cookie phpsessid will be soon treated as cross site cookie against with code examples

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The PHP session identifier (PHPSESSID) has long been a common way to maintain user sessions on the server side of web applications written in PHP. When a user logs in, a PHP script generates a unique PHPSESSID value and associates it with the user's session data stored on the server. Subsequent requests from the same user carry the PHPSESSID value as a cookie or a URL parameter, allowing the server to retrieve the session data and continue the user's context-sensitive interactions.

However, the PHPSESSID cookie is facing a significant change in its treatment by web browsers with regard to security and privacy. Starting from version 88, Google Chrome plans to classify the PHPSESSID cookie as a cross-site cookie, which means that it will be subject to the same restrictions as third-party cookies for tracking and sharing user activity across websites.

The main reason for this change is to prevent certain kinds of attacks that abuse PHPSESSID values for session fixation or cookie injection, both of which can be used to hijack user sessions and gain unauthorized access to sensitive resources on the server or other sites. As a cross-site cookie, the PHPSESSID cookie will be more closely scrutinized by default and may not be sent by browsers on cross-site requests unless certain conditions are met.

What does it mean for PHP developers and webmasters who rely on the PHPSESSID cookie? In short, they need to adapt their code and configuration to comply with the new browser policies and handle session management more carefully. Here are some examples of what needs to be changed or checked.

  1. Set the SameSite attribute for the PHPSESSID cookie

The SameSite attribute is a relatively new feature that allows a cookie to be marked as either "SameSite" or "None" with respect to its origin site. A SameSite cookie cannot be sent on cross-site requests, whereas a None cookie can be sent only if it has the Secure attribute and is delivered over HTTPS. By default, PHPSESSID cookies are not flagged with a SameSite attribute, which means they will be treated as left unspecified or "Lax" by browsers, depending on their interpretation.

To ensure that the PHPSESSID cookie is not considered a cross-site cookie, you can add the following line of code before any output is sent to the browser, typically in the session_start() function:

ini_set('session.cookie_samesite', 'Strict');

This sets the SameSite value to "Strict" for all PHPSESSID cookies created by the session functions. Alternatively, you can set it to "Lax" or "None" depending on the desired behavior and security requirements. Note that some older browsers or clients may not support the SameSite attribute and may fail to send PHPSESSID cookies altogether. In such cases, you may need to fall back to URL-based session management or use a more compatible session handling library.

  1. Secure the PHPSESSID cookie

As mentioned earlier, the None cookies can be sent only over a secure connection that uses HTTPS. Therefore, you should make sure that your website always uses HTTPS for sensitive pages and forms that involve user authentication or session management. You can enforce HTTPS by redirecting any HTTP requests to HTTPS using a redirect rule or modifying the server configuration (e.g., .htaccess for Apache).

Moreover, you should also enable the Secure attribute for the PHPSESSID cookie, which instructs the browser to send the cookie only over HTTPS connections. To do this, you can add the following line of code:

ini_set('session.cookie_secure', 1);

This applies to all session cookies generated by PHP, not just PHPSESSID, so you should make sure that your site does not rely on other insecure cookies that may be used in combination with PHPSESSID for tracking or attack purposes.

  1. Use appropriate session IDs

The PHPSESSID value itself can be targeted by attackers who try to guess or manipulate it to gain access to other users' sessions or impersonate them. Therefore, you should use cryptographically strong and unpredictable session IDs that cannot be easily guessed or brute-forced. You can achieve this by setting the session ID generator function to a custom one that employs a secure random number generator and a hashing algorithm.

Here is an example of how to use the random_bytes() function and the SHA-256 hash function to generate a 32-byte (256-bit) session ID:

ini_set('session.use_strict_mode', 1);
ini_set('session.cookie_httponly', 1);
ini_set('session.gc_maxlifetime', 3600);
ini_set('session.cookie_lifetime', 0);
session_set_save_handler(...); // custom session save handler

function generate_session_id() {
  return bin2hex(random_bytes(32));
}

session_id(generate_session_id());
session_start();

This code also enables other recommended session security options, such as strict mode, HTTP-only cookies, and no cookie lifetime, which minimize the exposure and persistence of session data on the client side. You can adjust the session lifetime and the save handler according to your application needs.

