Fix Word VSTO Add-In Certificate Not Trusted Error on Windows 11
Question details
The user is unable to install a ClickOnce VSTO Word add-in on Windows 11 due to a certificate trust error.
- Product
- Microsoft Word (VSTO Add-in)
- Device & OS
- Windows 11
- Scenario
- Installing a third-party or custom ClickOnce VSTO add-in for Microsoft Word.
- Observed behavior
- The installation fails with a System.Security.SecurityException, stating that the certificate used to sign the deployment manifest is not trusted. Reinstalling Office does not resolve the issue.
Ensure you have administrator privileges on your Windows 11 computer, as modifying the system's certificate trust store requires elevated permissions.
Install and Trust the Add-in Publisher Certificate
Manually add the publisher's certificate to your local machine's trusted certificate store to bypass the security exception.
Windows 11 blocks ClickOnce VSTO add-ins if the deployment manifest is signed by a certificate that is not present in the Trusted Publishers or Trusted Root Certification Authorities store. Manually installing this certificate establishes the necessary trust.
Right-click the add-in setup file or the associated .cer file and select 'Properties'. Navigate to the 'Digital Signatures' tab, select the signature from the list, and click the 'Details' button.
In the Digital Signature Details window, click on 'View Certificate' to open the certificate information.
Click 'Install Certificate'. When the Certificate Import Wizard opens, select 'Local Machine' as the store location and click 'Next'.
Choose 'Place all certificates in the following store', click 'Browse', and select 'Trusted Root Certification Authorities'. Click 'OK', then 'Next', and finally 'Finish'.
Restart your computer to ensure the certificate store updates are applied, then attempt to install the Word VSTO add-in again.
Contact Microsoft Support or IT Administrator
If the certificate is expired, revoked, or managed by strict organizational group policies, live support or IT intervention is required.
Try WPS Office for a Hassle-Free Document Experience
If complex Microsoft Word add-in errors, VSTO deployments, and certificate trust issues are disrupting your workflow, consider switching to WPS Office. It provides a lightweight, highly compatible, and easy-to-use alternative without the heavy configuration requirements of ClickOnce deployments.
- 1. Download the Installer: Visit the official WPS website and download the free version of WPS Office.
- 2. Install WPS Office: Run the lightweight installer and follow the on-screen instructions to complete the setup in minutes.
- 3. Open Your Documents: Launch WPS Writer and open your existing Word documents seamlessly without worrying about add-in trust errors.
Frequently Asked Questions
Why does reinstalling Office not fix the VSTO certificate error?
Reinstalling Microsoft Office only replaces the core application files. It does not modify the Windows certificate store or update the ClickOnce trust prompt settings, which are responsible for validating the add-in's publisher.
What is a System.Security.SecurityException in Word?
This exception occurs when the .NET Framework's security policy prevents code from running. In the context of VSTO add-ins, it usually happens because the application's deployment manifest is signed with an untrusted, expired, or revoked digital certificate.
How do I check if a certificate is revoked on Windows 11?
Open the Run dialog by pressing Windows Key + R, type 'certmgr.msc', and press Enter. Navigate to the Untrusted Certificates or Revoked Certificates folders in the left pane to see if the add-in's publisher is listed there.
