KSES: Conditionally remove the <form> element from $allowedposttags.

To avoid backwards compatibility issues, `<form>` is re-added if a custom filter has added the `<input>` or `<select>` elements to `$allowedposttags`.

Merges [43994] to the 4.9 branch.


git-svn-id: https://develop.svn.wordpress.org/branches/4.9@43997 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -184,15 +184,6 @@
 			'lang' => true,
 			'xml:lang' => true,
 		),
-		'form' => array(
-			'action' => true,
-			'accept' => true,
-			'accept-charset' => true,
-			'enctype' => true,
-			'method' => true,
-			'name' => true,
-			'target' => true,
-		),
 		'h1' => array(
 			'align' => true,
 		),
@@ -610,6 +601,7 @@ function wp_kses_one_attr( $string, $element ) {
  * Return a list of allowed tags and attributes for a given context.
  *
  * @since 3.5.0
+ * @since 5.0.1 `form` removed as allowable HTML tag.
  *
  * @global array $allowedposttags
  * @global array $allowedtags
@@ -638,7 +630,27 @@ function wp_kses_allowed_html( $context = '' ) {
 	switch ( $context ) {
 		case 'post':
 			/** This filter is documented in wp-includes/kses.php */
-			return apply_filters( 'wp_kses_allowed_html', $allowedposttags, $context );
+			$tags = apply_filters( 'wp_kses_allowed_html', $allowedposttags, $context );
+
+			// 5.0.1 removed the `<form>` tag, allow it if a filter is allowing it's sub-elements `<input>` or `<select>`.
+			if ( ! CUSTOM_TAGS && ! isset( $tags['form'] ) && ( isset( $tags['input'] ) || isset( $tags['select'] ) ) ) {
+				$tags = $allowedposttags;
+
+				$tags['form'] = array(
+					'action' => true,
+					'accept' => true,
+					'accept-charset' => true,
+					'enctype' => true,
+					'method' => true,
+					'name' => true,
+					'target' => true,
+				);
+
+				/** This filter is documented in wp-includes/kses.php */
+				$tags = apply_filters( 'wp_kses_allowed_html', $tags, $context );
+			}
+
+			return $tags;
 
 		case 'user_description':
 		case 'pre_user_description':