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Essential PHP Security by Chris Shiflett

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Chapter 4. Sessions and Cookies

This chapter discusses sessions and the inherent risks associated with stateful web applications. You will first learn the fundamentals of state, cookies , and sessions; then I will discuss several concerns—cookie theft, exposed session data, session fixation, and session hijacking—along with practices that you can employ to help prevent them.

The rumors are true: HTTP is a stateless protocol. This description recognizes the lack of association between any two HTTP requests. Because the protocol does not provide any method that the client can use to identify itself, the server cannot distinguish between clients.

While the stateless nature of HTTP has some important benefits—after all, maintaining state requires some overhead—it presents a unique challenge to developers who need to create stateful web applications. With no way to identify the client, it is impossible to determine whether the user is already logged in, has items in a shopping cart, or needs to register.

An elegant solution to this problem, originally conceived by Netscape, is a state management mechanism called cookies. Cookies are an extension of the HTTP protocol. More precisely, they consist of two HTTP headers: the Set-Cookie response header and the Cookie request header.

When a client sends a request for a particular URL, the server can opt to include a Set-Cookie header in the response. This is a request for the client to include a corresponding Cookie header in its future requests. Figure 4-1 illustrates this basic exchange.

A complete cookie exchange that involves two HTTP transactions

Figure 4-1. A complete cookie exchange that involves two HTTP transactions

If you use this concept to allow a unique identifier to be included in each request (in a Cookie header), you can begin to uniquely identify clients and associate their requests together. This is all that is required for state, and this is the primary use of the mechanism.

Tip

The best reference for cookies is still the specification provided by Netscape at http://wp.netscape.com/newsref/std/cookie_spec.html. This most closely resembles industry support.

The concept of session management builds upon the ability to maintain state by maintaining data associated with each unique client. This data is kept in a session data store, and it is updated on each request. Because the unique identifier specifies a particular record in the session data store, it's most often called the session identifier.

If you use PHP's native session mechanism, all of this complexity is handled for you. When you call session_start(), PHP first determines whether a session identifier is included in the current request. If one is, the session data for that particular session is read and provided to you in the $_SESSION superglobal array. If one is not, PHP generates a session identifier and creates a new record in the session data store. It also handles propagating the session identifier and updating the session data store on each request. Figure 4-2 illustrates this process.

While this convenience is helpful, it is important to realize that it is not a complete solution. There is no inherent security in PHP's session mechanism, aside from the fact that the session identifier it generates is sufficiently random, thereby eliminating the practicality of prediction. You must provide your own safeguards to protect against all other session attacks. I will show you a few problems and solutions in this chapter.

Cookie Theft

One risk associated with the use of cookies is that a user's cookies can be stolen by an attacker. If the session identifier is kept in a cookie, cookie disclosure is a serious risk, because it can lead to session hijacking .

PHP handles the complexity of session management for you

Figure 4-2. PHP handles the complexity of session management for you

The two most common causes of cookie disclosure are browser vulnerabilities and cross-site scripting (discussed in Chapter 2). While no such browser vulnerabilities are known at this time, there have been a few in the past—the most notable ones are in Internet Explorer Versions 4.0, 5.0, 5.5, and 6.0 (corrective patches are available for each of these vulnerabilities).

While browser vulnerabilities are certainly not the fault of web developers, you may be able to take steps to mitigate the risk to your users. In some cases, you may be able to implement safeguards that practically eliminate the risk. At the very least, you can try to educate your users and direct them to a patch to fix the vulnerability.

For these reasons, it is good to be aware of new vulnerabilities. There are a few web sites and mailing lists that you can keep up with, and many services are beginning to offer RSS feeds, so that you can simply subscribe to the feed and be alerted to new vulnerabilities. SecurityFocus maintains a list of software vulnerabilities at http://online.securityfocus.com/vulnerabilities, and you can filter these advisories by vendor, title, and version. The PHP Security Consortium also maintains summaries of the SecurityFocus newsletters at http://phpsec.org/projects/vulnerabilities/securityfocus.html.

Cross-site scripting is a more common approach used by attackers to steal cookies. An attacker can use several approaches, one of which is described in Chapter 2. Because client-side scripts have access to cookies, all an attacker must do is write a script that delivers this information. Creativity is the only limiting factor.

Protecting your users from cookie theft is therefore a combination of avoiding cross-site scripting vulnerabilities and detecting browsers with security vulnerabilities that can lead to cookie exposure. Because the latter is so uncommon (with any luck, these types of vulnerabilities will remain a rarity), it is not the primary concern but rather something to keep in mind.

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