GHSA-w2j7-f3c6-g8cw Flask-Security has an Open Redirect issue

Not scored Published 23 Jun 2026

Attackers can bypass redirect validation to send users to malicious sites via phishing when subdomain redirects are enabled.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: A user has to click or open something
Fix: Available

What is GHSA-w2j7-f3c6-g8cw?

GHSA-w2j7-f3c6-g8cw is a vulnerability in Flask-Security, published on 23 Jun 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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 = &#34;whitelist.com&#34;
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=&#34;poc-only&#34;,
    SERVER_NAME=&#34;whitelist.com&#34;,
    SECURITY_REDIRECT_ALLOW_SUBDOMAINS=True,
    SECURITY_REDIRECT_BASE_DOMAIN=None,
    SECURITY_REDIRECT_ALLOWED_SUBDOMAINS=[],
)

@app.get(&#34;/&#34;)
def index():
    return jsonify(
        flask_version=version(&#34;Flask&#34;),
        configured_server_name=app.config[&#34;SERVER_NAME&#34;],
        examples=[
            r&#34;http://evil.com\.whitelist.com&#34;,
            &#34;http://evil.com%5C.whitelist.com&#34;,
            &#34;http://sub.whitelist.com&#34;,
            &#34;http://sub.not-whitelist.com&#34;,
        ],
    )

@app.get(&#34;/check&#34;)
def check():
    next_url = request.args.get(&#34;next&#34;, &#34;&#34;)
    parsed = urlsplit(next_url)

    return jsonify(
        next=next_url,
        valid=validate_redirect_url(next_url),
        parsed={
            &#34;scheme&#34;: parsed.scheme,
            &#34;netloc&#34;: parsed.netloc,
            &#34;hostname&#34;: parsed.hostname,
            &#34;path&#34;: parsed.path,
        },
    )

@app.get(&#34;/redir&#34;)
def redir():
    next_url = request.args.get(&#34;next&#34;, &#34;&#34;)
    if not validate_redirect_url(next_url):
        return jsonify(error=&#34;blocked&#34;, next=next_url), 400

    return redirect(next_url)

if __name__ == &#34;__main__&#34;:
    app.run(host=&#34;127.0.0.1&#34;, 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:

Check the validation result:

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

Observed result:

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

Which versions are affected by GHSA-w2j7-f3c6-g8cw?

2 open source packages ship an affected version, including Flask-Security and flask-security.

Package	Affected	Fixed in
`Flask-Security` PyPI	All versions	`5.8.1`
`flask-security` PyPI	All versions	`5.8.1`

How do I fix GHSA-w2j7-f3c6-g8cw?

Upgrade Flask-Security to 5.8.1 or later.

Is GHSA-w2j7-f3c6-g8cw being exploited?

There is no known exploitation of GHSA-w2j7-f3c6-g8cw in the wild and no public exploit code.