  1. Avoid cookies in AJAX requests

Finally, you should avoid using the PHPSESSID cookie in AJAX requests that may cross site boundaries, as the cookie may trigger a CORS (Cross-Origin Resource Sharing) pre-flight request or be blocked by the client-side security policy. Instead, you can pass the session ID in a custom HTTP header or in the request body, or use a session token that is not directly associated with a cookie. For example:

$(document).ajaxSend(function(event, jqXHR, settings) {
  if (settings.url.startsWith('https://myapp.com/api/')) {
    jqXHR.setRequestHeader('X-MyApp-Session', '<?php echo session_id(); ?>');
  }
});

This code adds a custom header to every AJAX request sent from a page running on the myapp.com domain to the API endpoint that requires session authentication. The server can then retrieve the session ID from the header and validate it against the session data stored on the server. Note that you need to make sure that the X-MyApp-Session header is allowed by the server's CORS policy and does not expose the session ID to unintended parties.

Conclusion

The upcoming change in the treatment of the PHPSESSID cookie as a cross-site cookie is a significant challenge for PHP developers and businesses that rely on session-based web applications. However, by following the best practices outlined above and staying informed of the latest browser policies and security guidelines, you can mitigate the risks and ensure the continued reliability and security of your web applications. Remember to test your code thoroughly and consult with experienced professionals if you have any doubts or concerns.

here are some additional points that can be included in the article:

  1. Separate session cookies from other cookies

Another way to prevent session hijacking or fixation attacks is to isolate the PHPSESSID cookie from other cookies that your site may use for non-session-related purposes, such as tracking, advertising, or analytics. You can do this by setting a unique prefix for the PHPSESSID cookie name in the php.ini file, as follows:

session.name = MYAPPSESSID

This will rename the PHPSESSID cookie to MYAPPSESSID, which makes it distinct from other cookies and less likely to be tampered with or confused by user agents or third-party scripts. However, you need to update all your session handling code to use the new name consistently.

Alternatively, you can use the session_set_cookie_params() function to set a custom path and domain for the PHPSESSID cookie, which limits its scope and reduces the risk of exposure to other sites or subdomains:

session_set_cookie_params(0, '/myapp/', 'myapp.com');

This code sets the session cookie lifetime to 0 (i.e., "until the browser is closed"), the path to "/myapp/" (i.e., only accessible within the "/myapp" directory), and the domain to "myapp.com" (i.e., only accessible from the same origin or its subdomains). You can adjust these parameters according to your site's structure and access requirements.

  1. Secure the session data on the server side

While the PHPSESSID cookie itself is vulnerable to attacks, the session data stored on the server side can also be targeted if it is not protected properly. To minimize the risk of session hijacking or leaking, you should follow some common security practices, such as:

  • Encrypt the session data using a strong encryption algorithm and a secret key that is not exposed to users or attackers. You can use the openssl extension in PHP to encrypt and decrypt data, as shown in this example:
$key = openssl_random_pseudo_bytes(32); // generate a random key
$session_data = "Hello, world!";
$encrypted_data = openssl_encrypt($session_data, "AES-256-CBC", $key);
$decrypted_data = openssl_decrypt($encrypted_data, "AES-256-CBC", $key);

This code generates a 256-bit key and uses it to encrypt and decrypt the session data using the AES-256-CBC cipher. You can store the encrypted data in a database or a file, or in memory if you have a dedicated server or cluster with no shared variables.

  • Validate the session data against the user's IP address, user agent, or other fingerprinting factors that can verify the authenticity of the user. You can use the session_regenerate_id() function with the true parameter to regenerate the session ID and create a new session data file on the server side, which ensures that the old session data is not reused by attackers:
session_regenerate_id(true);

This code generates a new session ID and creates a new session data file on the server side. You should call this function after the user has authenticated or reauthenticated, or when the user's configuration or behavior has changed significantly (e.g., changing the device, network, or location).

