- QTBUG-113574 – Buffer overflow via SVG
- NVD entry CVE-2023-32763
- Qt patch discussion

Qt 6.5.1

Patch summary: The fix ensures that the buffer size calculation is correct and untrusted SVG-specified values cannot lead to overflows.

Conclusion

CVE-2023-32763 is a critical vulnerability affecting SVG rendering in Qt. Even if you don’t use SVGs directly, many libraries and apps do so behind the scenes. Update now, audit your usage, and keep an eye out for suspicious SVG files in the wild.

Stay safe, and keep your dependencies patched!

*This post was written exclusively for you. Stay tuned for more security breakdowns!*

Timeline

Published on: 05/28/2023 23:15:00 UTC
Last modified on: 06/03/2023 03:58:00 UTC