Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
Summary
JSONTaggedDecoder.decode_obj() in nltk/jsontags.py calls itself
recursively without any depth limit. A deeply nested JSON structure
exceeding sys.getrecursionlimit() (default: 1000) will raise an
unhandled RecursionError, crashing the Python process.
Affected code
File: nltk/jsontags.py, lines 47–52
@classmethod
def decode_obj(cls, obj):
if isinstance(obj, dict):
obj = {key: cls.decode_obj(val) for (key, val) in obj.items()}
elif isinstance(obj, list):
obj = list(cls.decode_obj(val) for val in obj)Proof of Concept
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)Impact
Any code path that passes externally-supplied JSON to
JSONTaggedDecoder is vulnerable to denial of service.
The severity depends on whether such a path exists in the
calling code (e.g. nltk/data.py).
Suggested Fix
Add a depth parameter with a hard limit:
@classmethod
def decode_obj(cls, obj, _depth=0):
if _depth > 100:
raise ValueError("JSON nesting too deep")
if isinstance(obj, dict):
obj = {key: cls.decode_obj(val, _depth + 1)
for (key, val) in obj.items()}
elif isinstance(obj, list):
obj = list(cls.decode_obj(val, _depth + 1) for val in obj)Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | nltk | < 3.9.4 | 3.9.4 |
References
- https://www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder
- https://github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw
- https://nvd.nist.gov/vuln/detail/CVE-2026-66393
- https://github.com/nltk/nltk/commit/00cdcd392142e6c745e7120c8d50a24127df5fad
- https://github.com/nltk/nltk
- https://github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3724.yaml
- CVSS vector
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-674- Platform
- Python (PyPI)
- Package
nltk- Disclosed
- 18.03.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-3724 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-rf74-v2fm-23pw - CC BY 4.0
Vulnerability history: nltk
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 52
- First → latest
- 08.2019 09.2026
- Without a fix
- 6
- Exploited in attacks
- 0
Severity breakdown
- Critical 5
- High 32
- Medium 13
- Low 2
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 2
- 2022: 1
- 2023: 0
- 2024: 1
- 2025: 0
- 2026: 47
-
Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS this entry
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