The option xfer.file.auto_accept_nicks is a list of nicks for which incoming files and chats are automatically accepted, with the format server.nick (for a specific server) or nick (for all servers).
For the format server.nick, only the beginning of the server name is compared, so the nick is also auto-accepted on any other server whose name starts with the same chars.
For example with libera.alice, files and chats sent by nick alice are automatically accepted on servers libera, libera2 or libera_test.
On these other servers, any user able to use this nick (which may not be registered there) can send files that are automatically saved in the download directory, or start DCC chats, without confirmation.
Mitigation
There are multiple ways to mitigate this issue:
Keep the option xfer.file.auto_accept_nicks empty (default), see: /help xfer.file.auto_accept_nicks
Check that no server name in this option is the beginning of another server name, see: /server list