Welcome to another insightful entry in our “Code of the Week” series, where we unravel the subtle intricacies and hidden vulnerabilities within our code. This week marks our fourth exploration into code pitfalls that are often overlooked but can have significant implications if left unaddressed.

Today, we delve into a critical aspect of web application security – Cross-Site Request Forgery (CSRF) protections. Our focus will be on a commonly employed but occasionally flawed implementation of CSRF token validation. By scrutinizing this segment, we aim to unveil how even well-intentioned security measures can be bypassed, leading to potential exploits.

Without further ado, let’s examine the code, spot the vulnerability, and discuss the measures needed to rectify this overlooked security gap.

The code

The rule of the game is straightforward: spot the bug! Keep in mind, for this snippet and any others in the series, if something isn’t explicitly shown, you can assume it’s either not within the user’s control or it’s not a source of vulnerability.

def isValid(token: str, context: str) -> bool:
    """Check if a token is valid for a given context/action"""
    db = get_db_cursor()
    query = "SELECT owner_id, token, context, expiry FROM tokens WHERE token = ? AND context = ? AND expiry > ?"
    db.execute(query, (token, context, datetime.now()))
    return db.fetchone() is not None

@app.route("/user/update", methods=["POST"])
def update_user():
    context = "user_update"
    token = request.form.get("csrf_token", None)
    if not isValid(token, context):
        return "Invalid CSRF token", 400

The bug

The current implementation allows a user to submit a CSRF token that could be tied to a different user’s session. Since the validation function isValid only checks whether the token is valid and not whether the token belongs to the user making the request, an attacker could exploit this by using a valid token from another session they’ve compromised.

The fix

To address this issue, we need to modify the isValid function to ensure that the token not only exists, but also belongs to the user making the request. We can do this by adding a user identification parameter (like user_id) to the function and including this in the database query.

The detection

Detecting such vulnerabilities requires understanding both the application’s flow and the importance of matching session tokens to the correct user in this situation. Static analysis tools might not catch this kind of logical issue, highlighting the importance of manual code reviews and security-focused testing.

The conclusion

This episode underscores the importance of thorough validation checks in security-sensitive operations like CSRF protection. It’s a reminder that security is not just about preventing unauthorized access, but also ensuring that each component of the system behaves as intended, especially in terms of user session and identity verification.

Through vigilant code review and testing, we can fortify our applications against these types of vulnerabilities. Let this be a reminder of the ongoing need for security in our coding practices. Stay tuned for more insights and remember, a secure application is a journey, not a one-time destination.