Discourse is an open source discussion platform. Prior to version 3.1.0.beta7 of the beta and tests-passed branches, a CSP (Content Security Policy) nonce reuse vulnerability was discovered could allow cross-site scripting (XSS) attacks to bypass CSP protection for anonymous (i.e. unauthenticated) users. There are no known XSS vectors at the moment, but should one be discovered, this vulnerability would allow the XSS attack to bypass CSP and execute successfully. This vulnerability isn't applicable to logged-in users. Version 3.1.0.beta7 contains a patch. The stable branch doesn't have this vulnerability. A workaround to prevent the vulnerability is to disable Google Tag Manager, i.e., unset the gtm container id setting.
Exploitability
AV:NAC:HPR:LUI:NScope
S:UImpact
C:HI:HA:N6.8/CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:NOther