Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 3.5.14, 4.0.10, 4.1.8, and 4.2.0-rc2, under certain circumstances, attackers can exploit a flaw in domain name normalization to spoof domains they do not own. Versions 3.5.14, 4.0.10, 4.1.8, and 4.2.0-rc2 contain a patch for this issue.
https://github.com/mastodon/mastodon/security/advisories/GHSA-v3xf-c9qf-j667
https://github.com/mastodon/mastodon/commit/eeab3560fc0516070b3fb97e089b15ecab1938c8