Monday, April 11, 2011

Cross site scripting Flaw

Cross site Scripting (XSS) attacks are a type of injection problem, in which malicious scripts are injected into otherwise benign and trusted web sites. Cross site scripting flaws are the most prevalent flaw in web applications today. Cross site scripting 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.
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.

Environments Affected

All web servers, application servers, and web application environments are susceptible to cross site scripting.
How to check the vulnerability ??? 
There are three known types of cross site scripting: reflected, stored, and DOM injection. Reflected XSS is the easiest to exploit – a page will reflect user supplied data directly back to the user:
echo $_REQUEST['userinput'];
Stored XSS takes malicious  data, stores it in a file, a database, or other back end system, and then at a later stage, displays the data to the user, unfiltered. This is extremely dangerous in systems such as CMS, blogs, or forums, where a large number of users will see input from other individuals.
With DOM based XSS attacks, the site’s JavaScript code and variables are manipulated rather than HTML elements. Alternatively, attacks can be a blend or hybrid of all three types. The danger with cross site scripting is not the type of attack, but that it is possible

Attacks are usually implemented in JavaScript, which is a powerful scripting language. Using JavaScript   
allows attackers to manipulate any aspect of the rendered page, including adding new elements (such as 
adding a login tile which forwards credentials to a malicious site), manipulating any aspect of the internal 
DOM tree, and deleting or changing the way the page looks and feels. JavaScript allows the use of 
XmlHttpRequest, which is typically used by sites using AJAX technologies, even if victim site does not use 
AJAX today.

Using XmlHttpRequest (AJAX), it is sometimes possible to get around a browser’s same source origination 

policy - thus forwarding victim data to hostile sites, and to create complex worms and malicious zombies that 

last as long as the browser stays open. AJAX attacks do not have to be visible or require user interaction to 

perform dangerous cross site request forgery (CSRF) attacks ( CSRF WILL BE EXPLAINED IN MY NEXT POST )

How to Get protected from Cross - Site Scripting ?? 

XSS flaws can be difficult to identify and remove from a web application. The best way to find flaws is to 

perform a security review of the code and search for all places where input from an HTTP request could 

possibly make its way into the HTML output. Note that a variety of different HTML tags can be used to 

transmit a malicious JavaScript. Nessus, Nikto, and some other available tools can help scan a website for 

these flaws, but can only scratch the surface.


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