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 attacks such as cross-site scripting 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 preventmalicious code from being injected. Sanitization is typically performed by using either a whitelist or a blacklistapproach. Leaving a safe HTML element off a whitelist is not so serious; it simply means that that feature will not be included post-sanitation. On the other hand, if an unsafe element is left off a blacklist, then the vulnerability will not be sanitized out of the HTML output. An out-of-date blacklist can therefore be dangerous if new, unsafe features have been introduced to the HTML Standard. Further sanitization can be performed based on rules which specify what operation is to be performed on the subject tags. Typical operations include removal of the tag itself while preserving the content, preserving only the textual content of a tag or forcing certain values on attributes.
Implementations
In PHP, HTML sanitization can be performed using the strip_tags function at the risk of removing all textual content following an unclosed less-than symbol or angle bracket. The HTML Purifier library is another popular option for PHP applications. In Java, sanitization can be achieved by using the OWASP Java HTML Sanitizer Project. In.NET, a number of sanitizers use the Html Agility Pack, an HTML parser. In JavaScript there are "JS-only" sanitizers for the back end, and browser-based implementations that use browser's own DOM parser to parse the HTML.
