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1. GraphiQL introspection schema template injection attack: Advisory Statement

This is a security advisory for an XSS vulnerability in graphiql.

A similar vulnerability affects graphql-playground, a fork of graphiql. There is a corresponding graphql-playground advisory and Apollo Server advisory.

1.1. Impact

All versions of graphiql older than graphiql@1.4.7 are vulnerable to compromised HTTP schema introspection responses or schema prop values with malicious GraphQL type names, exposing a dynamic XSS attack surface that can allow code injection on operation autocomplete.

In order for the attack to take place, the user must load a vulnerable schema in graphiql. There are a number of ways that can occur.

By default, the schema URL is not attacker-controllable in graphiql or in its suggested implementations or examples, leaving only very complex attack vectors.

If a custom implementation of graphiql's fetcher allows the schema URL to be set dynamically, such as a URL query parameter like ?endpoint= in graphql-playground, or a database provided value, then this custom graphiql implementation is vulnerable to phishing attacks, and thus much more readily available, low or no privilege level xss attacks. The URLs could look like any generic looking graphql schema URL.

Because this exposes an XSS attack surface, it would be possible for a threat actor to exfiltrate user credentials, data, etc. using arbitrary malicious scripts, without it being known to the user.

1.2. Scope

This advisory describes the impact on the graphiql package. The vulnerability also affects other projects forked from graphiql such as graphql-playground and the graphql-playground fork distributed by Apollo Server. The impact is more severe in the graphql-playground implementations; see the graphql-playground advisory and Apollo Server advisory for details.

This vulnerability does not impact codemirror-graphql, monaco-graphql or other dependents, as it exists in onHasCompletion.ts in graphiql. It does impact all forks of graphiql, and every released version of graphiql.

It should be noted that desktop clients such as Altair, Insomnia, Postwoman, do not appear to be impacted by this.

1.3. Patches

graphiql@1.4.7 addresses this issue via defense in depth.

  • HTML-escaping text that should be treated as text rather than HTML. In most of the app, this happens automatically because React escapes all interpolated text by default. However, one vulnerable component uses the unsafe innerHTML API and interpolated type names directly into HTML. We now properly escape that type name, which fixes the known vulnerability.

  • Validates the schema upon receiving the introspection response or schema changes. Schemas with names that violate the GraphQL spec will no longer be loaded. (This includes preventing the Doc Explorer from loading.) This change is also sufficient to fix the known vulnerability. You can disable this validation by setting dangerouslyAssumeSchemaIsValid={true}, which means you are relying only on escaping values to protect you from this attack.

  • Ensuring that user-generated HTML is safe. Schemas can contain Markdown in description and deprecationReason fields, and the web app renders them to HTML using the markdown-it library. As part of the development of graphiql@1.4.7, we verified that our use of markdown-it prevents the inclusion of arbitrary HTML. We use markdown-it without setting html: true, so we are comfortable relying on markdown-it's HTML escaping here. We considered running a second level of sanitization over all rendered Markdown using a library such as dompurify but believe that is unnecessary as markdown-it's sanitization appears to be adequate. graphiql@1.4.7 does update to the latest version of markdown-it (v12, from v10) so that any security fixes in v11 and v12 will take effect.

1.3.1 CDN bundle implementations may be automatically patched

Note that if your implementation is depending on a CDN version of graphiql, and is pointed to the latest tag (usually the default for most cdns if no version is specified) then this issue is already mitigated, in case you were vulnerable to it before.

1.4. Workarounds for Older Versions

If you cannot use graphiql@1.4.7 or later

  • Always use a static URL to a trusted server that is serving a trusted GraphQL schema.

  • If you have a custom implementation that allows using user-provided schema URLs via a query parameter, database value, etc, you must either disable this customization, or only allow trusted URLs.

1.5. How to Re-create the Exploit

You can see an example on codesandbox. These are both fixed to the last graphiql release 1.4.6 which is the last vulnerable release; however it would work with any previous release of graphiql.

Both of these examples are meant to demonstrate the phishing attack surface, so they are customized to accept a url parameter. To demonstrate the phishing attack, add ?url=https://graphql-xss-schema.netlify.app/graphql to the in-codesandbox browser.

Erase the contents of the given query and type {u. You will see an alert window open, showing that attacker-controlled code was executed.

Note that when React is in development mode, a validation exception is thrown visibly; however that exception is usually buried in the browser console in a production build of graphiql. This validation exception comes from getDiagnostics, which invokes graphql validate() which in turn will assertValidSchema(), as apollo-server-core does on executing each operation. This validation does not prevent the exploit from being successful.

Note that something like the url parameter is not required for the attack to happen if graphiql's fetcher is configured in a different way to communicate with a compromised GraphQL server.

1.6. Credit

This vulnerability was discovered by @Ry0taK, thank you! 🥇

Others who contributed:

1.7. References

The vulnerability has always been present

In the first commit

And later moved to onHasCompletion.js in 2016 (now .ts after the typescript migration)

1.8. For more information

If you have any questions or comments about this advisory:

2. More Details on the Vulnerability

This section provides more details in addition to the advisory.

An installation of the GraphiQL web app is vulnerable if two conditions are met:

  • The web app trusts information from its corresponding GraphQL server by interpolating information such as GraphQL type names directly into HTML instead of appropriately escaping or sanitizing the information.
  • The victim can load the web app in a way where it speaks to a GraphQL server controlled by the attacker.

All versions of graphiql prior to 1.4.7 inappropriately trust type names provided by the GraphQL server. They additionally rely on XSS filtering in the markdown-it package to try to protect themselves from XSS attacks in GraphQL descriptions and deprecation reasons.

By default, graphiql does not allow the attacker to control what GraphQL server it speaks to. Therefore, many installations of graphiql are not affected by this advisory. Installations are only affected if the fetcher argument provided to GraphiQL allows arbitrary customization of the GraphQL endpoint (eg, reading a GraphQL URL from an URL parameter), or if the attacker has another way of affecting the introspection schema returned by the GraphQL server. (Note that graphql-playground, a project which started as a fork of graphiql, does this sort of URL parsing by default, so graphql-playground installations are affected by a corresponding vulnerability in their default configuration.)

One example of "another way of affecting the introspection schema" would be if you served graphiql as part of a PAAS platform that allows users to define their own GraphQL schemas. In this case, even though the graphiql installation might be hard-wired to a single GraphQL endpoint, the attacker has control over that GraphQL endpoint and could use it to inject scripts into graphiql. In this case, your graphiql installation could be vulnerable if it responds to introspection requests without first validating its schema. GraphQL servers can prevent this by refusing to execute operations (including introspection operations) on invalid schemas; any server built with graphql-js properly validates its schema prior to execution.

3. Compromised introspection Schema Example

You can view the code for the exploited schema on codesandbox or in the repository

As you can see, the introspection schema must contain items with a compromised name value. this could be fields, input object names, enum names, variable names, etc any graphql NamedType in the schema with it's name rendered in the autocomplete list.

{
  "kind": "OBJECT",
  "name": "<img src=x onerror=alert(document.domain)>",
  "description": null,
  "fields": [
    {
      "name": "name",
      "description": null,
      "args": [],
      "type": {
        "kind": "NON_NULL",
        "name": null,
        "ofType": {
          "kind": "SCALAR",
          "name": "String",
          "ofType": null
        }
      },
      "isDeprecated": false,
      "deprecationReason": null
    }
  ],
  "inputFields": null,
  "interfaces": [],
  "enumValues": null,
  "possibleTypes": null
}