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.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