CVE-2021-41084

Description

### Impact http4s is vulnerable to response-splitting or request-splitting attacks when untrusted user input is used to create any of the following fields: * Header names (`Header.name`å * Header values (`Header.value`) * Status reason phrases (`Status.reason`) * URI paths (`Uri.Path`) * URI authority registered names (`URI.RegName`) (through 0.21) The following backends render invalid carriage return, newline, or null characters in an unsafe fashion. | | blaze-server | ember-server | blaze-client | ember-client | jetty-client | |:---------------|:-------------|:-------------|:-------------|--------------|--------------| | header names | ⚠ | ⚠ | ⚠ | ⚠ | ⚠ | | header values | ⚠ | ⚠ | ⚠ | ⚠ | | | status reasons | ⚠ | ⚠ | | | | | URI paths | | | ⚠ | ⚠ | | | URI regnames | | | ⚠ < 0.22 | ⚠ < 0.22 | | For example, given the following service: ```scala import cats.effect._ import org.http4s._ import org.http4s.dsl.io._ import org.http4s.server.blaze.BlazeServerBuilder import scala.concurrent.ExecutionContext.global object ResponseSplit extends IOApp { override def run(args: List[String]): IO[ExitCode] = BlazeServerBuilder[IO](global) .bindHttp(8080) .withHttpApp(httpApp) .resource .use(_ => IO.never) val httpApp: HttpApp[IO] = HttpApp[IO] { req => req.params.get("author") match { case Some(author) => Ok("The real content") .map(_.putHeaders(Header("Set-Cookie", s"author=${author}"))) case None => BadRequest("No author parameter") } } } ``` A clean `author` parameter returns a clean response: ```sh curl -i 'http://localhost:8080/?author=Ross' ``` ```http HTTP/1.1 200 OK Content-Type: text/plain; charset=UTF-8 Set-Cookie: author=Ross Date: Mon, 20 Sep 2021 04:12:10 GMT Content-Length: 16 The real content ``` A malicious `author` parameter allows a user-agent to hijack the response from our server and return different content: ```sh curl -i 'http://localhost:8080/?author=hax0r%0d%0aContent-Length:+13%0d%0a%0aI+hacked+you' ``` ```http HTTP/1.1 200 OK Content-Type: text/plain; charset=UTF-8 Set-Cookie: author=hax0r Content-Length: 13 I hacked you ``` ### Patches Versions 0.21.29, 0.22.5, 0.23.4, and 1.0.0-M27 perform the following: * If a status reasoon phrase is invalid, it is dropped. Rendering is optional per spec. * If a header name is invalid in a request or response, the header is dropped. There is no way to generically sanitize a header without potentially shadowing a correct one. * If a header value is invalid in a request or response, it is sanitized by replacing null (`\u0000`), carriage return (`\r`), and newline (`\n`) with space (` `) characters per spec. * If a URI path or registered name is invalid in a request line, the client raises an `IllegalArgumentException`. * If a URI registered name is invalid in a host header, the client raises an `IllegalArgumentException`. ### Workarounds http4s services and client applications should sanitize any user input in the aforementioned fields before returning a request or response to the backend. The carriage return, newline, and null characters are the most threatening. Not all backends were affected: jetty-server, tomcat-server, armeria, and netty on the server; async-http-client, okhttp-client, armeria, and netty as clients. ### References * https://owasp.org/www-community/attacks/HTTP_Response_Splitting * https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#fields.values ### 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)