Cross-site Scripting (XSS) via chat excerpts when content security policy (CSP) disabled in Discourse

Vulnerability Description

Discourse is an open source platform for community discussion. An attacker can execute arbitrary JavaScript on users' browsers by sending a maliciously crafted chat message and replying to it. This issue only affects sites with CSP disabled. This problem is patched in the latest version of Discourse. All users are advised to upgrade. Users unable to upgrade should ensure CSP is enabled on the forum. Users who do upgrade should also consider enabling a CSP as well as a proactive measure.

References

• https://github.com/discourse/discourse/security/advisories/GHSA-67mh-xhmf-c56h
• https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP