diff --git a/src/gssapi_client.cpp b/src/gssapi_client.cpp
index 942c25fa..68e33d9b 100644
--- a/src/gssapi_client.cpp
+++ b/src/gssapi_client.cpp
@@ -125,6 +125,9 @@ int zmq::gssapi_client_t::process_handshake_command (msg_t *msg_)
     }
 
     if (state != recv_next_token) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND);
         errno = EPROTO;
         return -1;
     }
diff --git a/src/gssapi_mechanism_base.cpp b/src/gssapi_mechanism_base.cpp
index 65d9b124..43d4f79b 100644
--- a/src/gssapi_mechanism_base.cpp
+++ b/src/gssapi_mechanism_base.cpp
@@ -128,8 +128,15 @@ int zmq::gssapi_mechanism_base_t::decode_message (msg_t *msg_)
     const uint8_t *ptr = static_cast <uint8_t *> (msg_->data ());
     size_t bytes_left = msg_->size ();
 
+    int rc = check_basic_command_structure (msg_);
+    if (rc == -1)
+        return rc;
+
     // Get command string
     if (bytes_left < 8 || memcmp (ptr, "\x07MESSAGE", 8)) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND);
         errno = EPROTO;
         return -1;
     }
@@ -138,6 +145,9 @@ int zmq::gssapi_mechanism_base_t::decode_message (msg_t *msg_)
 
     // Get token length
     if (bytes_left < 4) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_MESSAGE);
         errno = EPROTO;
         return -1;
     }
@@ -148,6 +158,9 @@ int zmq::gssapi_mechanism_base_t::decode_message (msg_t *msg_)
 
     // Get token value
     if (bytes_left < wrapped.length) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_MESSAGE);
         errno = EPROTO;
         return -1;
     }
@@ -168,11 +181,16 @@ int zmq::gssapi_mechanism_base_t::decode_message (msg_t *msg_)
     maj_stat = gss_unwrap(&min_stat, context, &wrapped, &plaintext,
                           &state, (gss_qop_t *) NULL);
 
+    //  TODO I don't think it is a good idea to use zmq_assert here. If 
+    //  decryption fails, gss_unwrap returns GSS_S_BAD_SIG. This opens up 
+    //  to DoS attacks by clients! Instead, a 
+    //  ZMQ_PROTOCOL_ERROR_ZMTP_CRYPTOGRAPHIC event should be emitted.
+
     zmq_assert(maj_stat == GSS_S_COMPLETE);
     zmq_assert(state);
 
     // Re-initialize msg_ for plaintext
-    int rc = msg_->close ();
+    rc = msg_->close ();
     zmq_assert (rc == 0);
 
     rc = msg_->init_size (plaintext.length-1);
@@ -190,6 +208,9 @@ int zmq::gssapi_mechanism_base_t::decode_message (msg_t *msg_)
     free(wrapped.value);
 
     if (bytes_left > 0) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_MESSAGE);
         errno = EPROTO;
         return -1;
     }
@@ -231,8 +252,15 @@ int zmq::gssapi_mechanism_base_t::process_initiate (msg_t *msg_, void **token_va
     const uint8_t *ptr = static_cast <uint8_t *> (msg_->data ());
     size_t bytes_left = msg_->size ();
 
+    int rc = check_basic_command_structure (msg_);
+    if (rc == -1)
+        return rc;
+
     // Get command string
     if (bytes_left < 9 || memcmp (ptr, "\x08INITIATE", 9)) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND);
         errno = EPROTO;
         return -1;
     }
@@ -241,6 +269,9 @@ int zmq::gssapi_mechanism_base_t::process_initiate (msg_t *msg_, void **token_va
 
     // Get token length
     if (bytes_left < 4) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_INITIATE);
         errno = EPROTO;
         return -1;
     }
@@ -250,6 +281,9 @@ int zmq::gssapi_mechanism_base_t::process_initiate (msg_t *msg_, void **token_va
 
     // Get token value
     if (bytes_left < token_length_) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_INITIATE);
         errno = EPROTO;
         return -1;
     }
@@ -264,6 +298,9 @@ int zmq::gssapi_mechanism_base_t::process_initiate (msg_t *msg_, void **token_va
     }
 
     if (bytes_left > 0) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_MALFORMED_COMMAND_INITIATE);
         errno = EPROTO;
         return -1;
     }
@@ -317,14 +354,28 @@ int zmq::gssapi_mechanism_base_t::process_ready (msg_t *msg_)
     const unsigned char *ptr = static_cast <unsigned char *> (msg_->data ());
     size_t bytes_left = msg_->size ();
 
+    int rc = check_basic_command_structure (msg_);
+    if (rc == -1)
+        return rc;
+
     if (bytes_left < 6 || memcmp (ptr, "\x05READY", 6)) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND);
         errno = EPROTO;
         return -1;
     }
     ptr += 6;
     bytes_left -= 6;
-    return parse_metadata (ptr, bytes_left);
+    rc = parse_metadata (ptr, bytes_left);
+    if (rc == -1)
+              session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_INVALID_METADATA);
+
+    return rc;
 }
+
 const gss_OID zmq::gssapi_mechanism_base_t::convert_nametype (int zmq_nametype)
 {
     switch (zmq_nametype) {
diff --git a/src/gssapi_server.cpp b/src/gssapi_server.cpp
index 936c4499..dbb065d7 100644
--- a/src/gssapi_server.cpp
+++ b/src/gssapi_server.cpp
@@ -114,6 +114,9 @@ int zmq::gssapi_server_t::process_handshake_command (msg_t *msg_)
     }
 
     if (state != recv_next_token) {
+        session->get_socket ()->event_handshake_failed_protocol (
+          session->get_endpoint (),
+          ZMQ_PROTOCOL_ERROR_ZMTP_UNEXPECTED_COMMAND);
         errno = EPROTO;
         return -1;
     }