# CVE-2026-33231: NLTK unauthenticated remote shutdown in WordNet Browser

Source: https://junglewise.ai/threats/cve-2026-33231-nltk-unauthenticated-remote-shutdown-in-wordnet-browser  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-20. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-33231 · Severity: high · CVSS 7.5 · Published 2026-03-20

The Natural Language Toolkit (NLTK) is a popular Python library used for processing human language data. A vulnerability in its WordNet Browser component allows an unauthenticated person to remotely shut down the server with a single web request. This can lead to a denial of service, making the tool unavailable for researchers or applications relying on its web interface.

Technologies: [nltk (PyPI)](https://junglewise.ai/threats/technologies/nltk), [Red Hat Lightspeed Core](https://junglewise.ai/threats/technologies/lightspeed-core), [Red Hat OpenShift Lightspeed](https://junglewise.ai/threats/technologies/openshift-lightspeed). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi), [Red Hat](https://junglewise.ai/threats/vendors/red-hat).

## Quick answers

**Is CVE-2026-33231 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-33231 being exploited in the wild.

**How severe is CVE-2026-33231?** High severity, with a CVSS score of 7.5, published 20 March 2026.

**Which products and versions are affected?** nltk (PyPI) <= 3.9.3, ECOSYSTEM:introduced=0,fixed=3.9.4.

**Is there a fix for CVE-2026-33231?** Yes. nltk (PyPI): upgrade to 3.9.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-33231 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-33231?** Check whether you run nltk (PyPI), Red Hat Lightspeed Core or Red Hat OpenShift Lightspeed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- nltk (PyPI): upgrade to 3.9.4 or later

  ```sh
  pip install "nltk>=3.9.4"
  ```


## Technical details

The `nltk.app.wordnet_app` component in NLTK versions up to and including 3.9.3 fails to implement authentication for critical server functions. By default, the WordNet Browser HTTP server listens on all interfaces (`0.0.0.0`). An attacker can send a specially crafted HTTP GET request to the `/SHUTDOWN%20THE%20SERVER` endpoint, which triggers an immediate `os._exit(0)` call. This results in a complete process termination and denial of service. The issue is addressed in later versions by binding the server to localhost (`127.0.0.1`) by default and patching the shutdown logic.

## Affected products

- nltk nltk <= 3.9.3
- Red Hat Red Hat OpenShift AI 2.25
- Red Hat Red Hat OpenShift AI 3.3
- Red Hat Lightspeed Core
- Red Hat OpenShift Lightspeed
- Red Hat Red Hat Ansible Automation Platform 2

## Identifiers

- CVE-2026-33231
- GHSA-jm6w-m3j8-898g
- PYSEC-2026-2236

## Timeline

- 2026-03-18: advisory: GitHub Security Advisory published
- 2026-03-20: disclosed: CVE published to NVD
- 2026-05-20: patched: Red Hat released security advisory RHSA-2026:19712

## References

- [https://github.com/nltk/nltk/commit/bbaae83db86a0f49e00f5b0db44a7254c268de9b](https://github.com/nltk/nltk/commit/bbaae83db86a0f49e00f5b0db44a7254c268de9b)
- [https://github.com/nltk/nltk/security/advisories/GHSA-jm6w-m3j8-898g](https://github.com/nltk/nltk/security/advisories/GHSA-jm6w-m3j8-898g)
- [https://access.redhat.com/errata/RHSA-2026:19712](https://access.redhat.com/errata/RHSA-2026:19712)
- [https://access.redhat.com/errata/RHSA-2026:24977](https://access.redhat.com/errata/RHSA-2026:24977)
- [https://access.redhat.com/security/cve/CVE-2026-33231](https://access.redhat.com/security/cve/CVE-2026-33231)
- [https://bugzilla.redhat.com/show_bug.cgi?id=2449836](https://bugzilla.redhat.com/show_bug.cgi?id=2449836)
- [https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33231.json](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33231.json)

## Related threats

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- [NLTK RecursiveDescentParser uncontrolled recursion denial of service](https://junglewise.ai/threats/cve-2026-12876-nltk-recursivedescentparser-uncontrolled-recursion-denial-of)
- [NLTK tgrep regular expression denial of service](https://junglewise.ai/threats/nltk-tgrep-regular-expression-denial-of-service-02fca1b2)
- [NLTK Downloader hardlink filesystem containment bypass](https://junglewise.ai/threats/nltk-downloader-hardlink-filesystem-containment-bypass-522cb97d)
- [NLTK Pl196xCorpusReader quadratic ReDoS on malformed TEI blocks](https://junglewise.ai/threats/nltk-pl196xcorpusreader-quadratic-redos-on-malformed-tei-blocks-8473e6ef)