CORS in AMP

Many AMP components and extensions take advantage of remote endpoints by using Cross-Origin Resource Sharing (CORS) requests. This document explains the key aspects of using CORS in AMP. To learn about CORS itself, see the W3 CORS Spec.

Why do I need CORS for my own origin?

You might be confused as to why you'd need CORS for requests to your own origin, let's dig into that.

AMP components that fetch dynamic data (e.g., amp-form, amp-list, etc.) make CORS requests to remote endpoints to retrieve the data. If your AMP page includes such components, you'll need to handle CORS so that those requests do not fail.

Let's illustrate this with an example:

Let's say you have an AMP page that lists products with prices. To update the prices on the page, the user clicks a button, which retrieves the latest prices from a JSON endpoint (done via the amp-list component). The JSON is on your domain.

Okay, so the page is on my domain and the JSON is on my domain. I see no problem!

Ah, but how did your user get to your AMP page? Is it a cached page they access? It's quite likely that your user did not access your AMP page directly, but instead they discovered your page through another platform. For example, Google Search uses the Google AMP Cache to render AMP pages quickly; these are cached pages that are served from the Google AMP Cache, which is a different domain. When your user clicks the button to update the prices on your page, the cached AMP page sends a request to your origin domain to get the prices, which is a mismatch between origins (cache -> origin domain). To allow for such cross-origin requests, you need to handle CORS, otherwise, the request fails.

Okay, what should I do?

1. For AMP pages that fetch dynamic data, make sure you test the cached version of those pages; don't just test on your own domain. (See Testing CORS in AMP section below)
2. Follow the instructions in this document for handling CORS requests and responses.

Utilizing cookies for CORS requests

Most AMP components that use CORS requests either automatically set the credentials mode or allow the author to optionally enable it. For example, the amp-list component fetches dynamic content from a CORS JSON endpoint, and allows the author to set the credential mode through the credentials attribute.

Example: Including personalized content in an amp-list via cookies

<amp-list
  credentials="include"
  src="<%host%>/json/product.json?clientId=CLIENT_ID(myCookieId)"
>
  <template type="amp-mustache">
    Your personal offer: ${{price}}  </template>
</amp-list>

By specifying the credentials mode, the origin can include cookies in the CORS request and also set cookies in the response (subject to third-party cookie restrictions).

Third-party cookie restrictions

The same third-party cookie restrictions specified in the browser also apply to the credentialed CORS requests in AMP. These restrictions depend on the browser and the platform, but for some browsers, the origin can only set cookies if the user has previously visited the origin in a 1st-party (top) window. Or, in other words, only after the user has directly visited the origin website itself. Given this, a service accessed via CORS cannot assume that it will be able to set cookies by default.

CORS security in AMP

To ensure valid and secure requests and responses for your AMP pages, you must:

1. Verify the request.
2. Send the appropriate response headers.

If you're using Node in your backend, you can use the AMP CORS middleware, which is part of the AMP Toolbox.

Verify CORS requests

When your endpoint receives a CORS request:

1. Verify that the CORS Origin header is an allowed origin (publisher's origin + AMP caches).
2. If there isn't an Origin header, check that the request is from the same origin (via AMP-Same-Origin).

1) Allow requests for specific CORS origins

CORS endpoints receive the requesting origin via the Origin HTTP header. Endpoints should only allow requests from: (1) the publisher's own origin; and (2) every cacheDomain origin listed in https://cdn.ampproject.org/caches.json.

For example, endpoints should allow requests from:

• Google AMP Cache subdomain: https://<publisher's domain>.cdn.ampproject.org
  (for example, https://nytimes-com.cdn.ampproject.org)

2) Allow same-origin requests

For same-origin requests where the Origin header is missing, AMP sets the following custom header:

AMP-Same-Origin: true

This custom header is sent by the AMP Runtime when an XHR request is made on the same origin (i.e., document served from a non-cache URL). Allow requests that contain the AMP-Same-Origin:true header.

Send CORS response headers

After verifying the CORS request, the resulting HTTP response must contain the following headers:

Access-Control-Allow-Origin: <origin>

This header is a W3 CORS Spec requirement, where origin refers to the requesting origin that was allowed via the CORS Origin request header (for example, "https://<publisher's subdomain>.cdn.ampproject.org").

