How to Build a Permissions-Policy HTTP Header

A site that doesn’t need camera access, microphone access, or geolocation still has those browser features available by default unless explicitly restricted — and an embedded third-party script or iframe could potentially request them if nothing’s stopping it. The Permissions-Policy header exists specifically to close that gap, letting a site explicitly declare which browser features are allowed and which are blocked, for both the top-level page and anything embedded within it.

Writing this header’s syntax correctly by hand means getting the directive names, the allowlist syntax, and the overall header structure exactly right — a specific format documented by MDN that’s easy to get subtly wrong without regular practice writing it.

What building a correct Permissions-Policy header actually involves

The Permissions-Policy header lets a site specify, feature by feature, whether things like camera, microphone, and geolocation access are allowed at all, and if so, for which origins. Each feature gets its own directive with an allowlist specifying which sources can use it — an empty allowlist blocks the feature entirely, while a specific list of origins restricts it to just those sources. Getting the syntax right matters because a malformed header doesn’t necessarily fail loudly; browsers may simply ignore a header they can’t parse correctly, silently leaving the intended restriction unenforced rather than throwing an obvious error that would catch the mistake immediately. Building this correctly for several features at once, each with its own block-or-allow decision, means tracking multiple directives and their syntax together without a mistake in one affecting the others.

Getting this header right specifically matters for reducing the attack surface a site exposes — even if the top-level page doesn’t misuse a given browser feature, restricting what embedded or third-party content can access limits what a compromised or malicious embed could actually do.

Why people get stuck here

• Permissions-Policy syntax isn’t something most developers write regularly. The specific directive and allowlist format is easy to get subtly wrong without frequent hands-on practice with the header.
• A malformed header can fail silently. Browsers that can’t parse a broken header may simply ignore it, meaning the intended restriction never actually takes effect without any obvious error signal.
• Building the header for multiple features at once means tracking several directives correctly together. Camera, microphone, geolocation, and other features each need their own correctly formatted directive within the same header value.
• Getting this wrong undermines a real security benefit without being obvious that anything’s wrong. A site can believe it’s restricted certain browser features when a malformed header actually left them unrestricted.

What a good Permissions-Policy builder looks like

Generates correctly formatted directives for each feature

Producing valid syntax for every chosen feature is what actually makes the resulting header take effect as intended, rather than failing silently.

Lets you choose block or allow per feature clearly

A straightforward choice for each browser feature makes building a header covering several features at once manageable, rather than error-prone.

Produces a header ready to deploy directly

A correctly formatted, complete header value that can be added directly to your server configuration removes the need to hand-assemble the syntax yourself.

Common mistakes to avoid

• Hand-writing Permissions-Policy syntax without regular experience with the header’s specific directive and allowlist format.
• Assuming a header that doesn’t throw an error is therefore correctly restricting the intended browser features.
• Building a multi-feature header manually and introducing a syntax mistake in one directive that affects the rest.
• Not testing that a deployed Permissions-Policy header actually restricts access as intended.

How to do it with Permissions Policy Builder

Online Tool Store’s Permissions Policy Builder lets you choose block or allow for browser features like camera and geolocation to generate a valid Permissions-Policy header, entirely in your browser.

1. Choose block or allow for each browser feature you want to configure.
2. Let it generate the correctly formatted header value.
3. Review the complete Permissions-Policy header.
4. Add it to your server configuration or response headers.

Because the header syntax is generated correctly for every chosen feature, you get a header that actually takes effect as intended, without the risk of a silent parsing failure from hand-written syntax.

Frequently asked questions

What happens if my Permissions-Policy header has a syntax mistake?

Browsers that can’t parse a malformed header may simply ignore it entirely, which means the intended restriction never actually takes effect, often without any obvious error to catch the problem.

Why would I want to block a browser feature my site doesn’t use?

Restricting features your site doesn’t need reduces the attack surface available to embedded or third-party content, limiting what a compromised or malicious embed could potentially access even if your own code never misuses that feature.

Can I configure multiple browser features in one header?

Yes — the header can include a separate directive for each feature, like camera, microphone, and geolocation, each with its own block-or-allow configuration within the same header value.

Final thought

A Permissions-Policy header only protects what it actually restricts, and getting its specific syntax right by hand is easy to get subtly wrong. Build it correctly, and close off browser features your site genuinely doesn’t need.

Try the free Permissions Policy Builder