mod_proxy65 is a file transfer proxy provided with Prosody to facilitate the transfer of files and other data between XMPP clients.
It was discovered that the proxy65 component of Prosody allows open access by default, even if neither of the users have an XMPP account on the local server, allowing unrestricted use of the server’s bandwidth.

The default configuration does not enable mod_proxy65 and is not affected.
With mod_proxy65 enabled, all configurations without a ‘proxy65_acl’ setting configured are affected.

Reference:
https://prosody.im/security/advisory_20210512/