A new cache poisoning attack could be used by an attacker to force a targeted website into delivering error pages to most of its visitors instead of legitimate content or resources. The issue could affect sites running behind reverse proxy cache systems like Varnish and some widely-used Content Distribution Networks (CDNs) services. The attack resides in the way intermediate CDN servers are incorrectly configured to cache web resources or pages returned by the single HTTP request header. The problem arises when an attacker can generate an HTTP request for a cacheable resource where the request contains inaccurate fields.
Source: https://thehackernews.com/2019/10/cdn-cache-poisoning-dos-attack.html