CVE-2021-41084

EUVD-2021-1987 · Published Sep 22, 2021

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

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

Technologies

Typelevel Http4S

Weaknesses

CWE-74CWE-918

CVSS scores

v3.18.7CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access Execution Stealth Defense Impairment

T1190Exploit Public-Facing Application T1027Obfuscated Files or Information T1574.006Dynamic Linker Hijacking T1574.007Path Interception by PATH Environment Variable T1690Prevent Command History Logging

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.

CVE-2021-41084

Impact

Patches

Workarounds

References

For more information

Technologies

Weaknesses

CVSS scores

MITRE ATT&CK

References

Potentially impacted assets

Latest trending attack

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

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