In the context of the Microsoft Windows NT line of operating systems, a Security Identifier (SID) is a unique, immutable identifier of a user, user group, or other security principal. A security principal has a single SID for life (in a given domain), and all properties of the principal, including its name, are associated with the SID. This design allows a principal to be renamed (for example, from "Jane Smith" to "Jane Jones") without affecting the security attributes of objects that refer to the principal.
Windows grants or denies access and privileges to resources based on access control lists (ACLs), which use SIDs to uniquely identify users and their group memberships. When a user logs into a computer, an access token is generated that contains user and group SIDs and user privilege level. When a user requests access to a resource, the access token is checked against the ACL to permit or deny particular action on a particular object.
SIDs are useful for troubleshooting issues with security audits, Windows Server and domain migrations.
The format of a SID can be illustrated using the following example: "S-1-5-21-3623811015-3361044348-30300820-1013":
Known identifier authority values are:[1] [2]
| 0 | Null Authority | e.g. "Nobody" (S-1-0-0) | |||
| 1 | World Authority | (not shown) | e.g. well known groups such as "Everyone". (S-1-1-0) | ||
| 2 | Local Authority | (not shown) | e.g. flag SIDs like "CONSOLE LOGON" | ||
| 3 | Creator Authority | ||||
| 4 | Non-unique Authority | ||||
| 5 | NT Authority | NT AUTHORITY\ | Managed by the NT security subsystem. There are many sub-authorities such as "BUILTIN" and every Active Directory Domain | ||
| 7 | Internet$ | Internet$\ | Windows 7 | ||
| 9 | Resource Manager Authority | Windows Server 2003 | [3] [4] | ||
| 11 | Microsoft Account Authority | MicrosoftAccount\ | Windows 8 | [5] | |
| 12 | Azure Active Directory | AzureAD\ | Windows 10 | ||
| 15 | Capability SIDs | Windows 8Windows Server 2012 | [6] [7] [8] | All capability SIDs begin at S-1-15-3By design, a capability SID does not resolve to a friendly name. The most commonly used capability SID is the following: S-1-15-3-1024-1065365936-1281604716-3511738428-1654721687-432734479-3232135806-4053264122-3456934681 | |
| 16 | Mandatory Label\ | Windows Vista | Used as part of Mandatory Integrity Control | ||
| 18 | Asserted Identity |
Per Microsoft Support: Important - DO NOT DELETE capability SIDS from either the Registry or file system permissions. Removing a capability SID from file system permissions or registry permissions may cause a feature or application to function incorrectly. After you remove a capability SID, you cannot use the UI to add it back.
| Decimal | Name | Display Name | First Introduced | References | Notes | |
|---|---|---|---|---|---|---|
| 18 | LocalSystem | LocalSystem | NT 3.x | Ex: S-1-5-18 is the well-known-sid for LocalSystem | ||
| 19 | LocalService | Local Service | S-1-5-19 is the well-known SID for LocalService | |||
| 20 | NetworkService | Network Service | S-1-5-20 is the well-known SID for NetworkService | |||
| 21 | Domain | |||||
| 32 | Users | Windows 7 | Ex: S-1-5-32-568 is the group ID for IIS_IUSRS | |||
| 64 | Authentication | 10 - NTLM14 - SChannel 21 - Digest | ||||
| 80 | NT Service | NT SERVICE\ | Windows Vista | Can be "Virtual Account NT Service" such as for SQL Server installationsS-1-5-80-0 corresponds to "NT SERVICE\ALL SERVICES" | ||
| 82 | IIS AppPool | AppPoolIdentity\ | Windows 7 | |||
| 83 | Virtual Machines | NT VIRTUAL MACHINE\ | Windows 7 | "NT Virtual Machine\" where is the GUID of the Hyper-V VMS-1-5-83-0 is the group ID for "NT VIRTUAL MACHINE\Virtual Machines" | ||
| 88 | NT NFS | ??? | Windows 2003 | Owner SID for UID | ||
| 90 | Window Manager | Windows Manager Group (DWM) | Windows 7 | Window manager class | ||
| 96 | Font Driver | Windows 7 | Font Driver Host\UMFD-1 |
The machine SID (S-1-5-21) is stored in the SECURITY registry hive located at SECURITY\SAM\Domains\Account, this key has two values F and V. The V value is a binary value that has the computer SID embedded within it at the end of its data (last 96 bits).[11] (Some sources state that it is stored in the SAM hive instead.) A backup is located at SECURITY\Policy\PolAcDmS\@.
The machine SID subauthority format is used for domain SIDs too. A machine is considered its own local domain in this case.
The machine SID is stored in a raw-bytes form in the registry. To convert it into the more common numeric form, one interprets it as three little endian 32-bit integers, converts them to decimal, and add hyphens between them.
| Example | ||
|---|---|---|
| 1) Divide the bytes into 3 sections: | ||
| 2) Reverse the order of bytes in each section: | ||
| 3) Convert each section into decimal: | ||
| 4) Add the machine SID prefix: |
The machine SID is also used by some free-trial programs, such as Start8, to identify the computer so that it cannot restart the trial.
Service SIDs are a feature of service isolation, a security feature introduced in Windows Vista and Windows Server 2008.[12] Any service with the "unrestricted" SID-type property will have a service-specific SID added to the access token of the service host process. The purpose of Service SIDs is to allow permissions for a single service to be managed without necessitating the creation of service accounts, an administrative overhead.
Each service SID is a local, machine-level SID generated from the service name using the following formula:
S-1-5-80-<nowiki/>{SHA-1(service name in upper case encoded as [[UTF-16]])}
The [[sc (command)|sc.exe]] command can be used to generate an arbitrary service SID:
The service can also be referred to as NT SERVICE\
In a Workgroup of computers running Windows NT/2K/XP, it is possible for a user to have unexpected access to shared files or files stored on a removable storage. This can be prevented by setting access control lists on a susceptible file, such that the effective permissions are determined by the user SID. If this user SID is duplicated on another computer, a user of a second computer having the same SID could have access to the files that the user of a first computer has protected. This can often happen when machine SIDs are duplicated by a disk clone, common for pirate copies. The user SIDs are built based on the machine SID and a sequential relative ID.
When the computers are joined into a domain (Active Directory or NT domain for instance), each computer is provided a unique Domain SID which is recomputed each time a computer enters a domain. This SID is similar to the machine SID. As a result, there are typically no significant problems with duplicate SIDs when the computers are members of a domain, especially if local user accounts are not used. If local user accounts are used, there is a potential security issue similar to the one described above, but the issue is limited to the files and resources protected by local users, as opposed to by domain users.
Duplicated SIDs are usually not a problem with Microsoft Windows systems, although other programs that detect SIDs might have problems with its security.
Microsoft used to provide Mark Russinovich's "NewSID" utility as a part of Sysinternals to change a machine SID.[13] It was retired and removed from download on November 2, 2009. Russinovich's explanation is that neither him nor the Windows security team could think of any situation where duplicate SIDs could cause any problems at all, because machine SIDs are never responsible for gating any network access.[14]
At present, the only supported mechanism for duplicating disks for Windows operating systems is through use of SysPrep, which generates new SIDs.
Well-Known SID Structures]
. 2020-09-03. docs.microsoft.com. en-us.