Input WYSIWYG: Updated tests, Added simple html limiting

app/Util/HtmlDescriptionFilter.php

@@ -0,0 +1,75 @@
+<?php
+
+namespace BookStack\Util;
+
+use DOMAttr;
+use DOMElement;
+use DOMNamedNodeMap;
+use DOMNode;
+
+/**
+ * Filter to ensure HTML input for description content remains simple and
+ * to a limited allow-list of elements and attributes.
+ * More for consistency and to prevent nuisance rather than for security
+ * (which would be done via a separate content filter and CSP).
+ */
+class HtmlDescriptionFilter
+{
+    /**
+     * @var array<string, string[]>
+     */
+    protected static array $allowedAttrsByElements = [
+        'p' => [],
+        'a' => ['href', 'title'],
+        'ol' => [],
+        'ul' => [],
+        'li' => [],
+        'strong' => [],
+        'em' => [],
+        'br' => [],
+    ];
+
+    public static function filterFromString(string $html): string
+    {
+        $doc = new HtmlDocument($html);
+
+        $topLevel = [...$doc->getBodyChildren()];
+        foreach ($topLevel as $child) {
+            /** @var DOMNode $child */
+            if ($child instanceof DOMElement) {
+                static::filterElement($child);
+            } else {
+                $child->parentNode->removeChild($child);
+            }
+        }
+
+        return $doc->getBodyInnerHtml();
+    }
+
+    protected static function filterElement(DOMElement $element): void
+    {
+        $elType = strtolower($element->tagName);
+        $allowedAttrs = static::$allowedAttrsByElements[$elType] ?? null;
+        if (is_null($allowedAttrs)) {
+            $element->remove();
+            return;
+        }
+
+        /** @var DOMNamedNodeMap $attrs */
+        $attrs = $element->attributes;
+        for ($i = $attrs->length - 1; $i >= 0; $i--) {
+            /** @var DOMAttr $attr */
+            $attr = $attrs->item($i);
+            $name = strtolower($attr->name);
+            if (!in_array($name, $allowedAttrs)) {
+                $element->removeAttribute($attr->name);
+            }
+        }
+
+        foreach ($element->childNodes as $child) {
+            if ($child instanceof DOMElement) {
+                static::filterElement($child);
+            }
+        }
+    }
+}