CDN are said to absorb and mitigate the Denial of Service and DDOS attacks. Consider an application that uses a CDN provider to deliver its content. So if an attacker tries to bring down such an application using DOS or DDOS, the flood of requests made during such an attack will go to the CDN servers. Will such a DDOS attack have to completely bring down all the CDN servers serving this application's content before impairing the origin server completely?
Will such a DDOS attack have to completely bring down all the CDN servers serving this application's content before impairing the origin server completely?
The CDN provider might decide that this attacks is too costly and too much impacts other clients protected by the CDN that it will no longer serve your domain. In this case your domain will not be directly brought down by the attack passing through your system but will be unreachable anyway.
Apart from that your application might have bugs which make it possible for already few requests to cause a large load - like causing lots of requests to the database or algorithmic complexity attacks. In this case only a few requests will be enough to cause a high load and will make your server unreachable. While WAF capabilities in a CDN might protect against some known attacks like this it cannot protect against arbitrary bugs in the application.