CVE-2017-13720

What it actually does

Buffer over-read in the PatternMatch function caused by incorrect handling of '\0' characters when pattern matching involves '?' wildcards, allowing memory contents to be read past intended bounds or triggering a crash.

Why it matters to the business

X font services may leak sensitive memory contents or become unavailable, disrupting graphical applications and increasing operational recovery effort; the issue affects packages such as opa-fips where the vulnerable library is embedded.

How an attack unfolds

An attacker establishes a local X connection and supplies a crafted font pattern containing '?' characters that forces the library to skip null terminators during matching, resulting in an over-read that either discloses memory or crashes the process.

How to fix it

Upgrade affected packages to the versions that contain the corrected libXfont code; for opa-fips (Chainguard) this means version 0.65.0-r1 or later. Remove or isolate systems that cannot be updated until patches are applied.

How to confirm the fix worked

Verify the installed package version matches or exceeds the fixed release, recompile or relink any statically built components that embed libXfont, and confirm that font pattern requests no longer produce crashes or unexpected memory reads in test environments.

Typical attack path

1. Obtain local access or a valid X connection on the target system
2. Craft a font directory pattern containing '?' wildcards designed to trigger null-byte skipping
3. Submit the pattern through the X font protocol to the vulnerable libXfont instance
4. Observe resulting memory disclosure or service crash

How to detect it

Review X server and application logs for segmentation faults or abnormal font loading errors; correlate process crashes of X font utilities with user sessions that have direct X access; examine core dumps for patterns consistent with font directory buffer reads.

What your security team should do

• Prioritize patching opa-fips (Chainguard) to 0.65.0-r1 within standard change windows given the 2017 disclosure date and low exploitation signals
• Inventory all systems using X font libraries and schedule upgrades before the next maintenance cycle
• Disable unnecessary X font services on headless systems to reduce attack surface
• Add version checks for libXfont to continuous compliance scanning