CVE-2026-57204 Overview
CVE-2026-57204 is a denial-of-service vulnerability in pypdf, a widely used pure-Python PDF library. Versions prior to 6.13.3 fail to consistently enforce the MAX_DECLARED_STREAM_LENGTH safeguard when parsing content streams that lack a /Length value. An attacker can craft a malicious PDF that triggers excessive memory allocation during parsing, exhausting host resources. The issue is categorized under [CWE-400: Uncontrolled Resource Consumption]. The maintainers addressed the flaw in pypdf version 6.13.3.
Critical Impact
A remote, unauthenticated attacker can cause memory exhaustion and application denial of service in any service that parses attacker-supplied PDFs with pypdf.
Affected Products
- pypdf versions prior to 6.13.3
- Python applications and services that ingest untrusted PDF files using pypdf
- Downstream libraries and pipelines that embed vulnerable pypdf releases
Discovery Timeline
- 2026-06-30 - CVE-2026-57204 published to NVD
- 2026-07-02 - Last updated in NVD database
Technical Details for CVE-2026-57204
Vulnerability Analysis
The flaw resides in the content stream parsing logic of pypdf. PDF content streams typically declare their byte length via a /Length entry in the stream dictionary. When this entry is absent, pypdf falls back to alternative logic that reads stream data until an end marker is encountered. During this fallback path, the MAX_DECLARED_STREAM_LENGTH guard, which normally caps how much data the parser will accept, is not consistently applied.
An attacker who supplies a crafted PDF without a /Length value can force the parser to buffer very large amounts of data into memory. The parsing operation consumes host RAM proportional to the crafted payload, degrading or crashing the process. Because pypdf is commonly used inside document ingestion services, web upload handlers, and email attachment processors, a single malicious file can disrupt shared infrastructure.
Root Cause
The root cause is inconsistent enforcement of MAX_DECLARED_STREAM_LENGTH in the code path that handles content streams missing a declared length. The safety limit exists but is bypassed when the parser derives stream boundaries dynamically, allowing unbounded reads.
Attack Vector
Exploitation requires only that the target application parse a PDF supplied by the attacker. No authentication, user interaction beyond normal document handling, or local access is needed. Common delivery paths include file upload endpoints, email attachment processors, document conversion pipelines, and batch OCR workflows. See the GitHub Security Advisory GHSA-jm82-fx9c-mx94 for maintainer analysis.
Detection Methods for CVE-2026-57204
Indicators of Compromise
- Python processes running pypdf showing sudden, sustained memory growth followed by MemoryError exceptions or OOM kills
- Repeated crashes or restarts of PDF-processing worker services shortly after ingesting user-supplied files
- Inbound PDF files whose content streams omit the /Length dictionary entry while carrying oversized stream data
Detection Strategies
- Inventory Python environments to identify installations of pypdf at versions below 6.13.3 using pip list or software bill of materials (SBOM) tooling
- Instrument PDF-parsing services with memory ceilings and log allocation spikes correlated to specific input files
- Add PDF structural validation upstream of pypdf to flag streams lacking a /Length entry for manual review
Monitoring Recommendations
- Alert on process RSS exceeding a defined threshold for any worker that invokes pypdf
- Track OOM killer events and container restarts on hosts running document ingestion workloads
- Retain a sample of failing PDFs for offline analysis to distinguish malformed benign files from targeted abuse
How to Mitigate CVE-2026-57204
Immediate Actions Required
- Upgrade pypdf to version 6.13.3 or later in every affected environment
- Rebuild and redeploy container images, virtual environments, and serverless functions that pin older pypdf releases
- Audit dependency trees for transitive uses of pypdf via libraries such as document loaders and OCR wrappers
Patch Information
The maintainers fixed the flaw in pypdf6.13.3. See the pypdf 6.13.3 release notes for the exact commit and change details. Upgrade using pip install --upgrade pypdf==6.13.3 or a later version.
Workarounds
- Enforce strict file size limits on PDF uploads before invoking pypdf
- Run PDF parsing in isolated worker processes with hard memory limits using cgroups, container resource limits, or resource.setrlimit
- Pre-validate PDF structure and reject files whose content streams lack a /Length entry until patched versions are deployed
# Configuration example
pip install --upgrade 'pypdf>=6.13.3'
pip show pypdf | grep -i version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