Although the W3 CORS spec allows the value of * to be returned in the response, for improved security, you should:

• If the Origin header is present, validate and echo the value of the Origin header.

Processing state changing requests

Before processing requests that could change the state of your system (for example, a user subscribes to or unsubscribes from a mailing list), check the following:

If the Origin header is set:

1. If the origin does not match one of the following values, stop and return an error response:

   • <publisher's domain>.cdn.ampproject.org
   • the publisher's origin (aka yours)

   where * represents a wildcard match, and not an actual asterisk ( * ).

2. Otherwise, process the request.

If the Origin header is NOT set:

1. Verify that the request contains the AMP-Same-Origin: true header. If the request does not contain this header, stop and return an error response.
2. Otherwise, process the request.

Example walkthrough: Handing CORS requests and responses

There are two scenarios to account for in CORS requests to your endpoint:

1. A request from the same origin.
2. A request from a cached origin (from an AMP Cache).

Let's walk though these scenarios with an example. In our example, we manage the example.com site that hosts an AMP page named article-amp.html.The AMP page contains an amp-list to retrieve dynamic data from a data.json file that is also hosted on example.com. We want to process requests to our data.json file that come from our AMP page. These requests could be from the AMP page on the same origin (non-cached) or from the AMP page on a different origin (cached).

Allowed origins

Based on what we know about CORS and AMP (from Verify CORS requests above), for our example we will allow requests from the following domains:

• example.com --- Publisher's domain
• example-com.cdn.ampproject.org --- Google AMP Cache subdomain

Response headers for allowed requests

For requests from the allowed origins, our response will contain the following headers:

Access-Control-Allow-Origin: <origin>

These are additional response headers we might include in our CORS response:

Access-Control-Allow-Credentials: true
Content-Type: application/json
Access-Control-Max-Age: <delta-seconds>
Cache-Control: private, no-cache

Pseudo CORS logic

Our logic for handling CORS requests and responses can be simplified into the following pseudo code:

IF CORS header present
   IF origin IN allowed-origins
      allow request & send response
   ELSE
      deny request
ELSE
   IF "AMP-Same-Origin: true"
      allow request & send response
   ELSE
      deny request

CORS sample code

Here's a sample JavaScript function that we could use to handle CORS requests and responses:

function assertCors(req, res, opt_validMethods, opt_exposeHeaders) {
  var unauthorized = 'Unauthorized Request';
  var origin;
  var allowedOrigins = [
    'https://example.com',
    'https://example-com.cdn.ampproject.org',
    'https://cdn.ampproject.org',
  ];
  var allowedSourceOrigin = 'https://example.com'; //publisher's origin
  // If same origin
  if (req.headers['amp-same-origin'] == 'true') {
    origin = sourceOrigin;
    // If allowed CORS origin & allowed source origin
  } else if (
    allowedOrigins.indexOf(req.headers.origin) != -1 &&
    sourceOrigin == allowedSourceOrigin
  ) {
    origin = req.headers.origin;
  } else {
    res.statusCode = 403;
    res.end(JSON.stringify({message: unauthorized}));
    throw unauthorized;
  }

  res.setHeader('Access-Control-Allow-Credentials', 'true');
  res.setHeader('Access-Control-Allow-Origin', origin);
}

Note: For a working code sample, see amp-cors.js.

Scenario 1: Get request from AMP page on same origin

In the following scenario, the article-amp.html page requests the data.json file; the origins are the same.

If we examine the request, we'll find:

Request URL: https://example.com/data.json
Request Method: GET
AMP-Same-Origin: true

As this request is from the same origin, there is no Origin header but the custom AMP request header of AMP-Same-Origin: true is present. We can allow this request as it's from the same origin (https://example.com).

Our response headers would be:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: https://example.com

Scenario 2: Get request from cached AMP page

In the following scenario, the article-amp.html page cached on the Google AMP Cache requests the data.json file; the origins differ.

If we examine this request, we'll find:

Request URL: https://example.com/data.json
Request Method: GET
Origin: https://example-com.cdn.ampproject.org

As this request contains an Origin header, we'll verify that it's from an allowed origin. We can allow this request as it's from an allowed origin.

