CVE-2025-27363: FreeType Font Parsing RCE Vulnerability

CVE-2025-27363 Overview

An out of bounds write exists in FreeType versions 2.13.0 and below (newer versions of FreeType are not vulnerable) when attempting to parse font subglyph structures related to TrueType GX and variable font files. The vulnerable code assigns a signed short value to an unsigned long and then adds a static value causing it to wrap around and allocate too small of a heap buffer. The code then writes up to 6 signed long integers out of bounds relative to this buffer. This may result in arbitrary code execution. This vulnerability may have been exploited in the wild.

Critical Impact
High impact with potential for arbitrary code execution if exploited.

Affected Products

freetype freetype
debian debian_linux

Discovery Timeline

Not Available - Vulnerability discovered
Not Available - Responsible disclosure to freetype
Not Available - CVE CVE-2025-27363 assigned
Not Available - freetype releases security patch
2025-03-11 - CVE CVE-2025-27363 published to NVD
2025-10-27 - Last updated in NVD database

Technical Details for CVE-2025-27363

Vulnerability Analysis

The vulnerability arises from incorrect handling of font subglyph structures in TrueType GX and variable fonts. When a signed short is assigned to an unsigned long, and a static offset is added, a wraparound occurs. This incorrect handling leads to a buffer that is too small, allowing for an out-of-bounds write of up to six signed long integers, potentially enabling arbitrary code execution.

Root Cause

The root cause is the improper casting of a signed short to an unsigned long during buffer allocation, compounded by a wraparound due to an added static value.

Attack Vector

Network-based exploits are feasible, as the parsing process is triggered when vulnerable font files are processed.

// Example exploitation code (sanitized)
#include <stdio.h>
#include <freetype/freetype.h>

void parse_font(const char* font_file) {
    FT_Library library;
    FT_Init_FreeType(&library);
    FT_Face face;
    FT_New_Face(library, font_file, 0, &face);
    // Vulnerable parsing occurs here
}

Detection Methods for CVE-2025-27363

Indicators of Compromise

Unexpected crashes in applications using FreeType
Anomalous network activity tied to font file parsing
Unexplained system behaviors during font rendering activities

Detection Strategies

Leverage heuristic anomaly detection within applications utilizing FreeType, specifically during font handling procedures. Monitor logs for access to font file directories and track process crashes.

Monitoring Recommendations

Implement monitoring for buffer overflow attempts and abnormal heap memory allocation activities across network ingress points and parsing libraries.

How to Mitigate CVE-2025-27363

Immediate Actions Required

Update FreeType to the latest secure version above 2.13.0
Restrict font file processing in applications as a temporary measure
Disable unnecessary network services related to font rendering

Patch Information

Ensure that the updated FreeType library with the official patch is deployed across all systems using vulnerable versions.

Workarounds

As an immediate workaround, consider filtering and sanitizing incoming font files to preemptively block malicious payloads.

bash

# Configuration example
sudo apt-get update
sudo apt-get install --only-upgrade freetype