Nextcloud: Unauthenticated SSRF in 3rd party module "cerdic/csstidy"

Summary: The mail extension in nextcloud includes a module called "cerdic/csstidy" which basically ships with a publicly accessible test/example interface to play with the CSS formatter and optimiser /apps/mail/vendor/cerdic/css-tidy/cssoptimiser.php. This module allows contacting any remote serv...