[DevBlog MS] Microsoft is updating its author-signing certificate starting September

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Action required: If you validate that packages are author-signed by Microsoft using a NuGet client policy or the

Code:

dotnet nuget verify

command, follow the steps in this post as soon as possible to avoid potential disruptions during the transition. If you are unsure whether you are impacted, follow the steps below to check.

Microsoft uses an X.509 certificate to author-sign its NuGet packages. As soon as September 23, 2026, a new certificate will become the default Microsoft author-signing certificate for NuGet packages. Existing packages signed with an older certificate will retain their signatures, but the current certificate will no longer be used to sign new packages after the transition.

Current certificate SHA-256 fingerprint:

Code:

566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353

New certificate SHA-256 fingerprint:

Code:

9A1B131BEE0605433056A4EA3815478A8E177961A968C6C0027C1093D1FEB630

Who will be impacted?

• Customers who use a NuGet client policy to enforce an allow list of trusted signers that includes Microsoft.
  
  

To determine whether you have a NuGet client policy configured, check for the following elements in your

Code:

nuget.config

. Keep in mind that

Code:

nuget.config

files can exist in multiple locations with different scopes.

Code:

[code]<config> <add key="signatureValidationMode" value="require" /> </config> <trustedSigners> <author name="Microsoft"> <certificate fingerprint="3F9001EA83C560D712C24CF213C3D312CB3BFF51EE89435D3430BD06B5D0EECE" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> <certificate fingerprint="AA12DA22A49BCE7D5C1AE64CC1F3D892F150DA76140F210ABD2CBFFCA2C18A27" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> <certificate fingerprint="566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> </author> </trustedSigners>[/code]

• Customers who use
  Code:

  dotnet nuget verify
  to verify that signed packages are author-signed by Microsoft.
  
  

This may look like the following:

Code:

[code]dotnet nuget verify <PackagePath> --certificate-fingerprint 3F9001EA83C560D712C24CF213C3D312CB3BFF51EE89435D3430BD06B5D0EECE --certificate-fingerprint AA12DA22A49BCE7D5C1AE64CC1F3D892F150DA76140F210ABD2CBFFCA2C18A27 --certificate-fingerprint 566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353[/code]

If neither scenario applies to you, you should be unaffected by this certificate update. Microsoft NuGet packages signed with the new certificate should install in the same way as packages signed with older certificates.

Allow the new Microsoft certificate

Client policy

If you use a NuGet client policy to enforce an allow list of trusted signers, add the new Microsoft certificate to the allow list as soon as possible. Keep the older Microsoft certificates in the policy so that you can continue to install packages signed with those certificates. If you try to install a package signed with the new certificate without updating your trusted signers, the package installation will fail with an NU3034 error.

You can add the new Microsoft author-signing certificate by running the following command:

Code:

[code]dotnet nuget trust author Microsoft 9A1B131BEE0605433056A4EA3815478A8E177961A968C6C0027C1093D1FEB630 --algorithm SHA256[/code]

The

Code:

dotnet nuget trust

command is available in the .NET 6 SDK and later. It updates the applicable

Code:

nuget.config

file. Use

Code:

--configfile <Path>

to update a specific configuration file.

Alternatively, add the new certificate to the existing Microsoft entry in

Code:

nuget.config

. The resulting entry should include both the older certificates and the new certificate:

Code:

[code]<trustedSigners> <author name="Microsoft"> <certificate fingerprint="3F9001EA83C560D712C24CF213C3D312CB3BFF51EE89435D3430BD06B5D0EECE" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> <certificate fingerprint="AA12DA22A49BCE7D5C1AE64CC1F3D892F150DA76140F210ABD2CBFFCA2C18A27" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> <certificate fingerprint="566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> <certificate fingerprint="9A1B131BEE0605433056A4EA3815478A8E177961A968C6C0027C1093D1FEB630" hashAlgorithm="SHA256" allowUntrustedRoot="false" /> </author> </trustedSigners>[/code]

Package verification

If you use

Code:

dotnet nuget verify

to confirm that a signed package is author-signed by Microsoft, add the new fingerprint while retaining the older fingerprints:

Code:

[code]dotnet nuget verify <PackagePath> --certificate-fingerprint 3F9001EA83C560D712C24CF213C3D312CB3BFF51EE89435D3430BD06B5D0EECE --certificate-fingerprint AA12DA22A49BCE7D5C1AE64CC1F3D892F150DA76140F210ABD2CBFFCA2C18A27 --certificate-fingerprint 566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353 --certificate-fingerprint 9A1B131BEE0605433056A4EA3815478A8E177961A968C6C0027C1093D1FEB630[/code]

Each

Code:

--certificate-fingerprint

option adds an accepted SHA-256 signer certificate fingerprint. Keeping all four values allows the command to verify newly signed packages and existing packages signed with an older Microsoft certificate.

Feedback

If you have questions about how you may be impacted or run into issues while following these steps, please contact us.

For more general NuGet feedback and suggestions:

• Review our guidance for submitting bugs and suggestions.
  
  
• Start a discussion or open an issue in the NuGet/Home repository.
  
  

The post Microsoft is updating its author-signing certificate starting September 23, 2026 appeared first on .NET Blog.