diff --git a/src/shared/firewall-util.c b/src/shared/firewall-util.c
index eb4f5ff616..cba52fb419 100644
--- a/src/shared/firewall-util.c
+++ b/src/shared/firewall-util.c
@@ -50,8 +50,14 @@ static int entry_fill_basics(
         entry->ip.proto = protocol;
 
         if (in_interface) {
+                size_t l;
+
+                l = strlen(in_interface);
+                assert(l < sizeof entry->ip.iniface);
+                assert(l < sizeof entry->ip.iniface_mask);
+
                 strcpy(entry->ip.iniface, in_interface);
-                memset(entry->ip.iniface_mask, 0xFF, strlen(in_interface)+1);
+                memset(entry->ip.iniface_mask, 0xFF, l + 1);
         }
         if (source) {
                 entry->ip.src = source->in;