diff --git a/wp-admin/includes/ajax-actions.php b/wp-admin/includes/ajax-actions.php
index 605b36e6e1..96c9c22d19 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -1917,7 +1917,11 @@ function wp_ajax_upload_attachment() {
 		$post_id = null;
 	}
 
-	$post_data = isset( $_REQUEST['post_data'] ) ? $_REQUEST['post_data'] : array();
+	$post_data = ! empty( $_REQUEST['post_data'] ) ? _wp_get_allowed_postdata( _wp_translate_postdata( false, (array) $_REQUEST['post_data'] ) ) : array();
+
+	if ( is_wp_error( $post_data ) ) {
+		wp_die( $post_data->get_error_message() );
+	}
 
 	// If the context is custom header or background, make sure the uploaded file is an image.
 	if ( isset( $post_data['context'] ) && in_array( $post_data['context'], array( 'custom-header', 'custom-background' ) ) ) {
diff --git a/wp-admin/includes/post.php b/wp-admin/includes/post.php
index 2f97869a8e..185873996d 100644
--- a/wp-admin/includes/post.php
+++ b/wp-admin/includes/post.php
@@ -175,6 +175,27 @@ function _wp_translate_postdata( $update = false, $post_data = null ) {
 	return $post_data;
 }
 
+/**
+ * Returns only allowed post data fields
+ *
+ * @since 4.9.9
+ *
+ * @param array $post_data Array of post data. Defaults to the contents of $_POST.
+ * @return object|bool WP_Error on failure, true on success.
+ */
+function _wp_get_allowed_postdata( $post_data = null ) {
+	if ( empty( $post_data ) ) {
+		$post_data = $_POST;
+	}
+
+	// Pass through errors
+	if ( is_wp_error( $post_data ) ) {
+		return $post_data;
+	}
+
+	return array_diff_key( $post_data, array_flip( array( 'meta_input', 'file', 'guid' ) ) );
+}
+
 /**
  * Update an existing post with values provided in $_POST.
  *
@@ -243,6 +264,7 @@ function edit_post( $post_data = null ) {
 	$post_data = _wp_translate_postdata( true, $post_data );
 	if ( is_wp_error($post_data) )
 		wp_die( $post_data->get_error_message() );
+	$translated = _wp_get_allowed_postdata( $post_data );
 
 	// Post Formats
 	if ( isset( $post_data['post_format'] ) )
@@ -320,7 +342,7 @@ function edit_post( $post_data = null ) {
 		$attachment_data = isset( $post_data['attachments'][ $post_ID ] ) ? $post_data['attachments'][ $post_ID ] : array();
 
 		/** This filter is documented in wp-admin/includes/media.php */
-		$post_data = apply_filters( 'attachment_fields_to_save', $post_data, $attachment_data );
+		$translated = apply_filters( 'attachment_fields_to_save', $translated, $attachment_data );
 	}
 
 	// Convert taxonomy input to term IDs, to avoid ambiguity.
@@ -365,7 +387,7 @@ function edit_post( $post_data = null ) {
 				}
 			}
 
-			$post_data['tax_input'][ $taxonomy ] = $clean_terms;
+			$translated['tax_input'][ $taxonomy ] = $clean_terms;
 		}
 	}
 
@@ -373,18 +395,18 @@ function edit_post( $post_data = null ) {
 
 	update_post_meta( $post_ID, '_edit_last', get_current_user_id() );
 
-	$success = wp_update_post( $post_data );
+	$success = wp_update_post( $translated );
 	// If the save failed, see if we can sanity check the main fields and try again
 	if ( ! $success && is_callable( array( $wpdb, 'strip_invalid_text_for_column' ) ) ) {
 		$fields = array( 'post_title', 'post_content', 'post_excerpt' );
 
 		foreach( $fields as $field ) {
-			if ( isset( $post_data[ $field ] ) ) {
-				$post_data[ $field ] = $wpdb->strip_invalid_text_for_column( $wpdb->posts, $field, $post_data[ $field ] );
+			if ( isset( $translated[ $field ] ) ) {
+				$translated[ $field ] = $wpdb->strip_invalid_text_for_column( $wpdb->posts, $field, $translated[ $field ] );
 			}
 		}
 
-		wp_update_post( $post_data );
+		wp_update_post( $translated );
 	}
 
 	// Now that we have an ID we can fix any attachment anchor hrefs
@@ -544,9 +566,9 @@ function bulk_edit_posts( $post_data = null ) {
 			unset( $post_data['tax_input']['category'] );
 		}
 
+		$post_data['post_ID']        = $post_ID;
 		$post_data['post_type'] = $post->post_type;
 		$post_data['post_mime_type'] = $post->post_mime_type;
-		$post_data['guid'] = $post->guid;
 
 		foreach ( array( 'comment_status', 'ping_status', 'post_author' ) as $field ) {
 			if ( ! isset( $post_data[ $field ] ) ) {
@@ -554,14 +576,12 @@ function bulk_edit_posts( $post_data = null ) {
 			}
 		}
 
-		$post_data['ID'] = $post_ID;
-		$post_data['post_ID'] = $post_ID;
-
 		$post_data = _wp_translate_postdata( true, $post_data );
 		if ( is_wp_error( $post_data ) ) {
 			$skipped[] = $post_ID;
 			continue;
 		}
+		$post_data = _wp_get_allowed_postdata( $post_data );
 
 		$updated[] = wp_update_post( $post_data );
 
@@ -572,8 +592,8 @@ function bulk_edit_posts( $post_data = null ) {
 				unstick_post( $post_ID );
 		}
 
-		if ( isset( $post_data['post_format'] ) )
-			set_post_format( $post_ID, $post_data['post_format'] );
+		if ( isset( $shared_post_data['post_format'] ) )
+			set_post_format( $post_ID, $shared_post_data['post_format'] );
 	}
 
 	return array( 'updated' => $updated, 'skipped' => $skipped, 'locked' => $locked );
@@ -753,9 +773,10 @@ function wp_write_post() {
 	$translated = _wp_translate_postdata( false );
 	if ( is_wp_error($translated) )
 		return $translated;
+	$translated = _wp_get_allowed_postdata( $translated );
 
 	// Create the post.
-	$post_ID = wp_insert_post( $_POST );
+	$post_ID = wp_insert_post( $translated );
 	if ( is_wp_error( $post_ID ) )
 		return $post_ID;
 
@@ -1634,6 +1655,7 @@ function wp_create_post_autosave( $post_data ) {
 	$post_data = _wp_translate_postdata( true, $post_data );
 	if ( is_wp_error( $post_data ) )
 		return $post_data;
+	$post_data = _wp_get_allowed_postdata( $post_data );
 
 	$post_author = get_current_user_id();
 
diff --git a/wp-admin/post.php b/wp-admin/post.php
index f8e39e53e2..0e1596a9eb 100644
--- a/wp-admin/post.php
+++ b/wp-admin/post.php
@@ -234,7 +234,7 @@ case 'editattachment':
 
 	// Update the thumbnail filename
 	$newmeta = wp_get_attachment_metadata( $post_id, true );
-	$newmeta['thumb'] = $_POST['thumb'];
+	$newmeta['thumb'] = wp_basename( $_POST['thumb'] );
 
 	wp_update_attachment_metadata( $post_id, $newmeta );