An issue was discovered in Prosody before 0.11.9. The undocumented dialback_without_dialback option in mod_dialback enables an experimental feature for server-to-server authentication. It does not correctly authenticate remote server certificates, allowing a remote server to impersonate another server (when this option is enabled).
https://www.debian.org/security/2021/dsa-4916
https://security.gentoo.org/glsa/202105-15
https://blog.prosody.im/prosody-0.11.9-released/