Security Identifier

In the context of the Microsoft Windows NT line of operating systems, a Security Identifier (commonly abbreviated 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";

S 1 5 21-3623811015-3361044348-30300820 1013
The string is a SID. The revision level (the version of the SID specification). The identifier authority value. Subauthority value
In this case, a domain (21) with a unique identifier
A Relative ID (RID). Any group or user that is not created by default will have a Relative ID of 1000 or greater.

Identifier Authority Values

Known identifier authority values are:[1]

Decimal Name Display Name First Introduced References Notes
0 Null Authority e.g. "Nobody"
1 World Authority (not shown) e.g. well known groups such as "Everyone".
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
9 Resource Manager Authority Windows Server 2003 [2][3]
11 Microsoft Account Authority MicrosoftAccount\ Windows 8 [4]
12 Azure Active Directory AzureAD\ Windows 10
16 Mandatory Label\ Windows Vista Used as part of Mandatory Integrity Control

Machine SIDs

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).[5] (Some sources state that it is stored in the SAM hive instead.) A backup is located at SECURITY\Policy\PolAcDmS\@.

NewSID ensures that this SID is in a standard NT 4.0 format (3 32-bit subauthorities preceded by three 32-bit authority fields). Next, NewSID generates a new random SID for the computer. NewSID's generation takes great pains to create a truly random 96-bit value, which replaces the 96-bits of the 3 subauthority values that make up a computer SID.

NewSID readme

The machine SID subauthority format is used for domain SIDs too. A machine is considered its own local domain in this case.

Decoding Machine SID

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.

1) Divide the bytes into 3 sections:
2E,43,AC,40 - C0,85,38,5D - 07,E5,3B,2B
2) Reverse the order of bytes in each section:
40,AC,43,2E - 5D,38,85,C0 - 2B,3B,E5,07
3) Convert each section into decimal:
1085031214 - 1563985344 - 725345543
4) Add the machine SID prefix:

Other Uses

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

Service SIDs are a feature of service isolation, a security feature introduced in Windows Vista and Windows Server 2008.[6] 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-{SHA-1(service name in upper case)}

The sc.exe utility can be used to generate an arbitrary service SID:

sc.exe showsid dnscache

NAME: dnscache
SERVICE SID: S-1-5-80-859482183-879914841-863379149-1145462774-2388618682
STATUS: Active

The service can also be referred to as NT SERVICE\<service_name> (e.g. "NT SERVICE\dnscache").

Duplicated SIDs

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 is 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.[7] 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.[8]

At present, the only supported mechanism for duplicating disks for Windows operating systems is through use of SysPrep, which generates new SIDs.

See also


  1. "Well-known security identifiers in Windows operating systems". Retrieved 12 December 2019.
  2. See "Custom Principals" section on
  4. "Example impact of Microsoft Accounts on Windows APIs in Windows 8/8.1 – Windows SDK Support Team Blog".
  5. "MS TechNet NewSID Utility - How It Works". Knowledge Base. Microsoft. November 1, 2006. Retrieved 2008-08-05.
  6. "Windows Service Isolation Feature". Article. Windows IT Pro. June 6, 2012. Retrieved December 7, 2012.
  7. "NewSID v4.10". Windows Sysinternals. Microsoft. 2006-11-01.
  8. Russinovich, Mark (2009-11-03). "The Machine SID Duplication Myth". TechNet Blogs. Microsoft.
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