4.1. Cookie Theft
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 pastthe 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.