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Protecting the SAM with Syskey

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Roberta Bragg, co-author of MCSE Training Guide (70-244): Supporting and Maintaining a Windows NT Server 4.0 Network, describes the syskey command and shows you how to protect your Windows NT 4.0 system by using it. Syskey protects the NT system by further encrypting the NT password database and can be used to prevent book without the knowledge of a special system password. Syskey is also used in Windows 2000.
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The Security Accounts Manager (SAM) stores the user passwords in a protected database. The original Windows NT 4.0 database is protected by several techniques:

  • Permissions on relevant Registry keys are set to allow only the operating system access.

  • Permissions on the Registry folders and files are limited. When the system is in operation, the SAM cannot be copied, or accessed directly except by the system and administrators.

  • The passwords are obscured by a one-way function (OWF). This OWF is not decryptable. However, anyone obtaining a copy of the database can use dictionary and brute-force attacks in an attempt to crack or guess the passwords. In a dictionary attack, the same OWF is applied to each word in a dictionary listing, and then the result is compared to the obscured password. A match means the password equals the dictionary word. A brute-force attack compares the password OWF to an OWF of every possible combination of available characters.

Windows NT password-cracking programs have been available for several years. (You can download an evaluation version of the famous LophtCrack tool from http://www.@stake.com.) Although to use them directly against the SAM requires Administrator privileges, a backup of the SAM can be used offline by an attacker. (This is an excellent reason to practice good physical security!) Microsoft developed Syskey to protect the SAM from these types of attacks. Files that support Syskey as well as the program SYSKEY.EXE were incorporated in Service Pack 3 and all later service packs. Syskey uses a 128-bit key to encrypt the password portion of the user database in the SAM. When it was introduced, existing cracking programs could no longer be used to attack the password database.

Unfortunately, additional programs that can be used to provide a crackable database to LophtCrack are now available. These tools, pwdump2 and pwdump3, must be run by a member of the Administrators group in order to be successful. LophtCrack 3.0 does not need these tools; LophtCrack 3.0 can be used directly against a Syskey-protected SAM.

Nevertheless, you should use Syskey to protect the SAM for four reasons:

  • If you use appropriate security practices and limit administrative accounts and require the use of strong passwords, you will mitigate the threat of pwdump2 and Lophtcrack 3.0 being used interactively on your systems. Indeed, if an administrative account has been compromised, there may be little need for cracking passwords in the SAM at all because the administrative account can be used to access any resources protected by DACLs.

  • You have no way of knowing what the attacker is able to deal with, nor what weapons he has in his arsenal. Just because there are armor-piercing bullets should not prevent me from wearing armor if I may be shot at. The bullets fired at me may be of the regular kind, and I will survive the attack.

  • It is always a good idea to layer security on your system. Each problem that you throw in an attacker's way decreases your risk of compromise. If you make attacking your network difficult, many attackers will move on to "lower hanging fruit."

  • For a nonadministrative user to use these tools against your SAM, he must somehow obtain a copy of the SAM and use the tools offline. Good security practices can reduce the possibilities of an attacker obtaining a copy of the SAM. Servers, especially domain controllers, should be physically secured. Emergency Repair Disks and backups of the Registry need to be physically secured. The C:\WINNT\Repair directory (which holds a copy of the Registry when the RDISK program is run to create an ERD) needs to be protected, and Registry files can be removed from this location.

Implementation

The key used to encrypt the passwords is randomly generated by the Syskey utility. This Password Encryption Key (PEK) is itself encrypted with a randomly generated "System" key (Syskey) and stored in the Registry. Encrypting the PEK prevents compromise of the encrypted passwords. If the PEK were stored unencrypted in the Registry, it might be obtained and used to decrypt the passwords. The Syskey must be present for the system to boot. However, now there is a problem: how to protect the Syskey. This protection may be implemented in one of three ways:

  • The Syskey is obfuscated and stored in the Registry. System can boot without administrative action.

  • The Syskey is obfuscated and placed on a floppy disk that must be present when the system reboots. The Syskey is not stored anywhere on the system. The key is stored in a file call STARTKEY.KEY. Do not store the key on an ERD. To do so would be to provide two items needed to attack your system in one location. Do make copies of the disk. Without it you cannot boot your Windows NT system.

  • A passphrase is entered and then used to create encrypt the Syskey. An MD5 cryptographic hash (digest) of the Syskey is stored in the Registry. The password must be entered during system boot to make the system usable.

In either the floppy disk choice or the password choice, the Syskey is not stored anywhere on the system. Therefore, these choices are more secure. If the floppy disk is lost or becomes corrupt, however, or if the password is forgotten, the system cannot be booted.

To apply the additional security provided by using Syskey, follow the procedure listed in Step by Step 1.

STEP BY STEP 1 Applying Syskey Protection to the SAM

  1. Create a backup copy of the Registry prior to completing the additional steps. Be sure to label the backup as pre-Syskey, and store it forever. The only way to recover a Syskey-protected SAM if the Syskey is lost or corrupted is to restore from this pre-Syskey backup of the SAM.

  2. Check the service pack level. Apply the most current service pack. (Service Pack 3 was the first service pack to incorporate Syskey.) Applying the most current service pack adds the code necessary to use Syskey.

  3. If you applied a service pack in step 2, you might want to make another backup of the Registry. Label this one as post-SP and pre-Syskey.

  4. From a command prompt, enter the Syskey.exe command.

  5. In the pop-up window, check the radio button to enable strong encryption.

  6. Select the choice of Syskey operations by selecting the radio button that matches your choice on the windows as shown in Figure 1.

  7. If you have selected to enter a passphrase, do so now.

  8. If prompted, provide a floppy disk.

  9. Click OK.

  10. A pop-up window will indicate success.

  11. Reboot the system.

  12. Make a new backup of the Registry and label it post-Syskey.

  13. Repeat the process for each domain controller (the Syskey is not replicated) or Windows NT 4.0 workstation that is to be protected.

    Figure 1 Selecting the Syskey storage method.

The Syskey program may be used to change the Syskey option, or to generate a new Syskey at a later time. It may not be used to bypass Syskey security. If the key is stored on a floppy disk, the disk must be present. If the key is passphrase-derived, the administrator much know the passphrase before being able to rerun Syskey.

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