CVE-2024-56732: HarfBuzz Buffer Overflow Vulnerability

CVE-2024-56732 Overview

CVE-2024-56732 is a critical heap-based buffer overflow vulnerability affecting HarfBuzz, a widely-used open-source text shaping engine. The vulnerability exists in the hb_cairo_glyphs_from_buffer function and affects HarfBuzz versions 8.5.0 through 10.0.1. HarfBuzz is an essential component used across numerous platforms and applications for rendering complex text scripts, making this vulnerability particularly impactful.

Critical Impact

This heap-based buffer overflow can be triggered remotely via network-accessible attack vectors without requiring user interaction or authentication, potentially enabling arbitrary code execution or system compromise.

Affected Products

• HarfBuzz versions 8.5.0 through 10.0.1
• Applications and systems integrating vulnerable HarfBuzz library versions
• Text rendering pipelines utilizing HarfBuzz with Cairo integration

Discovery Timeline

• 2024-12-27 - CVE-2024-56732 published to NVD
• 2024-12-28 - Last updated in NVD database

Technical Details for CVE-2024-56732

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow). The flaw resides in the hb_cairo_glyphs_from_buffer() function, which is responsible for converting HarfBuzz glyph buffer data for Cairo rendering. The function fails to properly validate UTF-8 input boundaries, allowing malformed or maliciously crafted UTF-8 sequences to trigger out-of-bounds memory access on the heap.

The vulnerability is particularly dangerous because HarfBuzz is a foundational library used by major desktop environments, browsers, and document processing applications. An attacker who can supply malicious text content to an application using the affected function could potentially achieve remote code execution.

Root Cause

The root cause lies in the hb_utf_offset_to_pointer template function, which was calculating UTF-8 character offsets without proper boundary constraints. When processing glyph clusters, the function would traverse memory using start + utf_t::max_len or start - utf_t::max_len as boundaries instead of the actual text buffer boundaries. This allowed reads and writes beyond the allocated buffer when handling malformed UTF-8 input, particularly in scenarios where cluster calculations resulted in offsets that exceeded the legitimate buffer range.

Attack Vector

The attack can be conducted remotely over the network. An attacker could exploit this vulnerability by providing specially crafted UTF-8 text to any application using HarfBuzz with Cairo integration. Attack scenarios include:

• Malicious documents containing crafted font glyph data
• Web content processed by applications using HarfBuzz
• Any text input pipeline that processes untrusted UTF-8 data through the vulnerable function

The security patch addresses this by adding proper boundary validation:

 template <typename utf_t>
 static inline const typename utf_t::codepoint_t *
 hb_utf_offset_to_pointer (const typename utf_t::codepoint_t *start,
+			  const typename utf_t::codepoint_t *text,
+			  unsigned text_len,
 			  signed offset)
 {
   hb_codepoint_t unicode;
 
   while (offset-- > 0)
     start = utf_t::next (start,
-			 start + utf_t::max_len,
+			 text + text_len,
 			 &unicode,
 			 HB_BUFFER_REPLACEMENT_CODEPOINT_DEFAULT);
 
   while (offset++ < 0)
     start = utf_t::prev (start,
-			 start - utf_t::max_len,
+			 text,
 			 &unicode,
 			 HB_BUFFER_REPLACEMENT_CODEPOINT_DEFAULT);
 

Source: GitHub Commit Update

The fix introduces proper text buffer boundaries (text and text + text_len) instead of using relative offsets from the current position, ensuring that UTF-8 traversal never exceeds the allocated buffer.

Detection Methods for CVE-2024-56732

Indicators of Compromise

• Unexpected application crashes in text rendering components with heap corruption symptoms
• Memory access violations or segmentation faults in processes using HarfBuzz
• Abnormal memory consumption patterns in applications processing text with Cairo rendering

Detection Strategies

• Deploy memory corruption detection tools (AddressSanitizer, Valgrind) during development and testing to identify heap overflow conditions
• Monitor application logs for crashes related to HarfBuzz or Cairo text rendering functions
• Implement version auditing to identify systems running HarfBuzz versions 8.5.0 through 10.0.1

Monitoring Recommendations

• Configure crash reporting to capture stack traces involving hb_cairo_glyphs_from_buffer or hb_utf_offset_to_pointer
• Implement software composition analysis (SCA) to track HarfBuzz library versions across your environment
• Set up alerts for abnormal process behavior in applications known to use HarfBuzz text rendering

How to Mitigate CVE-2024-56732

Immediate Actions Required

• Upgrade HarfBuzz to version 10.0.2 or later which contains the security fix
• Audit all applications and systems for HarfBuzz dependencies and prioritize updates based on exposure
• Consider temporary isolation of systems that cannot be immediately updated and process untrusted text input

Patch Information

The vulnerability has been addressed in a security patch available via the official HarfBuzz repository. The patch modifies the hb_utf_offset_to_pointer function in src/hb-utf.hh to accept explicit buffer boundaries and updates the caller in src/hb-cairo.cc to pass proper boundary information. For complete details, see the GitHub Security Advisory GHSA-qmp9-xqm5-jh6m.

Workarounds

• Restrict processing of untrusted text input in applications using vulnerable HarfBuzz versions
• Implement input validation to sanitize UTF-8 text before passing to HarfBuzz rendering functions
• Deploy application sandboxing to limit the impact of potential exploitation

# Check installed HarfBuzz version
pkg-config --modversion harfbuzz

# For Debian/Ubuntu systems, check package version
dpkg -l | grep harfbuzz

# Update HarfBuzz from source to patched version
git clone https://github.com/harfbuzz/harfbuzz.git
cd harfbuzz
git checkout v10.1.0  # or latest stable release
meson build
ninja -C build
sudo ninja -C build install