System settings: Optional subsystems
Applies to
- Windows 11
- Windows 10
Describes the best practices, location, values, policy management, and security considerations for the System settings: Optional subsystems security policy setting.
Reference
This policy setting determines which subsystems support your applications. You can use this security setting to specify as many subsystems as your environment demands.
The subsystem introduces a security risk that is related to processes that can potentially persist across logons. If a user starts a process and then signs out, the next user who signs in to the system might access the process that the previous user started. This pattern is dangerous, because the process started by the first user can retain that user's system user rights; therefore, anything that the second user does using that process is performed with the user rights of the first user. This privileges rollover makes it difficult to trace who creates processes and objects, which is essential for post-security incident forensics.
Possible values
- User-defined list of subsystems
- Not defined
Best practices
- Set this policy setting to a null value. The default value is POSIX, so applications that rely on the POSIX subsystem will no longer run. For example, Microsoft Services for UNIX 3.0 installs an updated version of the POSIX subsystem. Reset this policy setting in Group Policy for any servers that use Services for UNIX 3.0.
Location
Computer Configuration\Windows Settings\Security Settings\Local Policies\Security Options
Default values
The following table lists the actual and effective default values for this policy. Default values are also listed on the policy’s property page.
| Server type or GPO | Default value |
|---|---|
| Default Domain Policy | Not defined |
| Default Domain Controller Policy | Not defined |
| Stand-Alone Server Default Settings | POSIX |
| DC Effective Default Settings | POSIX |
| Member Server Effective Default Settings | POSIX |
| Client Computer Effective Default Settings | POSIX |
Policy management
This section describes features and tools that are available to help you manage this policy.
Restart requirement
None. Changes to this policy become effective without a device restart when they're saved locally or distributed through Group Policy.
Security considerations
This section describes how an attacker might exploit a feature or its configuration, how to implement the countermeasure, and the possible negative consequences of countermeasure implementation.
Vulnerability
The POSIX subsystem is an Institute of Electrical and Electronic Engineers (IEEE) standard that defines a set of operating system services. The POSIX subsystem is required if the server supports applications that use that subsystem.
The POSIX subsystem introduces a security risk that relates to processes that can potentially persist across sign-ins. If a user starts a process and then signs out, there's a potential that the next user who signs in to the computer could access the previous user's process. This accessibility would allow the second user to take actions on the process by using the privileges of the first user.
Countermeasure
Configure the System settings: Optional subsystems setting to a null value. The default value is POSIX.
Potential impact
Applications that rely on the POSIX subsystem no longer operate. For example, Microsoft Services for UNIX (SFU) installs an updated version of the POSIX subsystem that is required, so you must reconfigure this setting in Group Policy for any servers that use SFU.