By specially crafting HTTP requests, the mod_md challenge handler would dereference a NULL pointer and cause the child process to segfault. This could be used to DoS the server. Fixed in Apache HTTP Server 2.4.34 (Affected 2.4.33).
https://security.netapp.com/advisory/ntap-20180926-0007/
https://httpd.apache.org/security/vulnerabilities_24.html#CVE-2018-8011