In the context of Windows Security, what is a 'null' user?

NULL sessions take advantage of “features” in the SMB (Server Message Block) protocol that exist primarily for trust relationships. You can establish a NULL session with a Windows host by logging on with a NULL user name and password. Using these NULL connections allows you to gather the following information from the host:

* List of users and groups

* List of machines

* List of shares

* Users and host SID' (Security Identifiers)

NULL sessions exist in windows networking to allow:

* Trusted domains to enumerate resources

* Computers outside the domain to authenticate and enumerate users

* The SYSTEM account to authenticate and enumerate resources

NetBIOS NULL sessions are enabled by default in Windows NT and 2000. Windows XP and 2003 will allow anonymous enumeration of shares, but not SAM accounts.