Updates the antimalware definitions on a computer.
Networking / Security · Risk: High · Documentation Verified
Last verified: 2026-10-26
Microsoft Defender reference: Updates the antimalware definitions on a computer. The syntax and selected example below are adapted from the pinned Windows Server 2025 reference. Module availability, target configuration and delegated permissions determine whether it can run in your environment. Documentation and parameter text: Microsoft, CC BY 4.0; formatting adapted by Cmdlet World. No execution testing is claimed.
Use an account with the target resource permissions documented for Update-MpSignature. Modifying administrative cmdlets may require elevation or delegated administration. This catalog does not assert that ordinary users can execute the command.
Supported environments
Windows / Defender · Windows Server 2025 documentation; feature/module prerequisites apply
Parameters
-AsJob
Runs the cmdlet as a background job. Use this parameter to run commands that take a long time to complete. The cmdlet immediately returns an object that represents the job and then displays the command prompt. You can continue to work in the session while the job completes. To manage the job, use the *-Job cmdlets. To get the job results, use the Receive-Job cmdlet. For more information about Windows PowerShell background jobs, see about_Jobs.
-CimSession
Runs the cmdlet in a remote session or on a remote computer. Enter a computer name or a session object, such as the output of a New-CimSession or Get-CimSession cmdlet. The default is the current session on the local computer.
-ThrottleLimit
Specifies the maximum number of concurrent operations that can be established to run the cmdlet. If this parameter is omitted or a value of 0 is entered, then Windows PowerShell® calculates an optimum throttle limit for the cmdlet based on the number of CIM cmdlets that are running on the computer. The throttle limit applies only to the current cmdlet, not to the session or to the computer.
-UpdateSource
Specifies an update source. This cmdlet downloads updated definitions from the server you specify. If you do not specify this parameter, the cmdlet uses the sources configured through SignatureFallbackOrder. If no signature fallback order is configured, the cmdlet uses Microsoft Update Server, then Microsoft Malware Protection Center (MMPC) as an update source. The acceptable values for this parameter are: - InternalDefinitionUpdateServer - MicrosoftUpdateServer - MMPC - FileShares If you specify the InternalDefinitionUpdateServer setting, the service checks for updates on the Windows Software Update Services (WSUS) server.
Prerequisites
Windows with the Defender module and the Windows feature or management tools that provide it. Confirm target connectivity and version compatibility using the linked Microsoft reference.
Security considerations
This cmdlet changes system state. Review every target and parameter in the official reference, test outside production, and prepare an appropriate backup or rollback. Do not execute the example automatically.
Examples
Example 1: Update signatures
Update-MpSignature
This command updates the antimalware definitions. By default, the cmdlet uses the sources configured through SignatureFallbackOrder. If no signature fallback order is configured, the cmdlet uses the default update source. Source: Microsoft, CC BY 4.0; transcript prompt/output removed. This example was not executed.