Logging Out Without Compromising System Security
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Logging out of a session conserves system resources and protects your files. Leaving a terminal on line represents one of the greatest sources of inside intrusions. When you leave your terminal on line and your office open, you have effectively given away your password and your privileges and have left your files and those of the other members of your group unprotected. Any user can easily and quickly transfer all files accessible through your account. A malicious insider could rename and delete your files and any other files to which you have write access. If you have special privileges, especially privileges in the Files or All category, a malicious user can do major damage.
Log out when you leave your office even for a brief period of time. If you have performed remote logins, you must log out of each node. The following sections describe security considerations for logging out of specific types of terminals or sessions.
You may want to clear your screen each time you log out from a terminal to ensure that your user name, node name, and operating system are not revealed to anyone else. If you are logging out after a remote login, the name of the node to which you return (the local node) is also revealed. If you access multiple accounts remotely (over the network), the final sequence of logout commands reveals all the nodes and user names that are accessible to you on each node (excluding the name of the furthest node reached). To those who can recognize the operating system from the prompt or a logout message, these displays also reveal the operating system.
At some sites, it may be important to leave nothing but the logout message on your screen, as follows:
If you are using a VT100-series terminal, press the Set-Up key. Then press the key marked for reset (the 0 key) followed by the Return key.
Alternatively, to preserve temporary parameters, press the Set-Up key, and then press the key marked 80/132 columns (the 9 key) twice.
After the screen clears, the cursor is positioned at the top of the screen, next to the DCL prompt. Enter the DCL command LOGOUT at the prompt. The only information remaining after you log out is your logout command and the logout completion message, for example:
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After you log out from a hardcopy terminal, properly remove, file, or dispose of all hardcopy output that might reveal sensitive information. Your security administrator should provide direction on preferred procedures. Many sites use paper shredders or locked receptacles for this purpose. Handle output that you plan to save just as carefully.
You should also dispose of hardcopy output if the system fails before you log out. In addition, if you will not be present when the system is initialized, turn your terminal off.
The system automatically removes your disconnected processes after a certain interval. You can conserve system resources, however, if you directly log out of any disconnected processes, as follows:
Enter the DCL command SHOW USERS to determine if you have other disconnected jobs.
Enter the DCL command CONNECT/LOGOUT to log out of the current process. Connect back through each of the associated virtual terminals (as noted by the terminal prefix of VTA) until you reach the last existing process.
Enter the DCL command LOGOUT.
Your security administrator may ask you to break the connection to a dialup line when you log out. If you anticipate no further immediate use of the line, use the LOGOUT command with the /HANGUP qualifier. The /HANGUP qualifier directs the system to automatically break the connection to the dialup line after you log out.
Breaking the connection to a dialup line prevents someone from taking advantage of an open access line. To access the line, someone must know the access number and must personally redial. Breaking the connection is especially important if the dialup line you use is in a public area or where someone might use the terminal after you.
This practice also saves resources by reducing the required number of dialup lines.
If your site has moderate or high security requirements, your security administrator may ask you to turn off your terminal after logging out. This resets terminal characteristics and clears memory buffers. Some Trojan horse attacks use hardware frame buffers and the answerback capabilities that are built into newer terminals.