The Common Vulnerabilities and Exposures (CVE) system has documented a critical vulnerability identified as CVE-2025-27363. This vulnerability affects the FreeType library versions 2.13. and below, particularly in the processing of TrueType GX and variable font files. This article provides an in-depth look at this security issue, detailing the background, code snippet, original references, and possible exploit scenarios.

Background

Before diving into the technical aspects, it is essential to understand the context of this problem. FreeType is a popular open-source software library for rendering fonts on various platforms. It is widely used in numerous applications, including web browsers, operating systems, and embedded systems. The FreeType project aims to provide a high-quality, portable, and efficient font engine to handle a range of font file formats.

The Vulnerability: Out of Bounds Write

CVE-2025-27363 refers to an out of bounds write vulnerability present in the FreeType library when parsing font subglyph structures associated with TrueType GX or variable font files. In simple terms, this happens when the problematic code attempts to write data outside the allocated memory buffer due to an incorrect size calculation. Consequently, this could lead to unintended consequences, such as data corruption or arbitrary code execution.

Here's a simplified version of the vulnerable code segment from the FreeType library

void process_subglyph(fixed_buf, subglyph_data, subglyph_len) {
  unsigned long siz = (signed short)subglyph_data[] * 2 + 42;
  byte *heap_buf = malloc(siz);
  
  for (unsigned int i = ; i < subglyph_len / sizeof(signed long); i++) {
    heap_buf[i * sizeof(signed long)] = subglyph_data[i * sizeof(signed long)];
  }

  // ...
}

In this code snippet, the variable siz holds the result of a signed short multiplied by two, then adds a static value (42). Due to the erroneous assignment of a signed short to an unsigned long, the resulting value may wrap around and cause the allocation of a smaller heap buffer than actually required.

Later on, the loop writes up to six signed long integers out of bounds relative to the allocated buffer. The absence of any boundary checks or proper size calculations exacerbates the problem, potentially leading to security issues.

Original References

The vulnerability has been disclosed through the FreeType project's security advisory, available here. The commit that resolved the issue also serves as a reference, mentioning the assignment to wrap around and the corresponding patch. The National Vulnerability Database (NVD) entry provides additional information about CVE-2025-27363, including the severity and affected versions.

Potential Exploits

This out of bounds write vulnerability is dangerous because it may allow an attacker to execute arbitrary code by creating a specially crafted font file designed to trigger the vulnerability. An unsuspecting user who opens the malicious file might unintentionally enable the attacker to gain control over their system.

There have been reports indicating that CVE-2025-27363 might have been exploited in the wild already, emphasizing its significance and the need for robust security measures.

Mitigation

Since the vulnerability affects FreeType versions 2.13. and below, users and developers are advised to update their library to the latest version to prevent potential security issues. A patch addressing the vulnerability has been applied to the FreeType project, which you can access here. Always ensure that you're using a secure and updated font rendering library to avoid falling victim to such attacks.

Conclusion

CVE-2025-27363 is a critical vulnerability in the FreeType library, emphasizing the importance of updating, testing, and securing all aspects of a software ecosystem. Placing a strong focus on secure development practices and timely patching can help mitigate the risk of such vulnerabilities being exploited by threat actors.

Timeline

Published on: 03/11/2025 14:15:25 UTC
Last modified on: 03/13/2025 23:15:36 UTC