GHSA-W2J7-F3C6-G8CW

Flask-Security has an Open Redirect issue

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

SERVER_NAME = "whitelist.com"
SECURITY_REDIRECT_ALLOW_SUBDOMAINS = True

Flask-Security: 5.8.0
Flask: 3.1.3
Werkzeug: 3.1.8

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

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.

Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.

GHSA-W2J7-F3C6-G8CW has a CVSS score of 4.7 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.8.1); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

1. What is GHSA-W2J7-F3C6-G8CW? GHSA-W2J7-F3C6-G8CW is a medium-severity open redirect vulnerability in Flask-Security (pip), affecting versions <= 5.8.0. It is fixed in 5.8.1. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
2. How severe is GHSA-W2J7-F3C6-G8CW? GHSA-W2J7-F3C6-G8CW has a CVSS score of 4.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of Flask-Security are affected by GHSA-W2J7-F3C6-G8CW? Flask-Security (pip) versions <= 5.8.0 is affected.
4. Is there a fix for GHSA-W2J7-F3C6-G8CW? Yes. GHSA-W2J7-F3C6-G8CW is fixed in 5.8.1. Upgrade to this version or later.
5. Is GHSA-W2J7-F3C6-G8CW exploitable, and should I be worried? Whether GHSA-W2J7-F3C6-G8CW is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-W2J7-F3C6-G8CW is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-W2J7-F3C6-G8CW? Upgrade Flask-Security to 5.8.1 or later.

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