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Response Splitting from unsanitized headers

High severity GitHub Reviewed Published Sep 21, 2021 in http4s/http4s • Updated Jan 29, 2023

Package

maven org.http4s:http4s-client (Maven)

Affected versions

<= 0.21.28
>= 0.22.0, <= 0.22.4
>= 0.23.0, <= 0.23.3

Patched versions

0.21.29
0.22.5
0.23.4
maven org.http4s:http4s-server (Maven)
<= 0.21.28
>= 0.22.0, <= 0.22.4
>= 0.23.0, <= 0.23.3
0.21.29
0.22.5
0.23.4

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.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

For more information

If you have any questions or comments about this advisory:

References

@rossabaker rossabaker published to http4s/http4s Sep 21, 2021
Reviewed Sep 21, 2021
Published by the National Vulnerability Database Sep 21, 2021
Published to the GitHub Advisory Database Sep 22, 2021
Last updated Jan 29, 2023

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N

EPSS score

0.157%
(53rd percentile)

CVE ID

CVE-2021-41084

GHSA ID

GHSA-5vcm-3xc3-w7x3

Source code

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