Software Engineering Institute

Cross-site scripting is a form of attack in which an intruder leverages the trust between a victim and a web-site the victim trusts. Quoting from CERT Advisory CA-2001-02:

Many Internet web sites overlook the possibility that a client may send malicious data intended to be used only by itself. This is an easy mistake to make. After all, why would a user enter malicious code that only the user will see?

However, this situation may occur when the client relies on an untrustworthy source of information when submitting a request. For example, an attacker may construct a malicious link such as

<A HREF="http://example.com/comment.cgi? mycomment=<SCRIPT>malicious code</SCRIPT>"> Click here</A>

When an unsuspecting user clicks on this link, the URL sent to example.com includes the malicious code. If the web server sends a page back to the user including the value of mycomment, the malicious code may be executed unexpectedly on the client. This example also applies to untrusted links followed in email or newsgroup messages.

Abuse of other tags

In addition to scripting tags, other HTML tags such as the <FORM> tag have the potential to be abused by an attacker. For example, by embedding malicious <FORM> tags at the right place, an intruder can trick users into revealing sensitive information by modifying the behavior of an existing form. Other HTML tags can also be abused to alter the appearance of the page, insert unwanted or offensive images or sounds, or otherwise interfere with the intended appearance and behavior of the page.

In this case, when IIS issues a redirect response message, it includes unsanitized derived from the URL in the resulting error message. If an intruder convinces a victim to follow a link with malicious content in it, he can cause the web server to return a page largely under the control of the intruder. If the victim trusts the web site (specifically if Javascript or other script from that site is permitted to run) the intruder can execute arbitrary script as if it came from the web site. Ironically, if the victim is using Microsoft Internet Explorer (IE), he is not vulnerable to this attack since IE recognizes the redirect response message and displays a message of its own, rather than the resulting HTML generated by the web site.

For more information, see Microsoft Security Bulletin MS02-018.

IIS is a very popular web server, and any client that has a trust relationship with an IIS web site may be vulnerable if that site issues redirect response messages.

For a description of the potential impact, see http://www.cert.org/advisories/CA-2000-02.html#impact.

For a description of the range of solutions to this problem, see http://www.cert.org/advisories/CA-2000-02.html#solution. In this instance, web site managers should apply a patch as described in MS02-018.