Detecting whether a SID is well-known SID

Raymond Chen

You might think that the Is­Well­Known­Sid function would tell you whether a SID is well-known, but it doesn’t. Rather, it tells you whether a SID exactly matches the well-known SID you specified. For example, you can ask, “Is this the Authenticated Users SID?” or “Is this the Everyone SID?” But you can’t ask, “Is this any type of well-known SID?”

I guess you could enumerate through all the well-known SIDs, and check if your SID matches any of them, but that’s getting kind of ugly.

If what you’re interested in is whether this is a machine-relative SID (or a domain-relative SID, which is the special case where the machine is the domain controller), as opposed to a universal SID, you can check whether the SID format is S-1-5-21-#-#-#-#. All machine-relative SIDs have that form.

#define SECURITY_NT_NON_UNIQUE          (0x00000015L) // decimal 21

If you want to exclude machine\Administrator and other predefined machine-relative SIDs, you can verify that the last number (the RID) is greater than or equal to 1000.



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