CVE-2023-2004 (Common Vulnerabilities and Exposures) is an integer overflow vulnerability in a core component of the open-source font rendering library, Freetype. Specifically, the vulnerability resides in the tt_hvadvance_adjust() function of the "src/truetype/ttgxvar.c" file. This article will take an in-depth look at the vulnerability, providing code snippets, links to original references, and exploit details. We will explore how an attacker could potentially take advantage of this vulnerability and discuss ways to mitigate its impact.

1. Overview of Freetype and tt_hvadvance_adjust()

Freetype (https://www.freetype.org/) is a widely used, open-source software library designed for rendering fonts. It is written in C and provides a clean, efficient, and high-quality way to render text on various platforms, including desktop, mobile, and embedded systems.

The tt_hvadvance_adjust() function is a part of the Freetype library and is responsible for adjusting the horizontal and vertical advance widths in TrueType's font rendering process. The source code for the vulnerable function can be found here:

Source file: https://git.savannah.gnu.org/cgit/freetype/freetype2.git/tree/src/truetype/ttgxvar.c

This vulnerability arises due to incorrect calculations when computing the advance widths in certain scenarios, which leads to an integer overflow.

2. Vulnerability Details

The vulnerability specifically occurs when the library is passed a maliciously crafted font file containing an oversized value. When the tt_hvadvance_adjust() function processes this value, it results in an integer overflow. The relevant code snippet from the function is:

/* ttgxvar.c */
void tt_hvadvance_adjust( TT_Face... )
{
  //...

  HVadvance[Index] = (FT_Int16)(HVadvance[Index] +
                                 FT_MulFix( variance, delta )
                                );
  //...
}

This is where the integer overflow can happen. When the value of variance * delta is too large for a signed 16-bit integer, it wraps around, causing an overflow.

This integer overflow can allow an attacker to manipulate the advance width values arbitrarily, potentially leading to memory corruption, application crashes, or even remote code execution.

3. Exploiting the Vulnerability

To exploit this vulnerability, an attacker would have to create a specially crafted font file containing oversized values that cause the integer overflow. They would then need to trick a user into loading the malicious font file, either through web content or an application that uses the Freetype library.

Once the font file is loaded and rendered, the integer overflow occurs within the tt_hvadvance_adjust() function. Depending on the specifics of the overflow and the environment, this could cause memory corruption, crashes, or even grant the attacker the ability to execute arbitrary code in the context of the affected application.

4. Mitigations and Patch

The best way to protect against this vulnerability is by always using the latest version of the Freetype library, as security patches and updates are released regularly. The vulnerability was first disclosed by security researchers, and a patch was subsequently provided in the official Freetype repository.

The patch addresses the vulnerability by introducing proper bounds checking for the calculations, ensuring that integer overflows are avoided. Users and developers are strongly encouraged to update their Freetype version or apply the patch to eliminate the risk of exploitation.

5. Conclusion

In this article, we have taken a deep dive into CVE-2023-2004, an integer overflow vulnerability in the Freetype's tt_hvadvance_adjust() function. The vulnerability can lead to significant security risks such as memory corruption and remote code execution. By understanding the vulnerability, staying informed about updates and patches, and using the latest version of the Freetype library, users can protect themselves from exploitation.

Timeline

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