[stable35] fix: Reject login with NUL bytes rather than converting

lib/private/User/Manager.php

@@ -246,7 +246,12 @@ public function checkPassword($loginName, $password) {
 	 * @return IUser|false the User object on success, false otherwise
 	 */
 	public function checkPasswordNoLogging($loginName, $password) {
-		$loginName = str_replace("\0", '', $loginName);
+		// A NUL byte in the login name must not turn it into a second name for an
+		// account: the checks done before the password check (e.g. 2FA enforcement
+		// in Session::logClientIn()) see the name as sent, so the name has to stay the same
+		if (str_contains($loginName, "\0")) {
+			return false;
+		}
 		$password = str_replace("\0", '', $password);
 
 		$cachedBackend = $this->cache->get($loginName);

tests/lib/User/ManagerTest.php

@@ -174,6 +174,26 @@ public function testCheckPasswordNotSupported(): void {
 		$this->assertFalse($this->manager->checkPassword('foo', 'bar'));
 	}
 
+	public function testCheckPasswordNullByteInLoginName(): void {
+		$backend = $this->createMock(\Test\Util\User\Dummy::class);
+		$backend->expects($this->never())
+			->method('checkPassword');
+
+		$backend->expects($this->any())
+			->method('implementsActions')
+			->willReturnCallback(function ($actions) {
+				if ($actions === BACKEND::CHECK_PASSWORD) {
+					return true;
+				} else {
+					return false;
+				}
+			});
+
+		$this->manager->registerBackend($backend);
+
+		$this->assertFalse($this->manager->checkPassword("foo\0", 'bar'));
+	}
+
 	public function testGetOneBackendExists(): void {
 		$backend = $this->createMock(\Test\Util\User\Dummy::class);
 		$backend->expects($this->once())