Group SMTP user emails are exposed in CC email header

Vulnerability Description

Discourse is an option source discussion platform. Prior to version 2.8.14 on the `stable` branch and version 2.9.0.beta15 on the `beta` and `tests-passed` branches, recipients of a group SMTP email could see the email addresses of all other users inside the group SMTP topic. Most of the time this is not an issue as they are likely already familiar with one another's email addresses. This issue is patched in versions 2.8.14 and 2.9.0.beta15. The fix is that someone sending emails out via group SMTP to non-staged users masks those emails with blind carbon copy (BCC). Staged users are ones that have likely only interacted with the group via email, and will likely include other people who were CC'd on the original email to the group. As a workaround, disable group SMTP for any groups that have it enabled.

References

• https://github.com/discourse/discourse/security/advisories/GHSA-8p7g-3wm6-p3rm
• https://github.com/discourse/discourse/pull/19724