diff -up kbd-1.15.5/src/vlock/auth.c.orig kbd-1.15.5/src/vlock/auth.c
--- kbd-1.15.5/src/vlock/auth.c.orig	2017-08-28 15:05:02.499936226 +0200
+++ kbd-1.15.5/src/vlock/auth.c	2017-09-05 14:56:00.856307724 +0200
@@ -160,6 +160,15 @@ get_password (pam_handle_t * pamh, const
 				break;
 
 			default:
+				if (!isatty(STDIN_FILENO))
+				{
+					/* Cancel locking. */
+					pam_end (pamh, rc);
+					syslog (LOG_NOTICE,
+						"Cancelled lock of %s on %s for %s by (uid=%u)",
+						locked_name (), tty, username, uid);
+					return EXIT_FAILURE;
+				}
 				printf ("%s.\n\n\n", pam_strerror (pamh, rc));
 				fflush (stdout);
 		}