Microsoft 365 Installation Problems

Fix Microsoft 365 Offline Installer Unknown Publisher Warning

Nimra Malik Oct 1, 2026 868 views

Question details

The user is receiving an 'Unknown Publisher' security warning from their browser or operating system when trying to open the Microsoft 365 offline installer.

How to Fix Microsoft 365 Offline Installer Unknown Publisher Warning

Product: Microsoft 365
Device & OS: Windows
Scenario: Downloading and executing the large offline installer image file for Microsoft 365 on a PC.
Observed behavior: Microsoft Edge or Windows displays an 'Unknown Publisher' warning, pausing the installation process and raising security concerns about the file's integrity.

Before you start

Before proceeding, verify that you downloaded the offline installer image directly from your official Microsoft account dashboard, as third-party hosting sites can distribute compromised files.

Solution 1Recommended

Verify the File's Digital Signature

Check the digital certificate of the installer to ensure it was genuinely signed by Microsoft and has not been tampered with.

An 'Unknown Publisher' warning does not automatically mean your computer is compromised. Large .img files can sometimes trigger false alarms in Microsoft Edge or Windows SmartScreen. Verifying the digital signature guarantees the file is safe to run.

Open File Properties

Locate the downloaded Microsoft 365 offline installer file, right-click on it, and select 'Properties' from the context menu.

Check Digital Signatures

Navigate to the 'Digital Signatures' tab. If this tab is missing, the file is not signed and should be deleted immediately.

Verify the Publisher

In the signature list, look for 'Microsoft Corporation'. Select it and click the 'Details' button to ensure the certificate status says 'This digital signature is OK'.

Scan with Microsoft Defender

Right-click the verified file again and select 'Scan with Microsoft Defender' to perform a final security check before double-clicking to run the installation.

Warning: Do not run the installer if the signature is missing, invalid, or belongs to an unexpected publisher. Delete the file and download it again from Microsoft.

Solution 2

Redownload from the Official Microsoft Source

If the file is corrupted or lacks a valid signature, you must download a fresh copy of the offline installer.

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Frequently Asked Questions

Why does Microsoft Edge flag the official Microsoft 365 installer as an unknown publisher?

Microsoft Edge's SmartScreen filter occasionally flags large .img files if it cannot instantly verify the certificate online, or if the download was slightly interrupted, causing the digital signature to be misread.

Can I proceed with the installation if the digital signature is missing?

No. If the digital signature tab is missing from the file's properties, it means the file is corrupted, incomplete, or potentially modified by a third party. Delete the file and download it again from the official website.

How do I check the hash value of the offline installer?

You can use the built-in Windows PowerShell command 'Get-FileHash' followed by the file path. You can then compare this output to the official hash values provided on the Microsoft download page, if available.

Installation & Activation