CVE-2026-42308 Overview
CVE-2026-42308 is an integer overflow vulnerability [CWE-190] in Python Pillow, a widely used Python imaging library. The flaw exists in versions prior to 12.2.0 and occurs when Pillow tracks the current rendering position while processing fonts. If a font advances each glyph by an exceedingly large amount, the cumulative position calculation can overflow an integer. The Pillow maintainers patched this issue in version 12.2.0. The vulnerability requires local access and carries limited availability impact.
Critical Impact
An attacker who can supply a crafted font file to an application using Pillow can trigger an integer overflow during text rendering, potentially causing a denial of service in the affected process.
Affected Products
- Python Pillow versions prior to 12.2.0
- Applications embedding Pillow for image and text rendering
- Python services that process untrusted font files via Pillow
Discovery Timeline
- 2026-05-09 - CVE-2026-42308 published to the National Vulnerability Database
- 2026-05-12 - Last updated in NVD database
Technical Details for CVE-2026-42308
Vulnerability Analysis
The vulnerability resides in the text rendering path of Pillow. When Pillow draws text, it iterates over glyphs in a font and advances a running position value for each glyph. The library uses a fixed-width integer to store this position. A font that declares very large per-glyph advance values causes the cumulative sum to exceed the maximum representable integer value.
When the overflow occurs, the position wraps around to a small or negative value. This breaks subsequent length and offset calculations that depend on the position variable. The resulting state can cause incorrect memory allocations, abnormal rendering, or process termination. The advisory tracked under GHSA-wjx4-4jcj-g98j confirms that the flaw is reachable through font input alone.
Root Cause
The root cause is missing bounds validation on cumulative glyph advance values. Pillow trusts the font metrics it parses without checking whether successive additions would exceed integer range. This is a classic instance of [CWE-190] Integer Overflow or Wraparound.
Attack Vector
The attack vector is local. An attacker must supply a malicious font file to an application that calls Pillow to render text with that font. Common exposure scenarios include image generation pipelines that accept user-supplied fonts, document conversion tools, and CAPTCHA or thumbnail services. No authentication or user interaction beyond loading the font is required. See the GitHub Security Advisory GHSA-wjx4-4jcj-g98j for technical details.
Detection Methods for CVE-2026-42308
Indicators of Compromise
- Unexpected crashes or OverflowError exceptions in Python processes that invoke Pillow text rendering functions.
- Presence of font files (.ttf, .otf, .woff) in user-writable directories consumed by Pillow workflows.
- Anomalous resource usage or termination of image-processing worker processes following ingestion of third-party fonts.
Detection Strategies
- Inventory all Python environments and identify installations of Pillow below version 12.2.0 using pip list or software composition analysis tooling.
- Review application code paths that pass externally sourced font files to ImageFont.truetype() or related Pillow APIs.
- Inspect logs of image-processing services for repeated worker restarts correlated with font-rendering operations.
Monitoring Recommendations
- Track Pillow version pins across CI/CD pipelines and container images to surface vulnerable releases.
- Monitor process exit codes and stack traces from services that perform text-on-image rendering for untrusted inputs.
- Subscribe to the python-pillow GitHub security advisories feed for follow-up fixes.
How to Mitigate CVE-2026-42308
Immediate Actions Required
- Upgrade Pillow to version 12.2.0 or later in all production, staging, and development environments.
- Audit application code to reject or validate user-supplied font files before passing them to Pillow.
- Rebuild and redeploy container images that bundle Pillow to incorporate the patched release.
Patch Information
The issue is fixed in Pillow Release 12.2.0. Refer to the GitHub Security Advisory GHSA-wjx4-4jcj-g98j for the official fix details and commit references.
Workarounds
- Restrict Pillow workflows to a curated allowlist of trusted font files until the upgrade is deployed.
- Run image-rendering workers under strict resource limits and isolation so that a crash cannot affect other tenants.
- Sanitize or strip font metadata from uploaded files using a separate, hardened font parser before invoking Pillow.
# Upgrade Pillow to the patched release
pip install --upgrade "Pillow>=12.2.0"
# Verify the installed version
python -c "import PIL; print(PIL.__version__)"
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
