diff --git a/api/net/ethernet.hpp b/api/net/ethernet.hpp
index 8dc1e20c40..20375c8f2a 100644
--- a/api/net/ethernet.hpp
+++ b/api/net/ethernet.hpp
@@ -73,8 +73,8 @@ namespace net {
 
       // hex string representation
       std::string str() const {
-        char eth_addr[17];
-        sprintf(eth_addr, "%1x:%1x:%1x:%1x:%1x:%1x",
+        char eth_addr[18];
+        snprintf(eth_addr, sizeof(eth_addr), "%1x:%1x:%1x:%1x:%1x:%1x",
                 part[0], part[1], part[2],
                 part[3], part[4], part[5]);
         return eth_addr;