Enforcing SID filtering over external trusts in Active Directory

SID filtering keeps your security identifiers from being passed to untrusted domains, this explains how to make sure it is turned on.

Security identifier (SID) filtering is the default process by which a domain controller automatically prevents

SIDs from any domain other than the specific trusted domain pair from being exchanged over the external trust link. SID filtering is enabled by default on Windows 2003 Server domain controllers as well as Windows Server 2000 SP4+ domain controllers.

However, a Windows Server 2000 domain controller with SP3 or earlier, who is a participant in an external trust, does not have SID filtering enabled by default. Also, even if a later SP is applied, SID filtering remains disabled by default.

If you suspect that SID filtering is not enabled (for any reason) and want to turn it on, follow this procedure:

  1. Verify that an external trust already exists.
  2. Log into the domain controller as a member of the Domain Admins for the trusting domain
  3. Type in the following command at a command prompt (or use via a script): netdom trust trusting domain name /domain:trusted domain name /userO:user_name [/passwordO:*] /Quarantine:yes

Be sure to replace the two domain name placeholders with the appropriate domain names. Also, using the asterisks as the password prevents the password from appearing in clear text in the command window or script. Plus, it prompts the user for the password, which is protected during the input process by displaying only asterisks.


James Michael Stewart is a partner and researcher for ITinfopros, a technology-focused writing and training organization.


This was first published in August 2004

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