HTML sanitization
HTML sanitization is the process of examining an HTML document and producing a new HTML document that preserves only whatever tags are designated "safe" and desired. HTML sanitization can be used to protect against cross-site scripting (XSS) attacks by sanitizing any HTML code submitted by a user.
Basic tags for changing fonts are often allowed, such as <b>, <i>, <u>, <em>, and <strong> while more advanced tags such as <script>, <object>, <embed>, and <link> are removed by the sanitization process. Also potentially dangerous attributes such as the onclick attribute are removed in order to prevent malicious code from being injected.
Sanitization is typically performed by using either a whitelist or a blacklist approach. An item left off a whitelist, makes the sanitization produce HTML code that lacks safe elements. If an item is left off a blacklist, a vulnerability will be present in the sanitized HTML output. New unsafe HTML features, introduced after a blacklist has been defined, causes the blacklist to become out of date.
