FreeType Heap Out-of-Bounds Write Vulnerability (CVE-2025-27363) Security Advisory

1. Vulnerability Overview & Root Cause

This vulnerability (CVE-2025-27363) is a critical heap-based out-of-bounds write (CWE-787) in FreeType, a widely used open-source font rendering engine. The root cause lies within the parsing logic of font subglyph structures associated with TrueType GX and variable font files. The vulnerable code assigns a signed short value to an unsigned long variable and subsequently adds a static value. This arithmetic operation triggers an integer wrap-around, leading to the allocation of an inadequately sized heap buffer.

2. Exploitation Mechanics & Attack Scenario

An attacker can exploit this vulnerability by crafting a malicious TrueType GX or variable font file and forcing the target application to parse it. When FreeType processes the subglyph data of this malformed font, the wrap-around condition causes the engine to allocate a heap buffer that is too small. The parser then writes up to six signed long integers out of bounds relative to this undersized buffer. This memory corruption allows attackers to overwrite adjacent heap structures, hijack the execution flow, and achieve arbitrary code execution (RCE). Evidence indicates this vulnerability has been actively exploited in the wild.

3. Infrastructure & Business Impact

Because FreeType is embedded in numerous operating systems (including Linux and Android), web browsers, and document rendering suites, the attack surface is exceptionally broad. A successful exploit could compromise endpoints or servers simply by rendering a malicious font on a webpage or within a document. Given its inclusion in the CISA KEV catalog, failure to patch poses an immediate threat of system compromise, lateral movement, and potential data exfiltration.