python-markdownify (aka markdownify) before 0.14.1 allows large headline prefixes such as <h9999999> in addition to <h1> through <h6>. This causes memory consumption.
https://github.com/matthewwithanm/python-markdownify/issues/143
https://github.com/matthewwithanm/python-markdownify/compare/0.14.0...0.14.1