Summary
Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
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 toJSONTaggedDecoder is vulnerable to denial of service.
The severity depends on whether such a path exists in the
calling code (e.g. nltk/data.py).
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
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)
Frequently Asked Questions
- What is CVE-2026-66393? CVE-2026-66393 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.9.3. It is fixed in 3.9.4.
- Which versions of nltk are affected by CVE-2026-66393? nltk (pip) versions <= 3.9.3 is affected.
- Is there a fix for CVE-2026-66393? Yes. CVE-2026-66393 is fixed in 3.9.4. Upgrade to this version or later.
- Is CVE-2026-66393 exploitable, and should I be worried? Whether CVE-2026-66393 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-66393 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-66393? Upgrade
nltkto 3.9.4 or later.