Preventing Broken Access Control in Python Flask Applications

Some security vulnerabilities can be easily detected by static analysis tools (like Snyk), but others are borderline logic concerns that can be deliberate or mistaken. Broken access control is one such concern, and if you code your Python Flask applications for modern SaaS-like business applications, it is surely a vulnerability.

Access control is a fundamental aspect of application security, ensuring that users can only access their own resources or those that they are permitted to and nothing otherwise. When access control mechanisms in Python applications are weak or improperly implemented through business logic, they can be exploited by malicious actors who seek to gain unauthorized access, leading to severe security breaches and escalated security risks.

To begin with, let’s distill some of the security terminology and better understand what broken access control is from a security perspective.

What is broken access control?

Broken access control is a critical security vulnerability that occurs when an application fails to enforce restrictions (authorization controls) on what authenticated users can do. This vulnerability is featured as the number one security risk in OWASP Top 10 for 2021. This flaw allows attackers to access unauthorized resources or perform actions outside their intended permissions, such as modifying, creating, or deleting records.

Highly recommended: take the free byte-size Snyk Lesson on Broken Access Control right now in your browser!

Example of broken access control in a Python Flask application

This section explores a common example of broken access control in a Python Flask application. The code snippet provided demonstrates a vulnerability that stems from insufficient access control checks in Python code. In this code example, the authorization checks are not just insufficient. They are totally lacking. If this Python application were to be deployed with public access, even authenticated users could exploit this type of vulnerability to gain unauthorized actions or data access that they should not be able to.

Example code: A vulnerable Python Flask application

Below is the code for a simple Flask application in Python that contains a broken access control vulnerability:

from flask import Flask, render_template, request, redirect, url_for

app = Flask(__name__)

# Service to handle user account information
class AccountManager:
    def retrieve_account_details(self, account_identifier):
        """Fetches the account details based on the provided identifier."""
        # Database interaction to retrieve account data
        pass

    def save_account_changes(self, modified_account):
        """Persists the updated account information to the database."""
        # Database interaction to update account data
        pass

account_manager = AccountManager()

def is_user_logged_in():
    """Determines if a user is currently authenticated."""
    # Logic to check user authentication status
    return True # Replace with actual authentication check

@app.route('/update-account', methods=['POST'])
def update_account():
    """Handles the submission of updated account information."""
    if not is_user_logged_in():
        return redirect(url_for('login'))

    account_name = request.form.get('username')
    account_record = account_manager.retrieve_account_details(account_name)

    if account_record.get_username() == account_name:
        account_manager.save_account_changes(account_record)

        return redirect(url_for('/user'))

@app.route('/user')
def user_home():
    """Displays the user's dashboard."""
    return render_template('dashboard.html')

if __name__ == '__main__':
    app.run(debug=False)

Analysis of the vulnerabilities in the code

The misused authorization vulnerability in this Python code is the lack of proper access control checks when updating a user profile. The `update-account` function retrieves the username from the form data (via the `username` form field) and fetches the corresponding user profile. It then checks if the username from the form matches the username in the retrieved profile before updating it.

However, this check is insufficient to ensure that the user making the request is actually the same user authorized to update the profile. An attacker could manipulate the form data to specify a different username, through which they gain and manifest unauthorized access that updates another user's profile.

To address this vulnerability, the application should implement stricter access control measures. For instance, it should verify that the user making the request is indeed the owner of the profile being updated. This can be achieved by associating the user’s session with the profile and ensuring that only the authenticated user can update their profile.

Common vulnerabilities associated with broken access control

As we’ve seen above, missing an authorization bug is easy because it is mostly a logic issue. Should everyone be able to view or change each other’s data, or is this a bug? At which level should this be enforced? Perhaps row-level security at the database level is already secure against this.

Let’s explore some other blast-radius vulnerabilities and security risks that occur due to broken access control:

• Insecure direct object references (IDOR): This occurs when an application exposes a reference to an internal implementation object, such as a file or database key (e.g., database primary key `id` in a table), without proper access control checks. Attackers can manipulate these references to directly access unauthorized data because they can easily guess the identifier.

• Missing function-level access control: This vulnerability arises when an application does not enforce access control at the function or method level, allowing unauthorized users to execute restricted actions.

• Privilege escalation: This occurs when an attacker gains higher-level permissions than intended, often by exploiting flaws in the access control logic. For example, if the attacker can modify the administrator user’s credentials or email, they can escalate access from their own user to this administrator account and gain higher privileges in the vulnerable system.

• Cross-site request forgery (CSRF): Although not strictly an access control issue, CSRF can perform unauthorized actions on behalf of an authenticated user.

Impact of broken access control on application security

The impact of broken access control on application security and Python Flask applications, in particular, can be devastating. Unauthorized access to sensitive data can lead to breaches, violating user privacy and regulatory compliance requirements. As we’ve enumerated in the above examples, attackers can additionally manipulate or delete data, disrupt application functionality, and even gain control over the entire application infrastructure.

Understanding and mitigating broken access control vulnerabilities is crucial for developers to maintain the security and integrity of their applications. Implementing strong access control mechanisms, conducting thorough security testing, and leveraging tools like Snyk Code to detect insecure coding patterns can help identify and remediate these vulnerabilities before malicious actors exploit them.

Next steps to prevent broken access control

For more information on preventing broken access control, refer to the OWASP guidelines and consider using tools like Snyk Code to identify and fix security vulnerabilities in your code. You can sign up for Snyk. It’s free!

The following are recommended educational resources on preventing broken access control and training your developer team to protect against unauthorized access:

• Go through the Snyk Learn path for OWASP Top 10, which begins with Broken Access Control's interactive curriculum, insights, and best practices.

• JavaScript and Node.js users can refer to the guide on Preventing broken access control in express Node.js applications.

• If you’re concerned about authentication and authorization, you’re most likely building API-related backends. If so, refer to OWASP API Security Top 10 Risks.