A classic example of this is with online message boards where users are allowed to post HTML formatted messages for other users to read.For example, suppose there is a dating website where members scan the profiles of other members to see if they look interesting.Stopping an XSS attack when accepting HTML input from users is much more complex in this situation.Untrusted HTML input must be run through an HTML sanitization engine to ensure that it does not contain XSS code.Although widely recommended, performing HTML entity encoding only on the five XML significant characters is not always sufficient to prevent many forms of XSS attacks.As encoding is often difficult, security encoding libraries are usually easier to use.) will not suffice since the user input needs to be rendered as HTML by the browser (so it shows as "very large", instead of "very large").Exploiting one of these, attackers fold malicious content into the content being delivered from the compromised site.
Cross-site scripting attacks use known vulnerabilities in web-based applications, their servers, or the plug-in systems on which they rely.There are several escaping schemes that can be used depending on where the untrusted string needs to be placed within an HTML document including HTML entity encoding, Java Script escaping, CSS escaping, and URL (or percent) encoding.Most web applications that do not need to accept rich data can use escaping to largely eliminate the risk of XSS attacks in a fairly straightforward manner.A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same-origin policy.Cross-site scripting carried out on websites accounted for roughly 84% of all security vulnerabilities documented by Symantec as of 2007.
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To do so, she writes a script designed to run from other people's browsers when they visit her profile.