HTML Purifier: Removing an element conditionally based on its attributes
- by pinkgothic
As per the HTML Purifier smoketest, 'malformed' URIs are occasionally discarded to leave behind an attribute-less anchor tag, e.g.
<a href="javascript:document.location='http://www.google.com/'">XSS</a> becomes <a>XSS</a>
...as well as occasionally being stripped down to the protocol, e.g.
<a href="http://1113982867/">XSS</a> becomes <a href="http:/">XSS</a>
While that's unproblematic, per se, it's a bit ugly. Instead of trying to strip these out with regular expressions, I was hoping to use HTML Purifier's own library capabilities / injectors / plug-ins / whathaveyou.
Point of reference: Handling attributes
Conditionally removing an attribute in HTMLPurifier is easy. Here the library offers the class HTMLPurifier_AttrTransform with the method confiscateAttr().
While I don't personally use the functionality of confiscateAttr(), I do use an HTMLPurifier_AttrTransform as per this thread to add target="_blank" to all anchors.
// more configuration stuff up here
$htmlDef = $htmlPurifierConfiguration->getHTMLDefinition(true);
$anchor = $htmlDef->addBlankElement('a');
$anchor->attr_transform_post[] = new HTMLPurifier_AttrTransform_Target();
// purify down here
HTMLPurifier_AttrTransform_Target is a very simple class, of course.
class HTMLPurifier_AttrTransform_Target extends HTMLPurifier_AttrTransform
{
public function transform($attr, $config, $context) {
// I could call $this->confiscateAttr() here to throw away an
// undesired attribute
$attr['target'] = '_blank';
return $attr;
}
}
That part works like a charm, naturally.
Handling elements
Perhaps I'm not squinting hard enough at HTMLPurifier_TagTransform, or am looking in the wrong place(s), or generally amn't understanding it, but I can't seem to figure out a way to conditionally remove elements.
Say, something to the effect of:
// more configuration stuff up here
$htmlDef = $htmlPurifierConfiguration->getHTMLDefinition(true);
$anchor = $htmlDef->addElementHandler('a');
$anchor->elem_transform_post[] = new HTMLPurifier_ElementTransform_Cull();
// add target as per 'point of reference' here
// purify down here
With the Cull class extending something that has a confiscateElement() ability, or comparable, wherein I could check for a missing href attribute or a href attribute with the content http:/.
HTMLPurifier_Filter
I understand I could create a filter, but the examples (Youtube.php and ExtractStyleBlocks.php) suggest I'd be using regular expressions in that, which I'd really rather avoid, if it is at all possible. I'm hoping for an onboard or quasi-onboard solution that makes use of HTML Purifier's excellent parsing capabilities.
Returning null in a child-class of HTMLPurifier_AttrTransform unfortunately doesn't cut it.
Anyone have any smart ideas, or am I stuck with regexes? :)
