Mastodon is a free, open-source social network server based on ActivityPub. Starting in version 4.2.0-beta1 and prior to version 4.2.0-rc2, by crafting specific input, attackers can inject arbitrary data into HTTP requests issued by Mastodon. This can be used to perform confused deputy attacks if the server configuration includes `ALLOWED_PRIVATE_ADDRESSES` to allow access to local exploitable services. Version 4.2.0-rc2 has a patch for the issue.
https://github.com/mastodon/mastodon/security/advisories/GHSA-hcqf-fw2r-52g4
https://github.com/mastodon/mastodon/commit/94893cf24fc95b32cc7a756262acbe008c20a9d2