Cache-Poisoned Denial of Service (CPDoS) is a new class of web cache poisoning attacks that could be used to deny users access to resources delivered through a content delivery network (CDN) The new method has several variations and works by sending an HTTP request with a malformed header. CDNs have the property of reducing the traffic footprint on origin servers using their services by caching resources that are requested frequently by clients. The attack works at the level of the intermediate cache system of a CDN.
Source: https://www.bleepingcomputer.com/news/security/new-cpdos-web-cache-poisoning-attacks-impact-sites-using-popular-cdns/