CVE-2022-33068: Harfbuzz Integer Overflow DoS Vulnerability

CVE-2022-33068 Overview

An integer overflow vulnerability exists in the component hb-ot-shape-fallback.cc of HarfBuzz v4.3.0. HarfBuzz is a widely-used text shaping library that processes OpenType fonts and is embedded in numerous applications including web browsers, document processors, and operating system font rendering pipelines. This vulnerability allows attackers to cause a Denial of Service (DoS) condition by providing specially crafted input that triggers the integer overflow, potentially crashing applications that rely on the library for font processing.

Critical Impact

Applications using HarfBuzz v4.3.0 for text shaping are vulnerable to DoS attacks through maliciously crafted fonts or text input, potentially causing application crashes and service disruption.

Affected Products

• HarfBuzz Project HarfBuzz v4.3.0
• Fedora Project Fedora 35
• Fedora Project Fedora 36

Discovery Timeline

• 2022-06-23 - CVE CVE-2022-33068 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-33068

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw occurs in the hb-ot-shape-fallback.cc component, which handles fallback shaping operations for OpenType fonts. When processing certain font data, the library fails to properly validate numeric boundaries before performing arithmetic operations, leading to an integer overflow condition.

The vulnerability requires local access and user interaction to exploit, as the attacker must convince a victim to process a malicious font file or document containing specially crafted font data. The impact is limited to availability—successful exploitation results in a denial of service condition without compromising data confidentiality or integrity.

The patch analysis reveals that the fix addresses boundary conditions in the SBIX (Standard Bitmap Graphics Table) handling code, specifically limiting glyph extents to prevent overflow conditions when processing PNG header dimensions.

Root Cause

The root cause is insufficient validation of glyph dimension values in the SBIX table processing code. The library processes PNG header width and height values without verifying they fall within safe numeric bounds before using them in calculations. When these values approach or exceed 65536, integer overflow can occur during subsequent arithmetic operations, leading to undefined behavior and potential crashes.

Attack Vector

The attack vector is local, requiring user interaction. An attacker would need to:

1. Craft a malicious font file or document containing SBIX table data with oversized PNG dimension values
2. Trick a victim into opening the malicious file with an application that uses HarfBuzz for text rendering
3. When the vulnerable code processes the malformed glyph data, the integer overflow triggers, causing the application to crash

This attack is particularly relevant for document viewers, web browsers, and design applications that process untrusted font files.

// Security patch from src/hb-ot-color-sbix-table.hh
// Source: https://github.com/harfbuzz/harfbuzz/commit/62e803b36173fd096d7ad460dd1d1db9be542593
 
       const PNGHeader &png = *blob->as<PNGHeader>();
 
+      if (png.IHDR.height >= 65536 | png.IHDR.width >= 65536)
+      {
+	hb_blob_destroy (blob);
+	return false;
+      }
+
       extents->x_bearing = x_offset;
       extents->y_bearing = png.IHDR.height + y_offset;
       extents->width     = png.IHDR.width;

Source: GitHub Commit 62e803b

Detection Methods for CVE-2022-33068

Indicators of Compromise

• Application crashes when rendering specific fonts or documents
• Unexpected termination of text processing services or font rendering engines
• Error logs indicating memory corruption or assertion failures in HarfBuzz-related code paths
• Suspicious font files with abnormally large PNG dimension values in SBIX tables

Detection Strategies

• Monitor for application crashes related to font rendering or text shaping operations
• Implement file integrity monitoring for font files in system and application directories
• Deploy static analysis rules to detect HarfBuzz library versions prior to the security patch
• Scan incoming font files for anomalous SBIX table structures with dimension values exceeding 65535

Monitoring Recommendations

• Configure application crash reporting to capture stack traces involving HarfBuzz library functions
• Implement resource usage monitoring to detect abnormal memory consumption during font processing
• Enable security logging for document and font file processing operations
• Monitor for repeated crashes in applications that process external font resources

How to Mitigate CVE-2022-33068

Immediate Actions Required

• Upgrade HarfBuzz to a version containing the security patch (commit 62e803b36173fd096d7ad460dd1d1db9be542593 or later)
• Apply vendor-provided security updates for affected operating systems including Fedora 35 and 36
• Review and update any applications bundling HarfBuzz to use patched versions
• Implement input validation for font files from untrusted sources

Patch Information

The vulnerability has been addressed in the HarfBuzz repository. The fix adds boundary validation to prevent processing of SBIX table entries with PNG dimensions at or above 65536 pixels. Organizations should apply the patch through one of the following methods:

• Direct from source: Apply commit 62e803b
• Fedora users: Install updated packages via Fedora package announcements
• Gentoo users: Follow GLSA 202209-11 advisory

Workarounds

• Restrict processing of font files from untrusted or unknown sources
• Implement application-level sandboxing for processes that handle external fonts
• Configure font loading policies to use only trusted system fonts where possible
• Deploy content filtering to block or quarantine suspicious font file attachments

# Check installed HarfBuzz version on Linux systems
pkg-config --modversion harfbuzz

# For Fedora systems, update to patched version
sudo dnf update harfbuzz

# Verify the update was applied
rpm -q harfbuzz