Content Security Policy (CSP) in SharePoint Online: What It Is and Why It Matters

Microsoft is rolling out stronger Content Security Policy (CSP) enforcement for SharePoint Online starting March 1, 2026. Today, SharePoint can run CSP in a reporting mode where nothing is blocked yet. Instead, potential issues are logged so organizations can identify and address risky patterns before enforcement begins.

This is a positive shift for the SharePoint ecosystem. CSP is one of the most practical browser-level protections available, and it helps reduce the impact of common web attacks by limiting what a page is allowed to load and execute.

TABLE OF CONTENTS

• What is Content Security Policy (CSP)
• Why you should care about CSP
• Examples of real attacks CSP helps prevent or reduce
  • Not technical? Here is what this means in plain terms
• What is changing in SharePoint Online
• How ShortPoint is Preparing
• What customers can do today
• Final takeaway

What is Content Security Policy (CSP)

Content Security Policy is a set of rules that a website sends to your browser. It defines what the page is allowed to load and run. These rules typically focus on resources such as JavaScript, CSS, images, frames, and connections to external services.

In simple terms, CSP answers questions like:

• Which websites are allowed to provide scripts for this page?
• Are inline scripts allowed?
• Can this page be embedded inside an iframe on another website?
• Can the page load images or fonts from external domains?

When CSP is enforced, the browser blocks anything that violates the policy. When CSP is in report-only mode, the browser reports violations without blocking them, which is useful for testing and remediation.

Why you should care about CSP

Even well-built applications can be exposed through a combination of edge cases: user-generated content, third-party libraries, complex integrations, or unexpected HTML injection paths. CSP provides an extra layer of protection by letting the browser refuse risky behavior even if a bug slips through. This approach is called defense in depth. You still validate inputs and follow secure coding practices, but you also use browser enforcement to reduce the blast radius if something goes wrong.

Examples of real attacks CSP helps prevent or reduce

Microsoft references threats like script injection and related risks. Here are practical examples of what CSP is designed to make harder in day-to-day web environments.

1) Cross-site scripting (XSS) that loads a malicious file

If an attacker manages to inject HTML into a page, they may try to load a script from a domain they control. With CSP, you can restrict scripts to a small list of trusted sources, which prevents the browser from fetching and running scripts from unknown domains.

2) Inline script injection

Inline scripts are a frequent target for attackers. CSP can block inline JavaScript by default unless it is explicitly allowed using stronger mechanisms like nonces or hashes. This is one of the biggest improvements CSP brings because it breaks many classic injection techniques.

3) Inline event handlers and javascript: links

Patterns like onclick="..." or links that start with javascript: can become injection surfaces if untrusted input is introduced into HTML. CSP can restrict these behaviors and push code toward safer patterns, like adding event listeners in code instead of embedding code in HTML.

4) Risky dynamic code execution

Some legacy patterns and libraries rely on APIs like eval(), new Function(), or string-based setTimeout(). CSP can block these unless a tenant explicitly allows them, which is generally discouraged for security reasons.

5) Clickjacking (UI trickery via iframes)

Clickjacking happens when a site is embedded inside an iframe, and a user is tricked into clicking something they do not intend to. CSP can help reduce this risk by controlling which sites are allowed to embed your pages using frame-ancestors.

6) Safer defaults for transport security

CSP can also help organizations tighten mixed-content behavior, for example, by upgrading insecure HTTP requests to HTTPS under certain directives. This contributes to a stronger security posture across complex pages.

Not technical? Here is what this means in plain terms

You do not need to be a developer to understand the core idea: many modern attacks are not about “breaking into servers”, they are about tricking a web page into running something it should never run. If an attacker finds a way to get their content onto a page (through a vulnerable form, a URL parameter, embedded content, or a third-party component), they may be able to make a visitor’s browser treat that content as trusted code.

When that happens, the attacker is effectively riding on the trust your users already have in your site. That can enable things like capturing clicks or keystrokes, reading information displayed on the page, or silently performing actions in the background as if the user did them. Content Security Policy reduces the risk of this entire category of attacks by telling the browser which script sources are allowed, and by blocking common execution patterns used in injection attempts, even if something suspicious manages to slip into the page.

What is changing in SharePoint Online

SharePoint Online is tightening its CSP enforcement with special focus on script execution and trusted script sources.

Key highlights:

• Enforcement begins March 1, 2026.
• Tenants can optionally delay enforcement by 90 days to June 1, 2026 using SharePoint Online Management Shell.
• SharePoint is designed to work smoothly with standard SPFx packaging patterns, including common approaches like loading your solution bundle from an SPFx CDN base path or defining external libraries through SPFx externals.
• The main area of concern for many solutions is any pattern that relies on inline scripts or dynamically injecting script tags into the page.

Microsoft also provides helpful ways to evaluate impact ahead of time, including reviewing CSP violation reports and testing enforcement behavior on demand.

How ShortPoint is Preparing

We support Microsoft’s direction here. CSP enforcement is an ecosystem-wide security improvement for SharePoint customers, and we are aligning ShortPoint with SharePoint Online’s CSP model ahead of the enforcement deadline.

1) Consolidating scripts under the SPFx CDN base path

We are updating our scripts to consistently fall under the same SPFx CDN base path used in the SPFx manifest configuration. This improves compatibility with SharePoint’s trusted script source model and ensures our scripts follow the standard SPFx delivery expectations.

2) Updating Custom Code areas to avoid script tag injection

We are rewriting Custom Code areas so they do not inject script tags into the page. The goal is to use CSP-friendly execution patterns that work cleanly with SharePoint Online enforcement while keeping customization flexible and maintainable.

3) Standardizing dynamic script loading using SPFx SPComponentLoader

In cases where scripts must be loaded conditionally at runtime, we are standardizing on the SPFx-recommended approach by using SPComponentLoader.loadScript() instead of injecting <script> tags or relying on ad-hoc loaders.

await SPComponentLoader.loadScript('https://some-external-site/script.js');

While consolidating scripts under the SPFx CDN base path is often sufficient for CSP alignment, updating all dynamic loaders to use the platform-native SPFx API gives us a more consistent and future-proof model. It improves predictability under CSP enforcement, reduces reliance on DOM-based script injection patterns, and keeps our loading strategy aligned with Microsoft’s guidance for SPFx solutions.

What customers can do today

If you want to proactively validate your environment, here are a few practical steps:

• Review CSP violation reports in the browser console on pages where you use custom solutions.
• Check Microsoft Purview audit logs for CSP violation events where applicable.
• Use Microsoft’s recommended testing approach to see how enforcement would behave on key pages before the deadline.

Final takeaway

CSP is not a signal that something is wrong. It is a modern security standard that raises the baseline for everyone by making common browser-side attack paths significantly harder to exploit.

ShortPoint is aligning with SharePoint Online’s CSP enforcement approach in a proactive way, so customers can continue to use ShortPoint confidently as Microsoft moves toward enforcement starting March 1, 2026.