Discourse is an open-source discussion platform. Prior to version 3.4.4 of the "stable" branch, version 3.5.0.beta5 of the "beta" branch, and version 3.5.0.beta6-dev of the "tests-passed" branch, certain invites via email may result in HTML injection in the email body if the topic title includes HTML. This includes inviting someone (without an account) to a PM and inviting someone (without an account) to a topic with a custom message. This issue is patched in version 3.4.4 of the "stable" branch, version 3.5.0.beta5 of the "beta" branch, and version 3.5.0.beta6-dev of the "tests-passed" branch. This can be worked around if the relevant templates are overridden without "{topic_title}".