PYSEC-2026-3724

NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decodeobj 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 RecursionErro...

GHSA-RF74-V2FM-23PW Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS

Summary JSONTaggedDecoder.decodeobj 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...