The HTTP/2 protocol allows a denial of service (server resource consumption) because request cancellation can reset many streams quickly, as exploited in the wild in August through October 2023.

The Command Line Interface (aka Server CLI or administration interface) in the master process in the reverse proxy server in Varnish before 2.1.0 does not require authentication for commands received through a TCP port, which allows remote attackers to (1) execute arbitrary code via a vcl.inline directive that provides a VCL configuration file containing inline C code; (2) change the ownership of the master process via param.set, stop, and start directives; (3) read the initial line of an arbitrary file via a vcl.load directive; or (4) conduct cross-site request forgery (CSRF) attacks that leverage a victim's location on a trusted network and improper input validation of directives. NOTE: the vendor disputes this report, saying that it is "fundamentally misguided and pointless.

Multiple cross-site scripting (XSS) vulnerabilities in the Varnish module 6.x-1.x before 6.x-1.2 and 7.x-1.x before 7.x-1.0-beta2 for Drupal allow remote attackers to inject arbitrary web script or HTML via crafted a (1) Watchdog message or (2) admin setting.