Q: What’s the reason for the existence of the Windows Guest account? And, more important, how can I secure it?

A: The Guest account is a low-privilege Windows account that exists on every Windows system. It's available to users that don’t have an account defined and need occasional access to a Windows system. Users logging on using the Guest account can access local data and applications but can't install software or hardware. By default, the Guest account is disabled and isn't password-protected. It also has the “User cannot change password” and “Password never expires” account properties set.

You must secure the guest account. Even though it has a limited set of privileges, malicious anonymous users can use it to access system resources. Here is some advice on how to secure the guest account.

  • Disable the guest account if you don’t use it. The guest account is disabled by default, but it might be worthwhile to double-check this. When you enable the Guest account occasionally, make sure that you disable it when it's not being used.
  • Password-protect the guest account. Windows Server 2003 and Windows XP include important restrictions to limit what anonymous users can do on a Windows system, so it also makes sense to password-protect the Guest account.

    To password-protect the guest account in XP, you must first password-enable the Guest account. You can do this from the Microsoft Management Console (MMC) Local Users and Groups snap-in. On standalone Windows 2003 and Windows 2000 machines, the Guest account is password-enabled by default, but it has a blank password. The same is true for Win2K and Windows 2003 domain environments. On standalone Windows 2003 and Win2K platforms, you can assign the guest account a password from the Local Users and Groups snap-in. In domain environments you must use the MMC AD Users and Computers snap-in.

    You don’t need to enable the Guest account to password-enable it or set its password. Also make sure that you assign a strong password and update it regularly (so you might want to remove the “Password never expires” default property of the Guest account.

  • Rename the guest account. Although this approach is security by obscurity (the guest SID always ends in 501), it can be a simple but effective measure to protect the Guest account. On systems where anonymous enumeration isn't allowed, renaming the Guest account forces hackers to guess both the Guest account’s name and password.
  • Prevent Guest account network logon. Make sure that the Guest account is given the “Deny Access to this Computer from the Network” user right.
  • Prevent a Guest account from shutting down the system. By default the guest account is prohibited from doing this, but to make sure, you can double-check the following:
    - The Guest account doesn't have the “Shut down the system” user right.
    - Systems must prohibit system shut down without logging on. You can control this by using the following Security Option in the local security policy or Group Policy Object (GPO) settings: “Shutdown: Allow System To Be Shut Down Without Having To Log On”.
  • Prevent a Guest account from accessing the system logs. To do so, open the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Eventlog registry key and check each of the Application and System subkeys to make sure they contain a REG_DWORD value named RestrictGuestAccess that's set to a value of 1. You don’t need to make this change for the security log: the Guest account is prohibited from accessing this log by default.

You should make these changes even if the guest account is disabled. Even though hackers with administrator access can easily enable the Guest account, it will make their lives and attacks more difficult if they need to change multiple security settings. Also remember security’s most fundamental security principle--defense in depth--which means you shouldn't rely on a single security solution but rather a combination of different security solutions.

Learn more from "Use Guest Accounts to Fight Malware" and  "How do I set a password for the Windows XP Guest account?"