CVE-2026-66393

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-rf74-v2fm-23pw. This link is maintained to preserve external references.

Original Description

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

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)

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.