CVE-2024-27083 - Critical XSS Vulnerability in Flask-AppBuilder OAuth Login Page Explained

Flask-AppBuilder (FAB) is a popular application development framework built on top of Flask, and it’s widely used in projects for building web apps with strong user and data management out-of-the-box. Recently, a high-severity security flaw was found in FAB—a Cross-Site Scripting (XSS) vulnerability on the OAuth login page. This post breaks down CVE-2024-27083 in simple American English with code snippets, PoC, and mitigation tips so you can protect your apps.

What is CVE-2024-27083?

CVE-2024-27083 is a cross-site scripting (XSS) vulnerability that impacts Flask-AppBuilder versions 4.1.4 up to, but not including, 4.2.1. Here’s the situation:

Why is this dangerous?

If exploited, attackers can steal session cookies, impersonate users, perform unauthorized actions, or deliver malware—all in the context of your trusted site!

Vulnerable Code Example

Flask-AppBuilder’s OAuth login view generally takes several query parameters. Let’s look at a simplified (vulnerable) example:

# snippet from appbuilder/views.py (simplified)
from flask import request, render_template

@app.route('/oauth-authorized')
def oauth_authorized():
    next_url = request.args.get('next')  # << NOT SANITIZED
    return render_template('oauth_authorized.html', next=next_url)

In early versions, the next parameter wasn’t properly validated or escaped.

Attacker's Payload Example

https://your-app.com/oauth-authorized?next=%3Cscript%3Ealert('XSS')%3C/script%3E

That decodes to

https://your-app.com/oauth-authorized?next=<script>alert('XSS')</script>;

When a victim follows the link, the injected script runs in their browser!

Proof of Concept (PoC)

Let’s simulate this XSS on a vulnerable Flask-AppBuilder deployment.

`

http://localhost:808/oauth-authorized?next=

The rendered template includes

<a href="{{ next }}">Continue</a>

If next is not escaped, arbitrary HTML/JS can be injected.

User, believing it’s a real login, clicks the link and goes through OAuth.

4. Malicious JavaScript executes in the user’s browser, letting the attacker steal cookies, tokens, or perform actions as that user.

Fix & Patch Details

The maintainers fixed this in version 4.2.1. The solution is to sanitize and escape user-supplied parameters, especially those ending up in templates or client-side code.

Patch Example

from markupsafe import escape

@app.route('/oauth-authorized')
def oauth_authorized():
    next_url = request.args.get('next')
    safe_next = escape(next_url)
    return render_template('oauth_authorized.html', next=safe_next)

Or, better—only allow safe, internal URLs and never let arbitrary HTML through.

Always validate and sanitize user-provided URLs in your Flask (or any web) app.

- Use Flask/Jinja2’s built-in auto-escaping features.

Official Advisory:

GitHub Security Advisory
CVE Record on MITRE

Report & Patch PR:

Patch Pull Request

Release Notes:

Flask-AppBuilder Releases

Final Thoughts

CVE-2024-27083 is a real-world reminder: even popular frameworks can have critical security bugs. Vulnerabilities like XSS can have big consequences if ignored. If you’re running Flask-AppBuilder, patch right away. Teach your team about these issues—they’re easy for attackers to exploit, but you can stop them with smart coding and timely upgrades!

Timeline

Published on: 02/29/2024 01:44:19 UTC
Last modified on: 02/29/2024 13:49:29 UTC