Note: This issue applies only to the open source version of Varnish Cache, in version 7.0.x and 7.1.x. Varnish Cache Plus is not affected. For more information please see the open source project’s advisory page.
A denial of service attack can be performed against Varnish Cache servers by specially formatting the reason phrase of the backend response status line. In order to execute an attack, the attacker would have to be able to influence the HTTP/1 responses that the Varnish Server receives from its configured backends. A successful attack would cause the Varnish Server to assert and automatically restart.
The potential impact is reduced or lost availability.