XSS
From TechWiki
XSS aka Cross Site Scripting
Cross site scripting is becoming a popular hacking method in which a hacker uses a web application to send malicious code to another user. The malicious code can do several things such as harming the user's computer, accessing the user's cookie information to obtain private information, and even rewrite web pages. These are only a few of the many harmful tasks XSS attacks can do.
XSS has been growing in popularity recently, and we believe it being fueled by the growing number of Web 2.0 sites on the internet. Because of the nature of Web 2.0 sites such as Myspace and Facebook which allows user's to edit web pages and input scripting code and to share information freely and interact with other user's XSS has grown in popularity. A more detailed explanation of XSS is provided below courtesy of Wikipedia:
"Cross-site scripting (XSS) attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user in the output it generates without validating or encoding it.
An attacker can use XSS to send malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by your browser and used with that site. These scripts can even rewrite the content of the HTML page.
XSS attacks can generally be categorized into two categories: stored and reflected. Stored attacks are those where the injected code is permanently stored on the target servers, such as in a database, in a message forum, visitor log, comment field, etc. The victim then retrieves the malicious script from the server when it requests the stored information. Reflected attacks are those where the injected code is reflected off the web server, such as in an error message, search result, or any other response that includes some or all of the input sent to the server as part of the request. Reflected attacks are delivered to victims via another route, such as in an e-mail message, or on some other web server. When a user is tricked into clicking on a malicious link or submitting a specially crafted form, the injected code travels to the vulnerable web server, which reflects the attack back to the user’s browser. The browser then executes the code because it came from a ‘trusted’ server.
The consequence of an XSS attack is the same regardless of whether it is stored or reflected. The difference is in how the payload arrives at the server. Do not be fooled into thinking that a “read only” or “brochureware” site is not vulnerable to serious reflected XSS attacks. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. The most severe XSS attacks involve disclosure of the user’s session cookie, allowing an attacker to hijack the user’s session and take over the account. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, and modifying presentation of content. An XSS vulnerability allowing an attacker to modify a press release or news item could affect a company’s stock price or lessen consumer confidence. An XSS vulnerability on a pharmaceutical site could allow an attacker to modify dosage information resulting in an overdose.
Attackers frequently use a variety of methods to encode the malicious portion of the tag, such as using Unicode, so the request is less suspicious looking to the user. There are hundreds of variants of these attacks, including versions that do not even require any < > symbols. For this reason, attempting to “filter out” these scripts is not likely to succeed. Instead we recommend validating input against a rigorous positive specification of what is expected. XSS attacks usually come in the form of embedded JavaScript. However, any embedded active content is a potential source of danger, including: ActiveX (OLE), VBscript, Shockwave, Flash and more.
XSS issues can also be present in the underlying web and application servers as well. Most web and application servers generate simple web pages to display in the case of various errors, such as a 404 ‘page not found’ or a 500 ‘internal server error.’ If these pages reflect back any information from the user’s request, such as the URL they were trying to access, they may be vulnerable to a reflected XSS attack.
The likelihood that a site contains XSS vulnerabilities is extremely high. There are a wide variety of ways to trick web applications into relaying malicious scripts. Developers that attempt to filter out the malicious parts of these requests are very likely to overlook possible attacks or encodings. Finding these flaws is not tremendously difficult for attackers, as all they need is a browser and some time. There are numerous free tools available that help hackers find these flaws as well as carefully craft and inject XSS attacks into a target site."
