Discourse is an open-source discussion platform. Prior to version 3.2.3 on the "stable" branch and version 3.3.0.beta3 on the "tests-passed" branch, an attacker can execute arbitrary JavaScript on users’ browsers by posting a specific URL containing maliciously crafted meta tags. This issue only affects sites with Content Security Polic (CSP) disabled. The problem has been patched in version 3.2.3 on the "stable" branch and version 3.3.0.beta3 on the "tests-passed" branch. As a workaround, ensure CSP is enabled on the forum.