CVE-2026-22693: HarfBuzz Buffer Overflow Vulnerability

CVE-2026-22693 Overview

CVE-2026-22693 is a Null Pointer Dereference vulnerability in HarfBuzz, a widely-used open-source text shaping engine. The vulnerability exists in the SubtableUnicodesCache::create function located in src/hb-ot-cmap-table.hh. The function fails to properly validate the return value of hb_malloc before using placement new to construct an object at the returned pointer address, leading to undefined behavior and potential denial of service when memory allocation fails.

Critical Impact

Applications using vulnerable HarfBuzz versions may crash with a Segmentation Fault when memory allocation fails, potentially enabling denial of service attacks in resource-constrained environments.

Affected Products

• HarfBuzz versions prior to 12.3.0
• Applications and libraries depending on vulnerable HarfBuzz versions
• Systems using custom memory allocators with HarfBuzz

Discovery Timeline

• 2026-01-10 - CVE CVE-2026-22693 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-22693

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference). The issue stems from improper error handling in memory allocation within the HarfBuzz text shaping engine. When the SubtableUnicodesCache::create function calls hb_malloc to allocate memory for a new cache object, it does not verify whether the allocation succeeded before proceeding to use placement new syntax on the returned pointer.

Under normal operating conditions with sufficient memory, hb_malloc returns a valid pointer and the code functions correctly. However, when memory allocation fails due to low-memory conditions or when using custom allocators that simulate allocation failures, hb_malloc returns NULL. The subsequent placement new operation on this null pointer results in undefined behavior, manifesting as a Segmentation Fault that can crash the application.

Root Cause

The root cause is a missing NULL check after the hb_malloc call in the SubtableUnicodesCache::create function. The C++ placement new operator does not inherently protect against null pointers—it assumes the provided memory address is valid. By failing to validate the allocation result, the code violates safe memory handling practices and introduces a crash condition that can be triggered when system resources are exhausted or artificially constrained.

Attack Vector

An attacker could potentially exploit this vulnerability through network-accessible attack vectors by forcing memory exhaustion conditions or by leveraging applications that use custom memory allocators. In scenarios where an attacker can influence memory pressure on a target system, they could trigger allocation failures that cause the vulnerable code path to execute, resulting in application crashes and denial of service.

The following patch demonstrates the security fix applied in version 12.3.0:

   {
     SubtableUnicodesCache* cache =
         (SubtableUnicodesCache*) hb_malloc (sizeof(SubtableUnicodesCache));

+    if (unlikely (!cache))
+      return nullptr;
+
     new (cache) SubtableUnicodesCache (source_table);
     return cache;
   }

Source: GitHub Commit Update

Detection Methods for CVE-2026-22693

Indicators of Compromise

• Unexpected application crashes with Segmentation Fault errors in processes using HarfBuzz
• Core dumps containing stack traces pointing to SubtableUnicodesCache::create or hb-ot-cmap-table.hh
• Increased memory pressure or allocation failures preceding application crashes
• Repeated crashes in text rendering or font processing operations

Detection Strategies

• Monitor application logs for Segmentation Fault signals (SIGSEGV) in processes linked to HarfBuzz
• Implement crash reporting to identify patterns related to memory allocation failures in text shaping operations
• Use static analysis tools to scan for NULL pointer dereference vulnerabilities in codebases using HarfBuzz
• Deploy runtime sanitizers (AddressSanitizer, UndefinedBehaviorSanitizer) in development and testing environments

Monitoring Recommendations

• Enable crash telemetry for applications using HarfBuzz to detect exploitation attempts
• Monitor system memory utilization to identify potential resource exhaustion attacks
• Set up alerts for abnormal application restart patterns that may indicate DoS attempts
• Review memory allocator logs if using custom allocators with HarfBuzz

How to Mitigate CVE-2026-22693

Immediate Actions Required

• Upgrade HarfBuzz to version 12.3.0 or later immediately
• Identify all applications and dependencies using vulnerable HarfBuzz versions
• Implement memory monitoring to detect potential exploitation attempts
• Review custom allocator implementations if used with HarfBuzz

Patch Information

The vulnerability has been patched in HarfBuzz version 12.3.0. The fix adds a proper NULL check after the hb_malloc call, returning nullptr gracefully when memory allocation fails instead of proceeding with undefined behavior. Organizations should update to the patched version as soon as possible. For detailed patch information, refer to the GitHub Security Advisory and the commit update.

Workarounds

• Ensure adequate system memory resources to reduce likelihood of allocation failures
• Implement process isolation and restart mechanisms to limit DoS impact
• Configure resource limits (cgroups, ulimits) to prevent memory exhaustion from affecting critical services
• Consider implementing application-level fault tolerance for text rendering failures

# Verify HarfBuzz version and upgrade
pkg-config --modversion harfbuzz
# If version < 12.3.0, upgrade via package manager
# Debian/Ubuntu
sudo apt update && sudo apt install libharfbuzz-dev
# Fedora/RHEL
sudo dnf update harfbuzz
# Or compile from source with patched version
git clone https://github.com/harfbuzz/harfbuzz.git
cd harfbuzz && git checkout v12.3.0
meson build && ninja -C build