diff --git a/lib/private/Authentication/Login/SetUserTimezoneCommand.php b/lib/private/Authentication/Login/SetUserTimezoneCommand.php
index b18851e61faeb..57859b032b693 100644
--- a/lib/private/Authentication/Login/SetUserTimezoneCommand.php
+++ b/lib/private/Authentication/Login/SetUserTimezoneCommand.php
@@ -43,7 +43,7 @@ public function __construct(IConfig $config,
 	}
 
 	public function process(LoginData $loginData): LoginResult {
-		if ($loginData->getTimeZoneOffset() !== '') {
+		if ($loginData->getTimeZoneOffset() !== '' && $this->isValidTimezone($loginData->getTimeZone())) {
 			$this->config->setUserValue(
 				$loginData->getUser()->getUID(),
 				'core',
@@ -58,4 +58,8 @@ public function process(LoginData $loginData): LoginResult {
 
 		return $this->processNextOrFinishSuccessfully($loginData);
 	}
+
+	private function isValidTimezone(?string $value): bool {
+		return $value && in_array($value, \DateTimeZone::listIdentifiers());
+	}
 }