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, Codepen is present in the default "allowed_iframes" site setting, and it can potentially auto-run arbitrary JS in the iframe scope, which is unintended. 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. As a workaround, the Codepen prefix can be removed from a site's "allowed_iframes".