The html.parser.HTMLParser class had worse-case quadratic complexity when processing certain crafted malformed inputs potentially leading to amplified denial-of-service.
https://mail.python.org/archives/list/[email protected]/thread/K5PIYLR6EP3WR7ZOKKYQUWEDNQVUXOYM/
https://github.com/python/cpython/pull/135464
https://github.com/python/cpython/issues/135462
https://github.com/python/cpython/commit/d851f8e258c7328814943e923a7df81bca15df4b
https://github.com/python/cpython/commit/6eb6c5dbfb528bd07d77b60fd71fd05d81d45c41
https://github.com/python/cpython/commit/4455cbabf991e202185a25a631af206f60bbc949