Cross-site scripting (XSS) enables an attacker to send a customized request to a Web site that causes modified Web or e-mail code to be sent to another user. In other words, it allows an attacker to send malicious code to another user by exploiting a flaw or weakness in an Internet server. XSS attacks are used to exploit vulnerabilities on a victim's system to traffic malicious code rather than attack the system itself.
While XSS is not the most severe problem affecting Internet servers, it is still important enough to take seriously. Script or code sent to a victim via an XSS attack runs within the security context of the browser or e-mail viewer employed on the victim's system. In many cases this allows full read and write access to all of the user's personal data files and a considerable portion of the OS itself, such as driver files and configuration settings.
A vulnerability was recently discovered in Microsoft Internet Security and Acceleration (ISA) Server 2000 that allows XSS. An attacker can alter the error pages for failed page requests or invalid data submissions that are sent to clients from ISA. The error pages can be altered so that they direct victims to download malicious code or access a malicious Web site. The compromised error pages can also force automatic download or URL activity on the victim's system.
This vulnerability in ISA is easily dealt with through a simple patch. If you are using ISA to protect your Internet server,
About the author
James Michael Stewart is a partner and researcher for ITinfopros, a technology-focused writing and training organization.
For more information on this topic, visit these resources:
- Web Security Tip: Deal with cross-site scripting
- Web Security Tip: Anatomy of a hack
- News & Analysis: Top Web application security problems identified
This was first published in August 2003