CVE-2026-126299: NLTK: Quadratic-time CPU denial of service in CCG lexicon parser (fromstring) on wide bracket bodies and operator chains

The public CCG lexicon parser nltk.ccg.lexicon.fromstring parses caller-influenced lexicon text with two pure-Python loops whose cost is quadratic in the input width. A service that parses an untrusted CCG lexicon string can be driven into tens of seconds of single-threaded CPU by a few-megabyte input, starving the request worker. No authentication or user interaction is required.