diff -Naur net-snmp-5.4.1/snmplib/scapi.c net-snmp-5.4.1.fix/snmplib/scapi.c
--- net-snmp-5.4.1/snmplib/scapi.c	2006-09-15 05:47:01.000000000 -0700
+++ net-snmp-5.4.1.fix/snmplib/scapi.c	2008-05-23 17:18:43.000000000 -0700
@@ -562,6 +562,9 @@
         QUITFUN(SNMPERR_GENERR, sc_check_keyed_hash_quit);
     }
 
+    if (maclen != USM_MD5_AND_SHA_AUTH_LEN) {
+        QUITFUN(SNMPERR_GENERR, sc_check_keyed_hash_quit);
+    }
 
     /*
      * Generate a full hash of the message, then compare