CVE-2021-41084: Response Splitting from unsanitized headers

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.nameå
• 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:

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:

curl -i 'http://localhost:8080/?author=Ross'

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:

curl -i 'http://localhost:8080/?author=hax0r%0d%0aContent-Length:+13%0d%0a%0aI+hacked+you'

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
• Contact us via the http4s security policy