  • Avoid storing sensitive or critical data in the session data, such as passwords, credit card numbers, or authentication tokens. Instead, you should store them separately and encrypt them using a strong and verified mechanism, such as the password_hash() function or a third-party library.
  1. Keep up with the browser policies and updates

Finally, you should stay informed of the latest browser policies and updates that affect the PHPSESSID cookie and other cookies on your site. Google Chrome is not the only browser that has changed or plans to change how cookies are handled and shared across sites; for example, Mozilla Firefox introduced some changes to cookie and tracking policies in late 2019, and Apple Safari also has its own rules regarding cookies and sessions.

You can check the official documentation and announcements from each browser vendor, join relevant forums or mailing lists, or consult with security experts and auditors to ensure that your site remains compliant and secure in the changing landscape of web technologies and threats.

Conclusion

The PHPSESSID cookie is a fundamental component of many PHP-based web applications that rely on session management and user authentication. However, its role in security and privacy is under scrutiny due to recent changes in browser policies and standards. To adapt to these changes and protect your users' sessions from attacks and breaches, you need to implement best practices and guidelines, such as setting the SameSite and Secure attributes, using secure and unpredictable session IDs, separating session cookies from other cookies, securing the session data on the server side, and keeping up with the browser policies and updates. By doing so, you can ensure that your site remains trustworthy, reliable, and compliant in the eyes of users, regulators, and partners.

Popular questions

  1. What is the main reason for the upcoming change in the treatment of the PHPSESSID cookie as a cross-site cookie?

The main reason for this change is to prevent certain kinds of attacks that abuse PHPSESSID values for session fixation or cookie injection, both of which can be used to hijack user sessions and gain unauthorized access to sensitive resources on the server or other sites.

  1. What is the SameSite attribute and why is it important for session cookies?

The SameSite attribute is a relatively new feature that allows a cookie to be marked as either "SameSite" or "None" with respect to its origin site. A SameSite cookie cannot be sent in cross-site requests, which helps prevent certain attacks and tracking methods. For session cookies like PHPSESSID, setting the SameSite attribute to "Strict" or "Lax" is recommended to ensure that they are not treated as cross-site cookies by browsers.

  1. How can PHP developers generate secure and unpredictable session IDs?

PHP developers can use cryptographically strong and unpredictable session IDs that cannot be easily guessed or brute-forced by generating them using a custom function that employs a secure random number generator and a hashing algorithm. For example, one can use the random_bytes() function and the SHA-256 hash function to generate a 32-byte (256-bit) session ID.

  1. What is the difference between SameSite cookies and None cookies?

SameSite cookies can only be sent in first-party (same-site) requests, whereas None cookies can be sent in cross-site requests, but only if they have the Secure attribute and are delivered over HTTPS. SameSite cookies are a stricter and more secure option for session cookies like PHPSESSID, as they prevent the cookie from being used in cross-site requests and reduce the risk of certain attacks.

  1. What should developers do to prepare for the change in the treatment of the PHPSESSID cookie?

Developers should adapt their code and configuration to comply with the new browser policies and handle session management more carefully. This can include setting the SameSite and Secure attributes for the cookie, using appropriate session IDs, isolating session cookies from other cookies, securing the session data on the server side, and avoiding the use of cookies in cross-site AJAX requests. Additionally, staying informed of the latest browser policies and updates that affect cookies and sessions is important to maintain compliance and security.

Tag

Security

Throughout my career, I have held positions ranging from Associate Software Engineer to Principal Engineer and have excelled in high-pressure environments. My passion and enthusiasm for my work drive me to get things done efficiently and effectively. I have a balanced mindset towards software development and testing, with a focus on design and underlying technologies. My experience in software development spans all aspects, including requirements gathering, design, coding, testing, and infrastructure. I specialize in developing distributed systems, web services, high-volume web applications, and ensuring scalability and availability using Amazon Web Services (EC2, ELBs, autoscaling, SimpleDB, SNS, SQS). Currently, I am focused on honing my skills in algorithms, data structures, and fast prototyping to develop and implement proof of concepts. Additionally, I possess good knowledge of analytics and have experience in implementing SiteCatalyst. As an open-source contributor, I am dedicated to contributing to the community and staying up-to-date with the latest technologies and industry trends.
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