Writing a Permissions Policy: Granting Only the Browser Features You Need
Writing a Permissions Policy is an exercise in least privilege: start from the assumption that no browser feature is granted, and add back only the features the site and its trusted embeds genuinely need. That order matters, because a policy written by copying an example and deleting the lines that look wrong tends to keep grants nobody reviewed, while a policy built from nothing keeps only grants somebody justified. The Permissions Policy Generator produces the header value from the choices you make, so the result can be pasted into the server configuration. It generates text from your inputs; it cannot know which features your site needs, so the decisions remain yours, and the generated policy should be reviewed before it is deployed.
The first step is an inventory, and it is shorter than most people expect. List the features the site's own pages use: a conferencing page needs the camera and microphone, a store checkout may need payment, a mapping page may need geolocation, and most content sites need none of these. Then list the features embedded content needs: a video embed may need fullscreen, a payment embed may need payment, and an advertising embed generally needs no powerful features at all. Anything not on either list is disabled, and anything on the lists is granted only to the origins that need it, never to all origins unless there is a specific reason that survives scrutiny.
How it works
A generator takes the per-feature choices and emits the header syntax: each directive names a feature and an allowlist, which may be empty to disable the feature, the self keyword for the site's own origin, or a list of specific origins. The syntax is unforgiving about details: origins must be quoted correctly, the self keyword has a specific spelling, and a malformed directive may be ignored by the browser, silently leaving the feature at its default. A good generator handles the quoting and formatting, but the author should still read the generated header once, slowly, and check each directive against the inventory, because a formatting tool cannot catch a decision error.
How it works
The order of work should be: generate, review, deploy to a test environment, test the features that should work, and then deploy live. Testing matters because a policy that is too strict breaks real functionality in ways users report vaguely, such as a video that will not go fullscreen or a checkout that cannot open a payment sheet, and those reports are hard to connect to a header change made days earlier. Test each granted feature in the test environment, confirm each disabled feature is genuinely unused, and only then promote the policy to production, where a second check of the live headers confirms it is actually being served.
Worked example
Worked example with sample values. Suppose a sample content site uses no camera, microphone, geolocation or payment features itself, but embeds a video player from one provider that needs fullscreen. The generated policy contains 5 directives: camera disabled, microphone disabled, geolocation disabled, payment disabled, and fullscreen allowed for the site itself and the video provider's origin. That is 4 disabled features and 1 granted feature, with 0 features allowed for all origins. If the site later adds a store checkout needing payment for its own origin, the policy gains 1 directive change, from payment disabled to payment for the site itself, and the count becomes 3 disabled features and 2 granted features. Each change is a single deliberate line, which is the advantage of building the policy from an inventory. These are sample values for illustration.
Worked example
The same example shows how to handle a request for a broad grant. Suppose an embed provider asks for the camera to be allowed for all origins, claiming it is needed for a feature the site does not use. The correct response is to ask which specific origin needs it and for which feature, and to grant only that origin, only that feature, and only after confirming the embed actually uses it. If no specific origin can be named, the grant is refused, because a grant to all origins for an unused feature is pure risk with no benefit. The inventory discipline turns an awkward negotiation into a simple question: which origin, which feature, and why.
Mistakes to avoid
One common mistake is generating a policy and pasting it into the wrong place, such as a meta tag, where it has no effect, instead of the server's response headers, where browsers actually read it. Another is deploying a policy to production without testing, and discovering the breakage from user reports. A third is never revisiting the policy: embeds are added and removed over a site's life, and a policy that was correct at launch drifts into a mixture of stale grants and missing ones. Review the policy whenever embeds change, and at least once a year even if nothing seems to have changed.
Mistakes to avoid
A further mistake is treating Permissions Policy as a substitute for other security controls. It governs browser features, not data access, and it does not authenticate users, validate input, or protect an API. A site with a perfect Permissions Policy and an unauthenticated admin endpoint is still insecure, and a team that celebrates the header while ignoring the endpoint has misallocated its effort. Use the policy as one layer among several, alongside content security policy, secure headers, authentication, and input validation, and keep each layer honest about what it does and does not cover.
Putting it into practice
Build the policy from an inventory, generate the header, review it line by line, test it in a staging environment, verify the live headers after deployment, and record the reason for each grant. When a new embed arrives, add its grants deliberately and remove them when the embed leaves. A policy maintained this way stays short, stays accurate, and stays defensible: anyone who reads it can see exactly which features the site grants, to whom, and, from the accompanying notes, why. That clarity is the real product of the exercise, and it is worth more than the header itself.
Related tools and reading: Related tools and reading: the Cross-Origin Policy Bundle Generator and the Iframe Sandbox Policy Builder cover neighbouring needs that pair naturally with this one, and the guide at [/blog/http-security-headers-guide/] extends the same topic with further background and examples for readers who want to go deeper.
Key points
- Build the policy from an inventory, starting from no grants
- Disable unused features entirely and grant the rest only to named origins
- Test granted features in staging before deploying the policy live
- Permissions Policy governs browser features, not authentication or data access
In short: b.uild the policy from an inventory, starting from no grants
One habit ties all of this together: re-check after every change. A result is only as current as the input it was run on, so a fix, a redesign, or a new version of the underlying material deserves a fresh run rather than reliance on an old report. Keep the previous result alongside the new one, note what changed between them, and treat any new finding as a question to investigate rather than a verdict to accept or dismiss. That loop of checking, fixing, re-checking and recording is what turns a one-off test into a reliable routine, and it costs far less time than debugging a problem that a second check would have caught early.