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MEDIUM severity

CVE-2023-36464 — pypdf

MEDIUMFix: py-pdf/pypdf#1828

CVE-2023-36464 is a medium-severity (CVSS 6.2) CWE-835 vulnerability in pypdf. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for pypdf — see the affected versions and patch details below.

Infinite Loop when a comment isn't followed by a character in pypdf

Also known asGHSA-4vvm-4w3v-6mr8PYSEC-2026-1835PYSEC-2026-3008
Published
Updated
Affected
2 pkgs
Patched
1 / 2
Exploits
1 known
Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.

Exploitation and automatability from CISA’s SSVC triage for CVE-2023-36464.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs29th percentile — riskier than 29% of all scored CVEsHighest risk
0.00%0.28%0.57%0.85%0.0%0.4%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected
🐍pypdf🐍pypdf2

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

An attacker who uses this vulnerability can craft a PDF which leads to an infinite loop if __parse_content_stream is executed. This infinite loop blocks the current process and can utilize a single core of the CPU by 100%. It does not affect memory usage. That is, for example, the case if the user extracted text from such a PDF.

Example Code and a PDF that causes the issue:

from pypdf import PdfReader

# https://objects.githubusercontent.com/github-production-repository-file-5c1aeb/3119517/11367871?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20230627%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20230627T201018Z&X-Amz-Expires=300&X-Amz-Signature=d71c8fd9181c4875f0c04d563b6d32f1d4da6e7b2e6be2f14479ce4ecdc9c8b2&X-Amz-SignedHeaders=host&actor_id=1658117&key_id=0&repo_id=3119517&response-content-disposition=attachment%3Bfilename%3DMiFO_LFO_FEIS_NOA_Published.3.pdf&response-content-type=application%2Fpdf
reader = PdfReader("MiFO_LFO_FEIS_NOA_Published.3.pdf")
page = reader.pages[0]
page.extract_text()

The issue was introduced with https://github.com/py-pdf/pypdf/pull/969

Patches

The issue was fixed with https://github.com/py-pdf/pypdf/pull/1828

Workarounds

It is recommended to upgrade to pypdf>=3.9.0. PyPDF2 users should migrate to pypdf.

If you cannot update your version of pypdf, you should modify pypdf/generic/_data_structures.py:

OLD: while peek not in (b"\r", b"\n"):
NEW: while peek not in (b"\r", b"\n", b""):

Affected Packages

2 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIpypdf≥ 3.1.0&&< 3.9.03.9.0pip install --upgrade 'pypdf==3.9.0'
🐍PyPIpypdf2≥ 2.2.0No fix

Affected Products

2 products · 2 configurations
Application
pypdfpy-pdf
< 3.9.0
range
Application
pypdf2pypdf2_project
≥ 2.2.0
range
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

pypdf is an open source, pure-python PDF library. In affected versions a…

github.comNVDJul 2023

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pypdf, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update pypdf to 3.9.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-36464 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate
ProductFixed inAdvisory
Red Hat Quay 3quay/quay-rhel8:v3.9.1-6RHBA-2023:4974

Frequently Asked Questions

### Impact An attacker who uses this vulnerability can craft a PDF which leads to an infinite loop if `__parse_content_stream` is executed. This infinite loop blocks the current process and can utilize a single core of the CPU by 100%. It does not affect memory usage. That is, for example, the case if the user extracted text from such a PDF. Example Code and a PDF that causes the issue: ```python from pypdf import PdfReader # https://objects.githubusercontent.com/github-production-repository-file-5c1aeb/3119517/11367871?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%
O3 Security · Impact-Aware SCA

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CVE-2023-36464: pypdf — Fixed in 3.9.0