<?php
namespace BookStack\Util;
use DOMAttr;
use DOMDocument;
use DOMElement;
use DOMNodeList;
use DOMXPath;
class HtmlContentFilter
{
/**
* Remove all the script elements from the given HTML.
*/
public static function removeScripts(string $html): string
{
if (empty($html)) {
return $html;
}
$html = '<body>' . $html . '</body>';
libxml_use_internal_errors(true);
$doc = new DOMDocument();
$doc->loadHTML(mb_convert_encoding($html, 'HTML-ENTITIES', 'UTF-8'));
$xPath = new DOMXPath($doc);
// Remove standard script tags
$scriptElems = $xPath->query('//script');
static::removeNodes($scriptElems);
// Remove clickable links to JavaScript URI
$badLinks = $xPath->query('//*[' . static::xpathContains('@href', 'javascript:') . ']');
static::removeNodes($badLinks);
// Remove forms with calls to JavaScript URI
$badForms = $xPath->query('//*[' . static::xpathContains('@action', 'javascript:') . '] | //*[' . static::xpathContains('@formaction', 'javascript:') . ']');
static::removeNodes($badForms);
// Remove meta tag to prevent external redirects
$metaTags = $xPath->query('//meta[' . static::xpathContains('@content', 'url') . ']');
static::removeNodes($metaTags);
// Remove data or JavaScript iFrames
$badIframes = $xPath->query('//*[' . static::xpathContains('@src', 'data:') . '] | //*[' . static::xpathContains('@src', 'javascript:') . '] | //*[@srcdoc]');
static::removeNodes($badIframes);
// Remove attributes, within svg children, hiding JavaScript or data uris.
// A bunch of svg element and attribute combinations expose xss possibilities.
// For example, SVG animate tag can exploit javascript in values.
$badValuesAttrs = $xPath->query('//svg//@*[' . static::xpathContains('.', 'data:') . '] | //svg//@*[' . static::xpathContains('.', 'javascript:') . ']');
static::removeAttributes($badValuesAttrs);
// Remove elements with a xlink:href attribute
// Used in SVG but deprecated anyway, so we'll be a bit more heavy-handed here.
$xlinkHrefAttributes = $xPath->query('//@*[contains(name(), \'xlink:href\')]');
static::removeAttributes($xlinkHrefAttributes);
// Remove 'on*' attributes
$onAttributes = $xPath->query('//@*[starts-with(name(), \'on\')]');
static::removeAttributes($onAttributes);
$html = '';
$topElems = $doc->documentElement->childNodes->item(0)->childNodes;
foreach ($topElems as $child) {
$html .= $doc->saveHTML($child);
}
return $html;
}
/**
* Create a xpath contains statement with a translation automatically built within
* to affectively search in a cases-insensitive manner.
*/
protected static function xpathContains(string $property, string $value): string
{
$value = strtolower($value);
$upperVal = strtoupper($value);
return 'contains(translate(' . $property . ', \'' . $upperVal . '\', \'' . $value . '\'), \'' . $value . '\')';
}
/**
* Remove all the given DOMNodes.
*/
protected static function removeNodes(DOMNodeList $nodes): void
{
foreach ($nodes as $node) {
$node->parentNode->removeChild($node);
}
}
/**
* Remove all the given attribute nodes.
*/
protected static function removeAttributes(DOMNodeList $attrs): void
{
/** @var DOMAttr $attr */
foreach ($attrs as $attr) {
$attrName = $attr->nodeName;
/** @var DOMElement $parentNode */
$parentNode = $attr->parentNode;
$parentNode->removeAttribute($attrName);
}
}
}