In April 2023, a critical security vulnerability was found in FreeType, one of the world's most popular open-source font rendering libraries. This vulnerability, identified as CVE-2023-2004, involves an integer overflow bug in the function tt_hvadvance_adjust() (located in src/truetype/ttgxvar.c). If exploited, this could allow an attacker to execute arbitrary code simply by getting a target to open or process a malicious font file. This long-read post breaks down what happened, why it’s dangerous, and how you can protect your systems.

Let’s make sense of it all in simple, straightforward language.

What is FreeType, and Why Should You Care?

FreeType is a widely used software library for font rendering. It’s present in tons of applications, operating systems, devices, and even major web browsers. Nearly every Linux desktop and various embedded devices rely on FreeType to handle fonts. When a bug appears in FreeType, it has a huge potential impact.

CVE-2023-2004 is an *integer overflow vulnerability* in the tt_hvadvance_adjust() function.

- It affects versions of FreeType before the patched release in April 2023 (v2.13.).

If an attacker crafts a *malicious font file*, they can trigger the overflow. This can lead to memory corruption and potentially allow arbitrary code execution.

Technical Details: How the Vulnerability Works

The bug was introduced by not properly checking for integer overflow during certain calculations involving horizontal or vertical advance adjustments in TrueType GX Font Variations.

Here is a simplified code snippet highlighting the problematic logic in tt_hvadvance_adjust()

// This is not the real code, but demonstrates the logic vulnerability
static void tt_hvadvance_adjust(/* args */) {
    FT_Long adj = axis->mmvar->num_axis * axis->n_points;
    
    // Wrong: adj * sizeof(FT_Long) could overflow
    FT_Long* values = (FT_Long*)malloc(adj * sizeof(FT_Long));
    if (!values)
        return;
    // ...followed by writing to values[..adj] without proper bounds checks
}

The main problem is when adj * sizeof(FT_Long) results in an integer greater than the maximum allowed by the system (SIZE_MAX), causing an integer overflow and leading to a buffer that's too small for the data written to it. This opens the door to out-of-bounds writes.

Attack Scenario

1. Preparation: The attacker creates a malicious font file that forces FreeType to go through the vulnerable code path, causing adj to be very large and triggering the overflow.
2. Delivery: The victim is tricked into opening/viewing the font (e.g., on a website, PDF, or through an image viewer).
3. Execution: FreeType, running in the application, allocates a too-small buffer due to the overflow, causing subsequent writes to corrupt memory. This can crash the app and potentially run attacker-controlled code.

Example Exploit (Conceptual)

While a full ready-to-use exploit isn’t available to the public, the general steps are as follows (“pseudo-exploit”):

# Pseudocode for generating a font file that triggers the bug
from fontTools.ttLib import TTFont

# Open a normal font, tweak the GX variation axes data structures
font = TTFont('legit.ttf')
# Create way too many variation axes
font['fvar'].axes = [{"tag": "wght", "minValue": , "defaultValue": 1, "maxValue": 2} for _ in range(x10000)]
font.save('malicious.ttf')
# Now, when loaded with a vulnerable FreeType, this can overflow 'tt_hvadvance_adjust'

Important: The actual exploit will be more sophisticated, dealing with system memory layouts, ASLR, and app sandboxing. But the core is to create a font that overflows the integer math.

Original References and Patch

- Upstream FreeType fix and discussion
- CVE Details page
- FreeType Changelog mentioning the fix

Patch Summary

The patch adds proper checks to make sure the integer multiplication does not overflow before allocation:

if (adj >  && adj <= (SIZE_MAX / sizeof (FT_Long))) {
    FT_Long* values = (FT_Long*)malloc(adj * sizeof(FT_Long));
    /* ... */
}

This ensures the buffer size calculation is safe and won’t wrap around.

Affected

- All software (and users) running FreeType before v2.13. (April 2023) that processes untrusted fonts.
- This includes desktop apps, browsers, Linux distros, embedded systems, game engines, and any software using the old FreeType.

Check your OS for security updates — *most major Linux distributions patched this quickly*.

- For example: sudo apt upgrade libfreetype6 (Debian/Ubuntu)

Be cautious with files from the Internet (PDFs, documents, zips with fonts).

- Use browser/application sandboxing features when possible.

Conclusion

CVE-2023-2004 is a serious font parsing vulnerability lurking in a cornerstone of graphical Linux and cross-platform software – FreeType. Thanks to the open-source community, it was patched before widespread exploitation. However, given the prevalence of the library and the ease with which users open files containing fonts (think PDFs, websites, even apps), *patching quickly is critical*.

Further Reading

- FreeType Homepage & Downloads
- Original Bug Report
- A breakdown of font vulnerabilities


*Copyright © 2024 - All content written for this exclusive post. Please credit if quoting.*

Timeline

Published on: 04/14/2023 21:15:00 UTC
Last modified on: 05/02/2023 03:15:00 UTC