A remote attacker could cause excessive resource consumption by supplying specially crafted request parameters, potentially resulting in a denial of service condition. Older unsupported versions may also be affected. Users are recommended to upgrade to versions 2.3.12, 2.3-next-M9, 3.0.4, 4.0.4, or 4.1.4, which fix this issue.
https://lists.apache.org/thread/q8zxrdhbmmx8ofgr9s7ymnqo8l2x8oy9