# CVE-2026-78683: NLTK unsafe pickle deserialization in TransitionParser

Source: https://junglewise.ai/threats/cve-2026-78683-nltk-unsafe-pickle-deserialization-in-transitionparser  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-25. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-78683 · Severity: critical · CVSS 9.6 · Published 2026-08-25

NLTK is a Python library used for natural language processing tasks like text parsing and tokenization. The TransitionParser.parse() method loads pre-trained parsing models from pickle files without proper safety checks, allowing an attacker to place a malicious model file and achieve remote code execution with the privileges of the application using NLTK. This affects researchers, data scientists, and automated machine learning pipelines.

Technologies: [nltk (PyPI)](https://junglewise.ai/threats/technologies/nltk). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-78683 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-78683 being exploited in the wild.

**How severe is CVE-2026-78683?** Critical severity, with a CVSS score of 9.6, published 25 August 2026.

**Which products and versions are affected?** nltk (PyPI) <= 3.9.4, ECOSYSTEM:introduced=0,fixed=3.10.0.

**Is there a fix for CVE-2026-78683?** Yes. nltk (PyPI): upgrade to 3.10.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-78683 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-78683?** Check whether you run nltk (PyPI) 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 3.10.0 or later

  ```sh
  pip install "nltk>=3.10.0"
  ```


## Technical details

The vulnerability is a deserialization of untrusted data (CWE-502) in NLTK's pickle-based model loading. The parse() method in nltk/parse/transitionparser.py calls pickle_load(f) without the restricted=True parameter (line 557), routing through WarningUnpickler which does not override find_class() and thus permits arbitrary class/function resolution during unpickling. An attacker can craft a malicious pickle file using standard Python gadgets (e.g., os.system, subprocess.Popen) that executes arbitrary code during deserialization. The vulnerability is reachable when an application calls TransitionParser().parse() with a path to an attacker-controlled or compromised pickle file. The fix requires changing pickle_load() calls at four locations to use restricted=True, enabling RestrictedUnpickler which maintains a strict allowlist and blocks all unsafe globals.

## Affected products

- NLTK Project NLTK <= 3.9.4

## Identifiers

- CVE-2026-78683
- GHSA-rhp5-r9x4-f5g2
- PYSEC-2026-3734

## Timeline

- 2026-08-11: disclosed
- 2026-09-08: advisory
- 2026-09-08: patched: Fix released in NLTK 3.10.0

## References

- [https://api.github.com/users/ekaf](https://api.github.com/users/ekaf)
- [https://github.com/ekaf](https://github.com/ekaf)
- [https://api.github.com/users/ekaf/gists%7B/gist_id%7D](https://api.github.com/users/ekaf/gists%7B/gist_id%7D)
- [https://api.github.com/users/ekaf/repos](https://api.github.com/users/ekaf/repos)
- [https://avatars.githubusercontent.com/u/4782556?v=4](https://avatars.githubusercontent.com/u/4782556?v=4)
- [https://api.github.com/users/ekaf/events%7B/privacy%7D](https://api.github.com/users/ekaf/events%7B/privacy%7D)

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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)