---
id: GHSA-w2j7-f3c6-g8cw
title: Flask-Security has an Open Redirect issue
summary: Flask-Security has an Open Redirect issue
severity: medium
cvss: 4.7
cwe:
- CWE-601
vendor: Flask-Security
product: Flask-Security
ecosystem: pip
affected:
- Flask-Security <= 5.8.0
patched:
- Flask-Security 5.8.1
published: '2026-06-23'
updated: '2026-06-23'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-w2j7-f3c6-g8cw'
references:
- url: >-
https://github.com/pallets-eco/flask-security/security/advisories/GHSA-w2j7-f3c6-g8cw
- url: 'https://nvd.nist.gov/vuln/detail/CVE-2023-49438'
- url: 'https://advisories.gitlab.com/pypi/flask-security-too/CVE-2023-49438'
- url: 'https://osv.dev/vulnerability/CVE-2023-49438'
- url: 'https://github.com/advisories/GHSA-w2j7-f3c6-g8cw'
tags:
- ghsa
- pip
ingestedAt: '2026-06-26T16:43:14.591Z'
---
## Overview
# 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:
Check the validation result:
```bash
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
## Affected packages
- `Flask-Security <= 5.8.0`
## Remediation
Upgrade to a patched release:
- `Flask-Security 5.8.1`