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 prior to 3.9.4 contain an unbounded recursion vulnerability in the JSONTaggedDecoder.decode_obj() function. Attackers can exploit this flaw by supplying deeply nested JSON structures that exceed Python's recursion limit. This triggers an unhandled RecursionError that crashes the Python process, resulting in a denial of service condition. The vulnerability requires no authentication and can be triggered remotely by crafting malicious JSON payloads. The fix is available in NLTK version 3.9.4 and later. Users of affected versions should upgrade immediately to mitigate the risk of process crashes and service disruption.