Prevent CSRF, XSSI, and cross-origin information leaks.
 Lukas Weichselbaum [X](https://twitter.com/we1x) [GitHub](https://github.com/lweichselbaum) [Homepage](https://webappsec.dev)
## Why should you care about isolating your web resources?
Many web applications are vulnerable to [cross-origin](https://web.dev/articles/same-site-same-origin#same-origin-and-cross-origin) attacks like [cross-site request forgery](https://portswigger.net/web-security/csrf) (CSRF), [cross-site script inclusion](https://portswigger.net/research/json-hijacking-for-the-modern-web) (XSSI), timing attacks, [cross-origin information leaks](https://arxiv.org/pdf/1908.02204.pdf) or speculative execution side-channel ([Spectre](https://developer.chrome.com/blog/meltdown-spectre)) attacks.
[Fetch Metadata](https://www.w3.org/TR/fetch-metadata/) request headers allow you to deploy a strong defense-in-depth mechanism---a Resource Isolation Policy---to protect your application against these common cross-origin attacks.
It is common for resources exposed by a given web application to only be loaded by the application itself, and not by other websites. In such cases, deploying a Resource Isolation Policy based on Fetch Metadata request headers takes little effort, and at the same time protects the application from cross-site attacks.
## Browser compatibility
Fetch Metadata request headers are supported in all modern browser engines.
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Site)
## Background
Many cross-site attacks are possible because the web is open by default and your application server cannot easily protect itself from communication originating from external applications.
A typical cross-origin attack is cross-site request forgery (CSRF) where an attacker lures a user onto a site they control and then submits a form to the server the user is logged in to. Since the server cannot tell if the request originated from another domain (cross-site) and the browser automatically attaches cookies to cross-site requests, the server will execute the action requested by the attacker on behalf of the user.
Other cross-site attacks like cross-site script inclusion (XSSI) or cross-origin information leaks are similar in nature to CSRF and rely on loading resources from a victim application in an attacker-controlled document and leaking information about the victim applications. Since applications cannot easily distinguish trusted requests from untrusted ones, they cannot discard malicious cross-site traffic.
> [!TIP]
> Apart from attacks on resources as described above, *window references* can also lead to cross-origin information leaks and Spectre attacks. You can prevent them by setting the `Cross-Origin-Opener-Policy` response header to `same-origin`.
## Introducing Fetch Metadata
Fetch Metadata request headers are a new web platform security feature designed to help servers defend themselves against cross-origin attacks. By providing information about the context of an HTTP request in a set of `Sec-Fetch-*` headers, they allow the responding server to apply security policies before processing the request. This lets developers decide whether to accept or reject a request based on the way it was made and the context in which it will be used, making it possible to respond to only legitimate requests made by their own application.
Same-Origin Requests originating from sites served by your own server (same-origin) will continue to work.  Cross-site Malicious cross-site requests can be rejected by the server because of the additional context in the HTTP request provided by `Sec-Fetch-*` headers. 
### `Sec-Fetch-Site`
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Site)
`Sec-Fetch-Site` tells the server which [site](https://web.dev/articles/same-site-same-origin) sent the request. The browser sets this value to one of the following:
- `same-origin`, if the request was made by your own application (e.g. `site.example`)
- `same-site`, if the request was made by a subdomain of your site (e.g. `bar.site.example`)
- `none`, if the request was explicitly caused by a user's interaction with the user agent (e.g. clicking on a bookmark)
- `cross-site`, if the request was sent by another website (e.g. `evil.example`)
### `Sec-Fetch-Mode`
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Mode)
`Sec-Fetch-Mode` indicates the [mode](https://developer.mozilla.org/docs/Web/API/Request/mode) of the request. This roughly corresponds to the type of the request and allows you to distinguish resource loads from navigation requests. For example, a destination of `navigate` indicates a top-level navigation request while `no-cors` indicates resource requests like loading an image.
### `Sec-Fetch-Dest`
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/HTTP/Reference/Headers/Sec-Fetch-Dest)
`Sec-Fetch-Dest` exposes a request's [destination](https://developer.mozilla.org/docs/Web/API/Request/destination) (e.g. if a `script` or an `img` tag caused a resource to be requested by the browser).
> [!IMPORTANT]
> **Important:** For these protections to take effect on GET requests, your server must include `Sec-Fetch-Dest, Sec-Fetch-Mode, Sec-Fetch-Site` in its `Vary` header.
## How to use Fetch Metadata to protect against cross-origin attacks
The extra information these request headers provide is quite simple, but the additional context allows you to build powerful security logic on the server-side, also referred to as a Resource Isolation Policy, with just a few lines of code.
### Implementing a Resource Isolation Policy
A Resource Isolation Policy prevents your resources from being requested by external websites. Blocking such traffic mitigates common cross-site web vulnerabilities such as CSRF, XSSI, timing attacks, and cross-origin information leaks. This policy can be enabled for all endpoints of your application and will allow all resource requests coming from your own application as well as direct navigations (via an HTTP `GET` request). Endpoints that are supposed to be loaded in a cross-site context (e.g. endpoints loaded using CORS) can be opted out of this logic.
#### Step 1: Allow requests from browsers which don't send Fetch Metadata
Since not all browsers support Fetch Metadata, you need to allow requests that don't set `Sec-Fetch-*` headers by checking for the presence of `sec-fetch-site`.
> [!NOTE]
> **Note:** All of the following examples are Python code.
if not req['sec-fetch-site']:
return True # Allow this request
#### Step 2: Allow same-site and browser-initiated requests
Any requests that do not originate from a cross-origin context (like `evil.example`) will be allowed. In particular, these are requests that:
- Originate from your own application (e.g. a same-origin request where `site.example` requests `site.example/foo.json` will always be allowed).
- Originate from your subdomains.
- Are explicitly caused by a user's interaction with the user agent (e.g. direct navigation or by clicking a bookmark, etc.).
if req['sec-fetch-site'] in ('same-origin', 'same-site', 'none'):
return True # Allow this request
> [!TIP]
> In case your subdomains are not fully trusted, you can make the policy stricter by blocking requests from subdomains by removing the `same-site` value.
#### Step 3: Allow simple top-level navigation and iframing
To ensure that your site can still be linked from other sites, you have to allow simple (`HTTP GET`) top-level navigation.
if req['sec-fetch-mode'] == 'navigate' and req.method == 'GET'
#