diff --git tests/phpunit/includes/functions.php tests/phpunit/includes/functions.php
index 696741166e..5fb08b03bf 100644
--- tests/phpunit/includes/functions.php
+++ tests/phpunit/includes/functions.php
@@ -190,3 +190,26 @@ function _wp_rest_server_class_filter() {
 // Skip `setcookie` calls in auth_cookie functions due to warning:
 // Cannot modify header information - headers already sent by ...
 tests_add_filter( 'send_auth_cookies', '__return_false' );
+
+/**
+ * glob() for a pattern in subdirectories.
+ *
+ * Does not support the `GLOB_BRACE` flag.
+ *
+ * Based on https://php.net/manual/en/function.glob.php#106595, which was licensed under Creative Commons Attribution 3.0.
+ *
+ * @since 5.0.0
+ *
+ * @param string $pattern The regular expression pattern to search for.
+ * @param int    $flags   Optional. Any flags that should be passed on to `glob()`.
+ * @return array
+ */
+function glob_recursive( $pattern, $flags = 0 ) {
+	$files = glob( $pattern, $flags );
+
+	foreach ( glob( dirname( $pattern ) . '/*', GLOB_ONLYDIR | GLOB_NOSORT ) as $dir ) {
+		$files = array_merge( $files, glob_recursive( $dir . '/' . basename( $pattern ), $flags ) );
+	}
+
+	return $files;
+}
diff --git tests/phpunit/tests/dependencies/mediaelementjs.php tests/phpunit/tests/dependencies/mediaelementjs.php
new file mode 100644
index 0000000000..45de4c15d5
--- /dev/null
+++ tests/phpunit/tests/dependencies/mediaelementjs.php
@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @group dependencies
+ * @group scripts
+ */
+class Tests_Dependencies_MediaElementjs extends WP_UnitTestCase {
+	/**
+	 * Test if the MediaElement.js Flash fallbacks have been re-added.
+	 *
+	 * MediaElement's Flash fallbacks were removed in WordPress 4.9.2 due to limited use cases and
+	 * a history of security vulnerabilities. It's unlikely that there'll ever be a need to
+	 * restore them in the future, and doing so could introduce security vulnerabilities. If you
+	 * want to re-add them, please discuss that with the Security team first.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @ticket 42720
+	 */
+	function test_exclusion_of_flash() {
+		$mejs_folder = dirname( ABSPATH ) . '/build/' . WPINC . '/js/mediaelement';
+		$js_files    = glob( $mejs_folder . '/*.js' );
+		$swf_files   = glob_recursive( $mejs_folder . '/*.swf' );
+
+		/*
+		 * The path in $mejs_folder is hardcoded, so this is just a sanity check to make sure the
+		 * correct directory is used, in case it gets renamed in the future.
+		 */
+		$this->assertGreaterThan( 0, count( $js_files ) );
+
+		// Make sure the Flash files haven't been re-added accidentally.
+		$this->assertCount( 0, $swf_files );
+	}
+}