kses is an HTML/XHTML filter written in PHP. It removes all unwanted HTML elements and attributes, no matter how malformed HTML input you give it. It also does several checks on attribute values. kses can be used to avoid Cross-Site Scripting (XSS), Buffer Overflows and Denial of Service attacks, among other things.
Some of kses' current features are:
* It will only allow the HTML elements and attributes that it was explicitly told to allow
* Element and attribute names are case-insensitive (a href vs A HREF)
* It will understand and process whitespace correctly
* Attribute values can be surrounded with quotes, apostrophes or nothing
* It will accept attributes with just names and no values (selected)
* It will accept XHTML's closing " /" marks
* Attribute values that are surrounded with nothing will get quotes to avoid producing non-W3C conforming HTML (<a href=http://sourceforge.net/projects/kses> works but isn't valid HTML)
* It handles lots of types of malformed HTML, by interpreting the existing code the best it can and then rebuilding new code from it. That is a better approach than trying to process existing code, as you are bound to forget about some weird special case somewhere. It handles problems like never-ending quotes and tags gracefully
* It will remove additional "<" and ">" characters that people may try to sneak in somewhere
* It supports checking attribute values for maximum length and maximum value, to protect against Buffer Overflows and Denial of Service attacks against WWW clients and various servers. You can stop <iframe src= width= height=> from having too high values for width and height, for instance
* It has got a system for white listing URL protocols. You can say that attribute values may only start with http:, https:, ftp: and gopher:, but no other URL protocols (javascript:, java:, about:, telnet:..). The functions that do this work handle white space, upper/lower case, HTML entities ("javascript:") and repeated entries ("javascript:javascript:alert(57)"). It also normalizes HTML entities as a nice side effect
* It removes Netscape 4's JavaScript entities ("&{alert(57)};")