Our response headers would be:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: https://example-com.cdn.ampproject.org

Working with cached fonts

Google AMP Cache caches AMP HTML documents, images and fonts to optimize the speed of the AMP page. While making the AMP page fast, we also want to be careful in securing the cached resources. We will be making a change in how AMP cache responds it’s cached resources, typically for fonts, by respecting the origin’s Access-Control-Allow-Origin value.

Past behavior (before October 2019)

When an AMP page was loading https://example.com/some/font.ttf from @font-face src attribute, AMP Cache will cache the font file and serve the resource as below with having the wild card Access-Control-Allow-Origin.

• URL https://example-com.cdn.ampproject.org/r/s/example.com/some/font.tff
• Access-Control-Allow-Origin: *

New behavior (October 2019 and after)

While the current implementation is permissive, this could lead to unexpected use of the fonts from cross-origin sites. In this change AMP Cache will start to respond with the exact same Access-Control-Allow-Origin value the origin server responds. To properly load the fonts from the cached AMP document, you will need to accept the AMP Cache origin via the header.

A sample implementation would be:

function assertFontCors(req, res, opt_validMethods, opt_exposeHeaders) {
  var unauthorized = 'Unauthorized Request';
  var allowedOrigins = [
    'https://example.com',
    'https://example-com.cdn.ampproject.org',
  ];
  // If allowed CORS origin
  if (allowedOrigins.indexOf(req.headers.origin) != -1) {
    res.setHeader('Access-Control-Allow-Origin', req.headers.origin);
  } else {
    res.statusCode = 403;
    res.end(JSON.stringify({message: unauthorized}));
    throw unauthorized;
  }
}

As an example, if you wanted to load /some/font.ttf in https://example.com/amp.html, the origin server should respond with the Access-Control-Allow-Origin header as below.

We are planning to make this change around mid October 2019 and would expect every AMP publishers using self-hosted fonts to check if it’s affected.

Roll out plan

• 2019-09-30: release contains more precise control over which domains this change applies to. This build should roll out over the course of this week.
• 2019-10-07: test domains will be enabled for manual testing.
• 2019-10-14: (but depending on how testing goes): the feature will be rolled out generally.

Follow the related issue here.

Testing CORS in AMP

When you are testing your AMP pages, make sure to include tests from the cached versions of your AMP pages.

Verify the page via the cache URL

To ensure your cached AMP page renders and functions correctly:

1. From your browser, open the URL that the AMP Cache would use to access your AMP page. You can determine the cache URL format from this tool on AMP By Example.

   For example:

   • URL: https://amp.dev/documentation/guides-and-tutorials/start/create/
   • AMP Cache URL format: https://www-ampproject-org.cdn.ampproject.org/c/s/www.ampproject.org/docs/tutorials/create.html

2. Open your browser's development tools and verify that there are no errors and that all resources loaded correctly.

Verify your server response headers

You can use the curl command to verify that your server is sending the correct HTTP response headers. In the curl command, provide the request URL and any custom headers you wish to add.

Syntax: curl <request-url> -H <custom-header> - I

Test request from same origin

In a same-origin request, the AMP system adds the custom AMP-Same-Origin:true header.

Here's our curl command for testing a request from https://amp.dev to the examples.json file (on the same domain):

curl 'https://amp.dev/static/samples/json/examples.json' -H 'AMP-Same-Origin: true' -I

The results from the command show the correct response headers (note: extra information was trimmed):

HTTP/2 200
access-control-allow-headers: Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token
access-control-allow-credentials: true
access-control-allow-origin: https://amp.dev
access-control-allow-methods: POST, GET, OPTIONS

Test request from cached AMP page

In a CORS request not from the same domain (i.e., cache), the origin header is part of the request.

Here's our curl command for testing a request from the cached AMP page on the Google AMP Cache to the examples.json file:

curl 'https://amp.dev/static/samples/json/examples.json' -H 'origin: https://ampbyexample-com.cdn.ampproject.org' -I

The results from the command show the correct response headers:

HTTP/2 200
access-control-allow-headers: Content-Type, Content-Length, Accept-Encoding, X-CSRF-Token
access-control-allow-credentials: true
access-control-allow-origin: https://ampbyexample-com.cdn.ampproject.org
access-control-allow-methods: POST, GET, OPTIONS