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

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). 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
    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

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