CVE-2025-24023: Timing Attack Vulnerability in Flask-AppBuilder Prior to Version 4.5.3 Enables Username Enumeration by Unauthenticated Users

Flask-AppBuilder (FAB) is a popular web application development framework based on Python and Flask, which enables rapid development of secure applications with minimal coding effort. However, before version 4.5.3, a vulnerability existed that allowed unauthenticated users to enumerate existing usernames by exploiting a timing attack.

This blog post will delve into the specifics of this vulnerability, provide a code snippet to demonstrate the exploit, and link to the original references and resources for further information. Finally, we'll discuss the fix provided by FAB in 4.5.3.

Vulnerability Details

The vulnerability in question, CVE-2025-24023, relates to a timing attack on the authentication system in FAB versions before 4.5.3. By comparing the server's response time to login requests with existing and nonexistent usernames, an attacker could enumerate existing usernames.

A timing attack is a type of side channel attack where an attacker gains information from the time it takes for a system to perform cryptographic operations. In the case of FAB, the attacker measures server response times when attempting to log in with known and unknown usernames. A faster response time for a specific username typically indicates an existing user, while a slower response time indicates a non-existent user.

The impact of this vulnerability is significant, as the ability to enumerate existing usernames in an application presents a dangerous vector for attackers to target specific accounts and further conduct brute-force attacks with the identified usernames.

Exploit Code Snippet

To better understand how this vulnerability can be exploited, let's take a look at the following Python code snippet, which demonstrates the timing attack:

import requests
import time

url = "http://example.com/login/";  # Replace with the target FAB URL
usernames_to_test = ["admin", "test", "johndoe"]

def login(username):
    data = {"username": username, "password": "randompass"}
    start_time = time.time()
    response = requests.post(url, data=data)
    end_time = time.time()

    return end_time - start_time

for username in usernames_to_test:
    response_time = login(username)
    print(f"Username: {username}, Response Time: {response_time}")

This code snippet sends login requests with a list of known and unknown usernames to a target FAB application. By measuring the response time (end_time - start_time), the attacker can infer the existence of a given username based on the server's response time, revealing vulnerable accounts.

References and Resources

For detailed information on this vulnerability, including recommendations and official statements from the Flask-AppBuilder team, refer to the following resources:

- Flask-AppBuilder GitHub Repository
- Flask-AppBuilder Security Advisory GHSA-3hq8-j7wq-4424

Fix and Recommendations

The FAB development team has addressed the vulnerability in version 4.5.3, by modifying the authentication mechanism to provide a consistent response time, regardless of whether the username exists within the application or not.

Developers using FAB for their applications are strongly encouraged to update their dependencies to Flask-AppBuilder 4.5.3 or newer to mitigate the risks associated with the vulnerability. Users can update their FAB versions by modifying their requirements.txt file or running pip install --upgrade flask-appbuilder.

By keeping up to date with the latest security fixes and remaining vigilant to possible vulnerabilities, developers can continue to build secure and robust applications using Flask-AppBuilder.

Timeline

Published on: 03/03/2025 16:15:41 UTC
Last modified on: 03/07/2025 19:37:57 UTC