Null Session

A null session is a session with a server that does not require authentication of the user. Thus, this user becomes an anonymous access to the service on the server. It is created, for example, if a computer needs information from another computer, but not a conventional login by username and password out of the question.

The computer, or even a user connects to a remote service and makes use of this anonymously, ie without prior authentication. Such a meeting allows thus an unknown user to collect information about the computer system.

The Windows Explorer is eg able to detect the file shares others' computers. Since in a foreign system often no user name and password can be used, this is about building a Null Session with the SYSTEM account. This is a standard account with very extensive rights, which is available on any Windows computer. Many local security services run with this account.

Building a Null Session in Windows means that a token is presented showing that includes a user ANONYMOUS LOGON and Everyone group. Thus, the user will receive with this service, the rights that are assigned to this group and this user. Many settings on many Windows computers, however, are too broad. So when setting up a release often carelessly "Everyone" access is allowed. It would be correct, the Authenticated Users group in most cases. This contains only the users who have logged in with user name and password.

Null sessions are often targets for hackers who want to gain access to a computer. They allow, for example, list the user accounts, so if necessary, the login name of the user or further information, such as membership in certain groups (administrator) or current security settings fall into the hands of the hacker. This information can be used to select potential targets.

Attacks on Windows computers than zero sessions are known for Windows NT / Windows 2000. With any Windows - generation opportunities are reduced, there are fewer API functions for unauthenticated users. Nevertheless SP1 attacks on zero sessions are also possible with Windows Vista.