fix(security): enforce scheme allow-list on runbackup return URL

The pre-update return URL is assigned to window.location.href, so a
javascript:/data: URI (empty host) would have passed the host-only check
and executed. Require an absolute http(s) same-host URL or a single-slash
root-relative path; reject javascript:/data:/vbscript: and protocol-relative
//host.

Claude-Session: https://claude.ai/code/session_01WbGBN9VyRK61zczYWcCQ2i

source/packages/com_mokosuitebackup/tmpl/runbackup/default.php

@@ -22,23 +22,36 @@ use Joomla\CMS\Uri\Uri;
 $ajaxToken = Session::getFormToken();
 $ajaxUrl   = Route::_('index.php?option=com_mokosuitebackup&format=json', false);
 
-/* Validate the return URL to prevent an open redirect: accept only a relative
-   URL or one whose host matches this site. Falls back to '' (no redirect). */
+/* Validate the return URL to prevent an open redirect / javascript: XSS
+   (the value ends up in window.location.href). Accept ONLY:
+     - an absolute http(s) URL whose host matches this site, or
+     - a root-relative path starting with a single "/" (not "//").
+   This rejects javascript:, data:, vbscript: (empty host) and
+   protocol-relative //evil.com. Falls back to '' (no redirect). */
 $safeReturnUrl = '';
 
 if ($this->returnUrl !== '') {
 	$decoded = base64_decode($this->returnUrl, true);
 	$raw     = ($decoded !== false && $decoded !== '') ? $decoded : $this->returnUrl;
 
-	try {
-		$rootHost = Uri::getInstance(Uri::root())->getHost();
-		$target   = new Uri($raw);
+	$isAbsoluteHttp = (bool) preg_match('#^https?://#i', $raw);
+	$isRootRelative = isset($raw[0]) && $raw[0] === '/' && (!isset($raw[1]) || $raw[1] !== '/');
 
-		if ($target->getHost() === '' || strcasecmp($target->getHost(), $rootHost) === 0) {
-			$safeReturnUrl = $raw;
+	if ($isAbsoluteHttp || $isRootRelative) {
+		try {
+			$rootHost = Uri::getInstance(Uri::root())->getHost();
+			$target   = new Uri($raw);
+			$scheme   = strtolower((string) $target->getScheme());
+
+			$schemeOk = $scheme === '' || $scheme === 'http' || $scheme === 'https';
+			$hostOk   = $target->getHost() === '' || strcasecmp($target->getHost(), $rootHost) === 0;
+
+			if ($schemeOk && $hostOk) {
+				$safeReturnUrl = $raw;
+			}
+		} catch (\Throwable $e) {
+			$safeReturnUrl = '';
 		}
-	} catch (\Throwable $e) {
-		$safeReturnUrl = '';
 	}
 }