XSS en el software wiki grande

Encontré un XSS vuln en un gran software wiki. Aquí está el código vuln:

<?php
/**
 * Mock load.php with pre-defined test modules.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @package MediaWiki
 * @author Lupo
 * @since 1.20
 */
header( 'Content-Type: text/javascript; charset=utf-8' );

$moduleImplementations = [
    'testUsesMissing' => "
mw.loader.implement( 'testUsesMissing', function () {
    mw.loader.testFail( 'Module usesMissing script should not run.' );
}, {}, {});
",

    'testUsesNestedMissing' => "
mw.loader.implement( 'testUsesNestedMissing', function () {
    mw.loader.testFail('Module testUsesNestedMissing script should not run.' );
}, {}, {});
",

    'testSkipped' => "
mw.loader.implement( 'testSkipped', function () {
    mw.loader.testFail( false, 'Module testSkipped was supposed to be skipped.' );
}, {}, {});
",

    'testNotSkipped' => "
mw.loader.implement( 'testNotSkipped', function () {}, {}, {});
",

    'testUsesSkippable' => "
mw.loader.implement( 'testUsesSkippable', function () {}, {}, {});
",

    'testUrlInc' => "
mw.loader.implement( 'testUrlInc', function () {} );
",
    'testUrlInc.a' => "
mw.loader.implement( 'testUrlInc.a', function () {} );
",
    'testUrlInc.b' => "
mw.loader.implement( 'testUrlInc.b', function () {} );
",
    'testUrlOrder' => "
mw.loader.implement( 'testUrlOrder', function () {} );
",
    'testUrlOrder.a' => "
mw.loader.implement( 'testUrlOrder.a', function () {} );
",
    'testUrlOrder.b' => "
mw.loader.implement( 'testUrlOrder.b', function () {} );
",
];

$response = '';

// Does not support the full behaviour of ResourceLoaderContext::expandModuleNames(),
// Only supports dotless module names joined by comma,
// with the exception of the hardcoded cases for testUrl*.
if ( isset( $_GET['modules'] ) ) {
    if ( $_GET['modules'] === 'testUrlInc,testUrlIncDump|testUrlInc.a,b' ) {
        $modules = [ 'testUrlInc', 'testUrlIncDump', 'testUrlInc.a', 'testUrlInc.b' ];
    } elseif ( $_GET['modules'] === 'testUrlOrder,testUrlOrderDump|testUrlOrder.a,b' ) {
        $modules = [ 'testUrlOrder', 'testUrlOrderDump', 'testUrlOrder.a', 'testUrlOrder.b' ];
    } else {
        $modules = explode( ',', $_GET['modules'] );
    }
    foreach ( $modules as $module ) {
        if ( isset( $moduleImplementations[$module] ) ) {
            $response .= $moduleImplementations[$module];
        } elseif ( preg_match( '/^test.*Dump$/', $module ) === 1 ) {
            $queryModules = $_GET['modules'];
            $queryVersion = isset( $_GET['version'] ) ? strval( $_GET['version'] ) : null;
            $response .= 'mw.loader.implement( ' . json_encode( $module )
                . ', function ( $, jQuery, require, module ) {'
                . 'module.exports.query = { '
                . 'modules: ' . json_encode( $queryModules ) . ','
                . 'version: ' . json_encode( $queryVersion )
                . ' };'
                . '} );';
        } else {
            // Default
            $response .= 'mw.loader.state(' . json_encode( $module ) . ', "missing" );' . "\n";
        }
    }
}

echo $response;

La salida de $_GET["modules"] no se filtra. ¿Puedes decirme si este XSS es vulnerable y también peligroso para ti?