--- a/sys/netgraph7/ng_source.c
+++ b/sys/netgraph7/ng_source.c
@@ NGM_SOURCE_SETIFACE @@
 		if (msg->header.arglen < 2) {
 			error = EINVAL;
 			break;
 		}
+		if (msg->data[msg->header.arglen - 1] != 0) { /* NUL-terminate */
+			error = EINVAL;
+			break;
+		}