OpenProject is open source project management software. Prior to version 14.3.0, using a forged HOST header in the default configuration of packaged installations and using the "Login required" setting, an attacker could redirect to a remote host to initiate a phishing attack against an OpenProject user's account. This vulnerability affects default packaged installation of OpenProject without any additional configuration or modules on Apache (such as mod_security, manually setting a host name, having a fallthrough VirtualHost). It might also affect other installations that did not take care to fix the HOST/X-Forwarded-Host headers. Version 14.3.0 includes stronger protections for the hostname from within the application using the HostAuthorization middleware of Rails to reject any requests with a host name that does not match the configured one. Also, all generated links by the application are now ensured to use the built-in hostname. Users who aren't able to upgrade immediately may use mod_security for Apache2 or manually fix the Host and X-Forwarded-Host headers in their proxying application before reaching the application server of OpenProject. Alternatively, they can manually apply the patch to opt-in to host header protections in previous versions of OpenProject.