ident (113)

The Ident Protocol is used over the Internet to associate a TCP connection with a specific user. Originally designed to aid in network management and security, it operates by allowing a server to query a client on port 113 to request information about the user of a particular TCP connection.

However, due to modern privacy concerns and the potential for misuse, its usage has decreased as it can inadvertently reveal user information to unauthorized parties. Enhanced security measures, such as encrypted connections and strict access controls, are recommended to mitigate these risks.

It's possible to enumerate and uncover credentials among other vulnerabilities.

https://pentestmonkey.net/tools/user-enumeration/ident-user-enum
https://github.com/brokensound77/identipy
https://book.hacktricks.wiki/en/network-services-pentesting/113-pentesting-ident.html#113---pentesting-ident