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In a Microsoft environment, the Security Identifier (SID) is a unique value assigned to a user, group, and accounts. The SID is issued by a security authority, which in most cases is the domain controller, and is pulled from the Active Directory database. At login, an access token is generated. It is composed of the SID with group SIDs and a list of privileges, as illustrated in the following figure:
Figure 3. Microsoft user access token
The SID is written in the following format: (SID)-(revision level)-(identifier-authority)-(subauthority1)-(subauthority2)-(etc)
As an example, consider the following SID: S-1-5-21-1004336348-1177238915-682003330-512
For more information about SIDs, see the following Microsoft article: What Are Security Identifiers?