TamperMonkey set strange cookies

2 min read 22-10-2024

the ifix

TamperMonkey is a popular userscript manager that allows you to customize the way web pages look and function. However, some users have reported experiencing strange cookie behaviors while using this tool. Let's delve into the issue and understand the potential implications and solutions.

Original Code Issue

To set the context, let's consider a scenario where a userscript in TamperMonkey appears to set cookies in an unintended manner. Here is a hypothetical code snippet that illustrates the issue:

// ==UserScript==
// @name         Strange Cookie Setter
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  A script that sets cookies strangely
// @author       You
// @match        http://example.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    document.cookie = "name=value; expires=Wed, 01 Jan 2025 00:00:00 UTC; path=/";
})();

The Problem with Cookie Management

In the above script, the line document.cookie is used to set a cookie, but this code may lead to unexpected behavior, such as overwriting existing cookies or setting cookies that aren't properly secured (for example, without the SameSite or Secure attributes). This can compromise user privacy or lead to unwanted tracking.

Potential Causes

Script Conflicts: TamperMonkey allows multiple scripts to run on the same page. If multiple scripts are trying to manage cookies simultaneously, it could lead to conflicts.
Domain and Path Mismatch: Cookies are domain-specific. If the script targets the wrong domain or path, the cookie may not behave as intended.
Browser Settings: Some browsers have settings that affect how cookies are stored and sent. It's essential to ensure that the browser is configured to allow the kind of cookie management your script intends.

Best Practices for Cookie Management

Here are some best practices for managing cookies in your TamperMonkey userscripts:

Use Proper Attributes: When setting cookies, consider adding attributes like Secure, HttpOnly, and SameSite to protect against common security vulnerabilities.
```
document.cookie = "name=value; expires=Wed, 01 Jan 2025 00:00:00 UTC; path=/; Secure; HttpOnly; SameSite=Lax";
```
Check for Existing Cookies: Before setting a new cookie, check if a cookie with the same name already exists to avoid overwriting crucial data.
Debugging Tools: Utilize browser developer tools to monitor cookies in real-time. You can view existing cookies, their attributes, and understand how your script interacts with them.

Practical Example

Let’s see how you can create a more robust userscript for cookie management:

// ==UserScript==
// @name         Improved Cookie Setter
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  A script that sets cookies safely
// @author       You
// @match        http://example.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    
    // Function to set a cookie safely
    function setCookie(name, value, days) {
        const expires = new Date(Date.now() + days * 864e5).toUTCString();
        document.cookie = `${name}=${encodeURIComponent(value)}; expires=${expires}; path=/; Secure; HttpOnly; SameSite=Lax`;
    }
    
    // Check if the cookie already exists
    if (!document.cookie.includes("name=")) {
        setCookie("name", "value", 365);  // Set cookie for 365 days
    }
})();

Conclusion

Strange cookie behaviors in TamperMonkey can lead to issues ranging from minor inconveniences to significant security concerns. By following best practices in cookie management and utilizing debugging tools, users can ensure that their scripts work as intended without unintended side effects.

Additional Resources

Understanding and managing cookies effectively is crucial for both user experience and web security. With proper script management and understanding of how cookies work, you can enhance your web browsing experience with TamperMonkey.