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Microsoft will make a new X.509 certificate its default author-signing certificate for NuGet packages on September 23, 2026. Existing packages will keep their current signatures, but teams that restrict trusted Microsoft signers need to update their configuration before installing newly signed packages.
Who needs to act
The change affects customers whose NuGet client policy requires packages to be signed by an allow-listed Microsoft certificate. It also affects teams using dotnet nuget verify with explicit Microsoft certificate fingerprints. Users who do not use either form of signer validation should be unaffected, and Microsoft says packages signed with the new certificate should install in the same way as earlier packages.
The certificate change
Microsoft identifies the current certificate with the SHA-256 fingerprint 566A31882BE208BE4422F7CFD66ED09F5D4524A5994F50CCC8B05EC0528C1353. The replacement fingerprint is 9A1B131BEE0605433056A4EA3815478A8E177961A968C6C0027C1093D1FEB630. The older certificate will no longer be used for new packages after the transition, while older signatures remain valid.
Required configuration updates
Teams using trustedSigners in nuget.config should add the new fingerprint to the existing Microsoft author entry and retain the older certificate values. Microsoft recommends using dotnet nuget trust author Microsoft with the new SHA-256 fingerprint; that command is available in the .NET 6 SDK and later and can target a specific configuration file with –configfile.
Verification failure risk
For direct package checks, the new fingerprint must be added as another –certificate-fingerprint value in dotnet nuget verify, alongside the three older Microsoft fingerprints. Microsoft warns that attempting to install a package signed with the new certificate without updating a required trusted-signer policy will produce an NU3034 error.
Source: Microsoft is updating its author-signing certificate starting September 23, 2026
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