Php hacking techniques Part 1
Posted by acstopstar
Last Updated: March 18, 2012
  2471

Techniques for securing user input, then extended those techniques to applications where database input and output is required, looking at some SQL security issues. next is to deal with file operations and file uploads, looking specifically at the security issues involved with accessing files based on some user supplied filename, and user-supplied files (uploaded files). 


What Are Sessions?
Sessions are a PHP construct allowing persistent data to be retained across HTTP connections. In English, sessions allow you to store the values of certain variables across page visits. This is achieved by serializing the data (converting it to some binary representation) and writing it out to a file (or a database, or wherever you tell it), when a page is finished processing in PHP. When the next page (or that same page some time later) is processed, and PHP is told to start a session, it will check if the user already has a session, and read their data back in, unserializing it and assigning the variables. This allows you to keep track of a user across multiple visits, or while browsing multiple pages on your site.

For example, you can create a shopping cart using sessions, storing an array of items added to the cart in a session variable, and loading it on every page. When the user clicks 'Add to cart' you can add the item to the array, and it will be saved for the next page the user goes to. The whole array can be fetched on your checkout page and appropriate processing will take place.

How Do Sessions Work?
As many probably know, HTTP is a stateless protocol. By stateless, I mean that any HTTP connection is unaware of previous connections made by the same client, to the same server (persistent connections excepting). There are two useful ways in which PHP can pass identification information between pages in order to uniquely associate a user with a session.

PHP can use cookies to store a session ID. The cookie value is sent on every request, so PHP can match that up to its session data and retrieve the correct set of variables for that user. Another way is to pass the session ID in URLs. In order to do this, URL rewriting must be enabled.

Passing session data in URLs is not recommended since it is possible to pass your session onto another user if you give them a link which contains your session ID, and the session ID data is more easily attackable than in a cookie. URL-based session tracking should be used only where cookies cannot.

Using $_SESSION
PHP provides a super-global variable named $_SESSION. By super-global I mean it is a global variable which you may access without going via $_GLOBALS or stating global $_SESSION within a function. In this way, it behaves like $_GET and $_POST.

$_SESSION is, in fact, an associative array. The keys are variable names, and the values are the stored session data for that variable name.

Using $_SESSION is preferred over the use of session_register() to register ordinary global variables as session variables, especially when register_globals is enabled, since global variables may be more easily changed inadvertently than the contents of $_SESSION. It is still possible to alias ordinary global variables to their equivalents within $_SESSION,

$username = &$_SESSION["username"];

Here, the & indicates a reference, or alias. It is then possible to use $username instead of $_SESSION["username"], but note that $username is an ordinary variable, and you will have to access as $_GLOBALS["username"] or global $username from within a function.

Trusting Session Data
Since a session ID can be spoofed, it is always wise to perform some extra validation where possible. The simplest mechanism would be to store the IP address of the client to whom the session ID was issued, and compare the client IP against that stored IP every session. This will prevent the basic security problems associated with passing links between computers (though not if the computers are on a private network and share a single public IP address).

Session data is also stored in files on the server. The default location is /tmp on UNIX, or the system temporary file directory on Windows. If /tmp is world-writable (or, in some cases, world-readable), or there are multiple websites hosted on a single server, storing session data in a public location is not secure. PHP provides a way to change the way session data is stored.

Changing The Session File Path
The location in which PHP saves session data can be set using the php.ini directive session.save_path, or the string below in httpd.conf or a virtual host configuration.

php_value session.save_path "/home/test/sessions/"

It is important to ensure that your session data path is included in the paths allowed by open_basedir, if you have open_basedir settings or PHP Safe Mode enabled.

The data representation used for saving session data to files can be controlled with the session.serialize_handler directive in php.ini. By default, PHP uses its own built in format, but the WDDX ( http://www.wddx.org ) format can be used also. Set the type using one of the lines below.

(in php.ini ...)

session.serialize_handler wddx

or

session.serialize_handler php

(or in httpd.conf ...)

php_value session.serialize_handler wddx

or

php_value session.serialize_handler php

Storing Session Data In A Database
When you use on-disk files to store session data, those files must be readable and writeable by PHP. On a multi-user hosting system, it is possible for other users to access your session data through the PHP process (but see the commentary on open_basedir in part 5 of this series. The best way to secure your session data is to store it in a database.

Unfortunately, there is no direct way to store session data in a database using the php.ini directives, but luckily PHP provides a mechanism for customised session storage handlers. The function session_set_save_handler() allows you to register handler functions for session management. These functions must be written in PHP (or made available as a PHP extension).

session_set_save_handler(open_fn, close_fn, read_fn, write_fn,

destroy_fn, gc_fn)

To use these user-supplied session storage handlers, you must set session.save_handler to the value user, and the value of session.save_path should be the name of the database into which you're saving session data (so that the session save handler functions you define can locate and use that database). The value of session.name can be used as the name of the table within the database.

(httpd.conf)



php_value session.save_handler user

php_value session.save_path dbname

php_value session.name session_data



Next, a table for storing session data must exist in the database. At the minimum, your session handler should keep track of the session ID, the expiration time, and the serialized session data. The SQL below creates a simple table for storing this data.

CREATE TABLE session_data (

sessionid text not null PRIMARY KEY,

expiration timestamp,

sessiondata text not null

);

The final task is to create the functions which manage this session store, and register them with session_set_save_handler(). The open_fn must open the database connection, the close_fn must close it and perform any associated cleanup tasks, and the read_fn and write_fn functions must read and write session data respectively. destroy_fn is called when a session ends and is destroyed, and gc_fn is called when session data is garbage collected. These operations must be mapped into database queries by your PHP code. The prototypes for the functions are given below, and parameters passed are explained.

function open_fn($save_path, $session_name)

$save_path is the value of session.save_path, $session_name is the value of session.name

function close_fn()

Takes no arguments

function read_fn($session_id, $data)

$session_id is the session ID for which PHP requests the associated session data to be returned

function write_fn($session_id)

$session_id is the session ID for which PHP requests that $data be associated with in the session store (database)

function destroy_fn($session_id)

$session_id is the ID of a session which may be removed from the store

function gc_fn($max_time)

$max_time is the oldest last modified time to retain in the session store. Sessions with an older modified time than this are to be removed from the store.
-- Continued in Next pPrt