CVE-2026-78683: Nltk

Critical severity, CVSS 9.6. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Nltk Nltk: before 3.10.0 (fixed in 3.10.0)

Published 2026-08-25. Last modified 2026-08-31.