Cross-Site Request Forgery (CSRF) Prevention

About CSRF

Cross-site request forgery (CSRF) attacks use side effects of "simple" requests to execute GraphQL operations from requests sent by sites that shouldn't be allowed to communicate with your server, based on the CORS policy of your GraphOS Router or Apollo Router Core.

Your router's CORS policy enables you to control which websites can talk to your server. In most cases, the browser checks your server's CORS policy by sending a preflight request before sending the actual operation. This is a separate HTTP request. Unlike most HTTP requests (which use the GET or POST method), this request uses a method called OPTIONS. The browser sends an Origin header, along with some other headers that start with Access-Control-. These headers describe the kind of request that the potentially untrusted JavaScript wants to make. Your server returns a response with Access-Control-* headers describing its policies (as described above), and the browser uses that response to decide whether it's OK to send the real request. Processing the OPTIONS preflight request never actually executes GraphQL operations.

However, in some circumstances, the browser will not send this preflight request. If the request is considered "simple", then the browser just sends the request without sending a preflight first. Your server's response can still contain Access-Control-* headers, and if they indicate that the origin that sent the request shouldn't be able to access the site, the browser will hide your server's response from the problematic JavaScript code.

Unfortunately, this means that your server might execute GraphQL operations from "simple" requests sent by sites that shouldn't be allowed to communicate with your server. These requests can even contain cookies. Although the browser will hide your server's response data from the malicious code, that might not be sufficient. If running the operation has side effects, then the attacker might not care if it can read the response or not, as long as it can use an unsuspecting user's browser (and cookies) to trigger those side effects. Even with a read-only query, the malicious code might be able to figure out something about the response based entirely on how long the query takes to execute.

In addition to CSRF, attacks that measure the timing of simple requests are called "cross-site search" attacks, or XS-Search.

Preventing CSRF

To avoid CSRF and XS-Search attacks, GraphQL servers should refuse to execute any operation coming from a browser that has not "preflighted" that operation. There's no reliable way to detect whether a request came from a browser, so GraphQL servers should not execute any operation in a "simple request".

The most important rule for whether or not a request is "simple" is whether it tries to set arbitrary HTTP request headers. Any request that sets the Content-Type header to application/json (or anything other than a list of three particular values) cannot be a simple request, and thus it must be preflighted. Because all POST requests recognized by Apollo Server must contain a Content-Type header specifying application/json, we can be confident that they are not simple requests and that if they come from a browser, they have been preflighted.

However, the router also handles GET requests which do not require a Content-Type header, so they can potentially be simple requests. So how can we ensure that we only execute GET requests that are not simple requests? If we require the request to include an HTTP header that is never set automatically by the browser, then that is sufficient: requests that set HTTP headers other than the handful defined in the spec must be preflighted.

Configuring CSRF prevention in the router

Enable CSRF prevention

The router enables CSRF prevention by default. When this feature is enabled, the router only executes GraphQL operations if at least one of the following conditions is true:

• The incoming request includes a Content-Type header that specifies a type other than text/plain, application/x-www-form-urlencoded, or multipart/form-data.

  ⓘ note

  A Content-Type of application/json is sufficient, including any suffix like application/json; charset=utf-8. Since all POST requests must use Content-Type: application/json, this means all POST requests will be executed. And because all versions of Apollo Client Web that support GET requests include Content-Type: application/json headers, any request from Apollo Client Web (POST or GET) will also be executed.

• There is a X-Apollo-Operation-Name header. This header is sent with all POST or GET operations by Apollo iOS (v0.13.0+) and Apollo Kotlin (all versions, including its former name "Apollo Android"), so any request from Apollo iOS or Apollo Kotlin will be executed.

• There is a Apollo-Require-Preflight header.

HTTP requests that satisfy none of the conditions above will be rejected with a 400 status code and a message clearly explaining which headers need to be added in order to make the request succeed.

This should have no impact on legitimate use of your graph, unless you have clients that send GET requests and are not Apollo Client Web, Apollo iOS, or Apollo Kotlin. If you do send GET requests with other clients, you should configure them to send a non-empty Apollo-Require-Preflight header along with all requests.

You can also configure the set of headers that allow execution. For example, if you use a GraphQL client that performs GET requests without sending Content-Type, X-Apollo-Operation-Name, or Apollo-Require-Preflight headers, but it does send a Some-Special-Header header:

1csrf:
2  required_headers:
3    - X-Apollo-Operation-Name
4    - Apollo-Require-Preflight
5    - Some-Special-Header

The check for Content-Type remains the same.

Disable CSRF prevention

Disabling CSRF in the router might be appropriate for a GraphQL server that only contains public data and allows no mutation, or on a closed private network where browsers cannot reach untrusted websites.

To disable CSRF prevention in the router, set csrf.unsafe_disabled to true:

1csrf:
2  unsafe_disabled: true