Office Settings & Configuration

How to Fix Content Security Policy Errors in Excel Web Add-Ins

Kushani Nimanthika Sep 25, 2026 869 views

Question details

The user needs to resolve Content Security Policy (CSP), CORS, frame-ancestors, and connect-src errors preventing a React-based Excel web add-in from functioning when hosted in a Rails application.

How to Fix Content Security Policy Errors in Excel Web Add-Ins

Product: Microsoft Excel
Device & OS: not provided
Scenario: Developing and running a React-based Excel web add-in that relies on a Rails backend for hosting and API responses.
Observed behavior: The Excel add-in fails to load or execute properly due to blocked requests, triggering Content Security Policy and CORS errors in the browser.

Before you start

Before troubleshooting, ensure you have access to your Rails application's web server configuration and the browser developer tools to inspect network requests.

Solution 1Recommended

Configure Rails Response Headers and CORS Settings

Adjust your Rails server configuration to allow trusted origins and required Microsoft endpoints.

Content Security Policy and CORS errors generally stem from strict server-side rules. You must explicitly instruct your Rails application to accept connections from your Excel add-in and permit Office to frame your content.

Inspect Blocked Requests

Open your browser's developer tools (F12) and navigate to the Console and Network tabs. Identify the first blocked request to see the exact CSP or CORS error.

Update CORS Configuration

In your Rails application, update the CORS middleware settings to allow cross-origin requests from the exact domains where your add-in is hosted.

Configure Content Security Policy

Modify the Rails response headers to properly set the frame-ancestors and connect-src directives, ensuring Microsoft Office endpoints (like officeapps.live.com) are permitted.

Verify Manifest Domains

Check your Excel add-in's XML manifest file to ensure all required domains, including your Rails app URL, are listed under the <AppDomains> element.

Server Restart Required: Remember to restart your Rails server after making changes to the CORS or CSP configuration for the new headers to take effect.

Solution 2

Request Specialist Assistance on Microsoft Learn

Since this is an advanced development issue, consult Microsoft's developer community for tailored guidance.

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Frequently Asked Questions

What causes Content Security Policy errors in Excel web add-ins?

These errors occur when the web server hosting the add-in restricts which resources can be loaded or framed. If the server's policy doesn't explicitly allow Microsoft Office endpoints or the domains required by your React application, the browser will block the add-in from loading.

How do I fix the frame-ancestors directive error?

You need to update your server's Content Security Policy headers. Ensure the frame-ancestors directive includes the domains where Excel on the web is hosted (such as https://*.officeapps.live.com) so the add-in can be rendered properly inside an iframe.

Can CORS issues prevent my Excel add-in from connecting to my Rails API?

Yes. Cross-Origin Resource Sharing (CORS) restricts web applications from making requests to a different domain. You must configure your Rails backend to explicitly accept requests from the origin where your Excel web add-in is hosted.

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