diff --git a/sys/netgraph7/ng_source.c b/sys/netgraph7/ng_source.c
--- a/sys/netgraph7/ng_source.c
+++ b/sys/netgraph7/ng_source.c
@@ -478,9 +478,15 @@
 		    }
 		case NGM_SOURCE_GET_COUNTER:
 		    {
-			uint8_t index = *(uint8_t *)msg->data;
+			uint8_t index;
 			struct ng_source_embed_cnt_info *embed;
 
+			if (msg->header.arglen < sizeof(index)) {
+				error = EINVAL;
+				goto done;
+			}
+			index = *(uint8_t *)msg->data;
+
 			if (index >= NG_SOURCE_COUNTERS) {
 				error = EINVAL;
 				goto done;