Sessions and security

External links: Session fixation

The session module cannot guarantee that the information you store in a session is only viewed by the user who created the session. You need to take additional measures to actively protect the integrity of the session, depending on the value associated with it.

Assess the importance of the data carried by your sessions and deploy additional protections -- this usually comes at a price, reduced convenience for the user. For example, if you want to protect users from simple social engineering tactics, you need to enable session.use_only_cookies. In that case, cookies must be enabled unconditionally on the user side, or sessions will not work.

There are several ways to leak an existing session id to third parties. A leaked session id enables the third party to access all resources which are associated with a specific id. First, URLs carrying session ids. If you link to an external site, the URL including the session id might be stored in the external site's referrer logs. Second, a more active attacker might listen to your network traffic. If it is not encrypted, session ids will flow in plain text over the network. The solution here is to implement SSL on your server and make it mandatory for users.

Requirements

No external libraries are needed to build this extension.

Installation

Session support is enabled in PHP by default. If you would not like to build your PHP with session support, you should specify the --disable-session option to configure. To use shared memory allocation (mm) for session storage configure PHP --with-mm[=DIR] .

The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.

Runtime Configuration

The behaviour of these functions is affected by settings in php.ini.

For further details and definition of the PHP_INI_* constants see ini_set().

The session management system supports a number of configuration options which you can place in your php.ini file. We will give a short overview.

The track_vars and register_globals configuration settings influence how the session variables get stored and restored.

Resource Types

This extension has no resource types defined.

Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

Examples

If register_globals is disabled, only members of the global associative array $_SESSION can be registered as session variables. The restored session variables will only be available in the array $_SESSION.

Use of $_SESSION (or $HTTP_SESSION_VARS with PHP 4.0.6 or less) is recommended for improved security and code readability. With $_SESSION, there is no need to use the session_register(), session_unregister(), session_is_registered() functions. Session variables are accessible like any other variables.

Example 1. Registering a variable with $_SESSION. <?php session_start(); // Use $HTTP_SESSION_VARS with PHP 4.0.6 or less if (!isset($_SESSION['count'])) {    $_SESSION['count'] = 0; } else {    $_SESSION['count']++; } ?>

Example 2. Unregistering a variable with $_SESSION and register_globals disabled. <?php session_start(); // Use $HTTP_SESSION_VARS with PHP 4.0.6 or less unset($_SESSION['count']); ?>

Example 3. Unregistering a variable with register_globals enabled, after registering it using $_SESSION. <?php session_start(); // With PHP 4.3 and later, you can also simply use the prior example. session_unregister('count'); ?>

If register_globals is enabled, then each global variable can be registered as session variable. Upon a restart of a session, these variables will be restored to corresponding global variables. Since PHP must know which global variables are registered as session variables, users need to register variables with session_register() function. You can avoid this by simply setting entries in $_SESSION.

Example 4. Registering a variable with register_globals enabled <?php if (! isset($_SESSION['count'])) {    $_SESSION['count'] = 1; } else {    $_SESSION['count']++; } ?>

If register_globals is enabled, then the global variables and the $_SESSION entries will automatically reference the same values which were registered in the prior session instance.

There is a defect in PHP 4.2.3 and earlier. If you register a new session variable by using session_register(), the entry in the global scope and the $_SESSION entry will not reference the same value until the next session_start(). I.e. a modification to the newly registered global variable will not be reflected by the $_SESSION entry. This has been corrected in PHP 4.3.

Passing the Session ID

There are two methods to propagate a session id:

• Cookies

• URL parameter

The session module supports both methods. Cookies are optimal, but because they are not always available, we also provide an alternative way. The second method embeds the session id directly into URLs.

PHP is capable of transforming links transparently. Unless you are using PHP 4.2 or later, you need to enable it manually when building PHP. Under Unix, pass --enable-trans-sid to configure. If this build option and the run-time option session.use_trans_sid are enabled, relative URIs will be changed to contain the session id automatically.

Alternatively, you can use the constant SID which is always defined. If the client did not send an appropriate session cookie, it has the form session_name=session_id. Otherwise, it expands to an empty string. Thus, you can embed it unconditionally into URLs.

The following example demonstrates how to register a variable, and how to link correctly to another page using SID.

Example 5. Counting the number of hits of a single user <?php if (!session_is_registered('count')) {    session_register('count');    $count = 1; } else {    $count++; } ?> <p> Hello visitor, you have seen this page <?php echo $count; ?> times. </p> <p> To continue, <a href="nextpage.php?<?php echo strip_tags(SID); ?>">click here</a>. </p>

The strip_tags() is used when printing the SID in order to prevent XSS related attacks.

Printing the SID, like shown above, is not necessary if --enable-trans-sid was used to compile PHP.

Custom Session Handlers

To implement database storage, or any other storage method, you will need to use session_set_save_handler() to create a set of user-level storage functions.