Security Best Practices for Office Add-ins Development

Introduction

Office add-ins run inside trusted productivity apps and often handle sensitive business data. Treating security as a first-class concern protects both your users and your reputation.

The Add-in Threat Model

Add-ins are essentially web apps embedded in Office. That means they inherit web threats — XSS, token leakage, insecure transport — plus host-specific risks around the data you can read from documents and mail.

Enforce HTTPS Everywhere

Office only loads add-in resources over HTTPS. Extend that discipline to every API call, and add a strict Content Security Policy to limit where scripts and connections can originate.

<meta
  http-equiv="Content-Security-Policy"
  content="default-src 'self'; connect-src 'self' https://api.yourdomain.com;"
/>

Authentication & Tokens

Prefer the modern identity flows. Office Single Sign-On (SSO) via getAccessToken issues a token for the signed-in Microsoft 365 user without a separate login prompt.

const token = await Office.auth.getAccessToken({
  allowSignInPrompt: true,
});
// Send this token to your backend; validate it there before trusting it.

Handling User Data

• Collect the minimum data required and document why you need it.
• Encrypt data in transit and at rest.
• Avoid storing secrets in client code or the manifest.
• Provide a clear privacy policy and honor deletion requests.

Securing Your Manifest

The manifest declares the URLs and permissions your add-in uses. Keep AppDomains tight, request the lowest permission level that works, and review it as part of every release.

Conclusion

Security is not a one-time checkbox. Bake HTTPS, server-side token validation, input sanitization, and least-privilege permissions into your workflow, and your add-ins will be ready for even the most demanding enterprise reviews.