CVE-2026-78683

Critical

Description

NLTK before 3.10.0 (affected versions <=3.9.4) contains an unsafe pickle deserialization vulnerability in the TransitionParser.parse() method (nltk/parse/transitionparser.py). The method calls pickle_load() with the default restricted=False, routing deserialization through WarningUnpickler, which does not override find_class() and therefore permits arbitrary class resolution. When an application loads an attacker-crafted model file, embedded pickle gadget chains execute arbitrary Python code with the privileges of the user running the application. NLTK provides a RestrictedUnpickler for safe deserialization, but it is not used by production code paths. Fixed in 3.10.0.

SeverityCritical

CVSS9.6

Exploit probability—

ProductNltk

PublishedAug 25, 2026

First seenAug 25, 2026

Weakness typeCWE-502Deserialization of untrusted data

How to fix

Update

nltk nltk3.10.0 or later
nltk3.10.0 or later

Vendor advisories

github.com/nltk/nltk/security/advisories/GHSA-rhp5-r9x4-f5g2

Affected products

Product	Affected versions	Fixed in
nltk nltk	v < 3.10.0	3.10.0
nltkPyPI	v < 3.10.0	3.10.0

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