diff --git a/wp-includes/collaboration/class-wp-http-polling-sync-server.php b/wp-includes/collaboration/class-wp-http-polling-sync-server.php
index 88554a48c7..a90821ab78 100644
--- a/wp-includes/collaboration/class-wp-http-polling-sync-server.php
+++ b/wp-includes/collaboration/class-wp-http-polling-sync-server.php
@@ -37,6 +37,30 @@ class WP_HTTP_Polling_Sync_Server {
 	 */
 	const COMPACTION_THRESHOLD = 50;
 
+	/**
+	 * Maximum total size (in bytes) of the request body.
+	 *
+	 * @since 7.0.0
+	 * @var int
+	 */
+	const MAX_BODY_SIZE = 16 * MB_IN_BYTES;
+
+	/**
+	 * Maximum number of rooms allowed per request.
+	 *
+	 * @since 7.0.0
+	 * @var int
+	 */
+	const MAX_ROOMS_PER_REQUEST = 50;
+
+	/**
+	 * Maximum length of a single update data string.
+	 *
+	 * @since 7.0.0
+	 * @var int
+	 */
+	const MAX_UPDATE_DATA_SIZE = MB_IN_BYTES;
+
 	/**
 	 * Sync update type: compaction.
 	 *
@@ -96,8 +120,9 @@ class WP_HTTP_Polling_Sync_Server {
 		$typed_update_args = array(
 			'properties' => array(
 				'data' => array(
-					'type'     => 'string',
-					'required' => true,
+					'type'      => 'string',
+					'required'  => true,
+					'maxLength' => self::MAX_UPDATE_DATA_SIZE,
 				),
 				'type' => array(
 					'type'     => 'string',
@@ -149,12 +174,14 @@ class WP_HTTP_Polling_Sync_Server {
 				'methods'             => array( WP_REST_Server::CREATABLE ),
 				'callback'            => array( $this, 'handle_request' ),
 				'permission_callback' => array( $this, 'check_permissions' ),
+				'validate_callback'   => array( $this, 'validate_request' ),
 				'args'                => array(
 					'rooms' => array(
 						'items'    => array(
 							'properties' => $room_args,
 							'type'       => 'object',
 						),
+						'maxItems' => self::MAX_ROOMS_PER_REQUEST,
 						'required' => true,
 						'type'     => 'array',
 					),
@@ -223,6 +250,30 @@ class WP_HTTP_Polling_Sync_Server {
 		return true;
 	}
 
+	/**
+	 * Validates that the request body does not exceed the maximum allowed size.
+	 *
+	 * Runs as the route-level validate_callback, after per-arg schema
+	 * validation has already passed.
+	 *
+	 * @since 7.0.0
+	 *
+	 * @param WP_REST_Request $request The REST request.
+	 * @return true|WP_Error True if valid, WP_Error if the body is too large.
+	 */
+	public function validate_request( WP_REST_Request $request ) {
+		$body = $request->get_body();
+		if ( is_string( $body ) && strlen( $body ) > self::MAX_BODY_SIZE ) {
+			return new WP_Error(
+				'rest_sync_body_too_large',
+				__( 'Request body is too large.' ),
+				array( 'status' => 413 )
+			);
+		}
+
+		return true;
+	}
+
 	/**
 	 * Handles request: stores sync updates and awareness data, and returns
 	 * updates the client is missing.
@@ -278,24 +329,47 @@ class WP_HTTP_Polling_Sync_Server {
 	 *
 	 * @param string      $entity_kind The entity kind, e.g. 'postType', 'taxonomy', 'root'.
 	 * @param string      $entity_name The entity name, e.g. 'post', 'category', 'site'.
-	 * @param string|null $object_id   The object ID / entity key for single entities, null for collections.
+	 * @param string|null $object_id   The numeric object ID / entity key for single entities, null for collections.
 	 * @return bool True if user has permission, otherwise false.
 	 */
 	private function can_user_sync_entity_type( string $entity_kind, string $entity_name, ?string $object_id ): bool {
-		// Handle single post type entities with a defined object ID.
-		if ( 'postType' === $entity_kind && is_numeric( $object_id ) ) {
-			return current_user_can( 'edit_post', (int) $object_id );
+		if ( is_string( $object_id ) ) {
+			if ( ! ctype_digit( $object_id ) ) {
+				return false;
+			}
+			$object_id = (int) $object_id;
+		}
+		if ( null !== $object_id && $object_id <= 0 ) {
+			// Object ID must be numeric if provided.
+			return false;
 		}
 
-		// Handle single taxonomy term entities with a defined object ID.
-		if ( 'taxonomy' === $entity_kind && is_numeric( $object_id ) ) {
-			$taxonomy = get_taxonomy( $entity_name );
-			return isset( $taxonomy->cap->assign_terms ) && current_user_can( $taxonomy->cap->assign_terms );
-		}
+		// Validate permissions for the provided object ID.
+		if ( is_int( $object_id ) ) {
+			// Handle single post type entities with a defined object ID.
+			if ( 'postType' === $entity_kind ) {
+				if ( get_post_type( $object_id ) !== $entity_name ) {
+					// Post is not of the specified post type.
+					return false;
+				}
+				return current_user_can( 'edit_post', $object_id );
+			}
 
-		// Handle single comment entities with a defined object ID.
-		if ( 'root' === $entity_kind && 'comment' === $entity_name && is_numeric( $object_id ) ) {
-			return current_user_can( 'edit_comment', (int) $object_id );
+			// Handle single taxonomy term entities with a defined object ID.
+			if ( 'taxonomy' === $entity_kind ) {
+				$term_exists = term_exists( $object_id, $entity_name );
+				if ( ! is_array( $term_exists ) || ! isset( $term_exists['term_id'] ) ) {
+					// Either term doesn't exist OR term is not in specified taxonomy.
+					return false;
+				}
+
+				return current_user_can( 'edit_term', $object_id );
+			}
+
+			// Handle single comment entities with a defined object ID.
+			if ( 'root' === $entity_kind && 'comment' === $entity_name ) {
+				return current_user_can( 'edit_comment', $object_id );
+			}
 		}
 
 		// All the remaining checks are for collections. If an object ID is provided,
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 0fc627fc66..8e12371f19 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.1-alpha-62196';
+$wp_version = '7.1-alpha-62198';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.