=== modified file 'client/mysqltest.cc'
--- a/client/mysqltest.cc	2012-11-08 13:24:35 +0000
+++ b/client/mysqltest.cc	2013-01-24 23:17:39 +0000
@@ -3908,7 +3908,10 @@
                       cur_con->name, ds_user.str, ds_passwd.str, ds_db.str));
 
   if (mysql_change_user(mysql, ds_user.str, ds_passwd.str, ds_db.str))
-    die("change user failed: %s", mysql_error(mysql));
+    handle_error(command, mysql_errno(mysql), mysql_error(mysql),
+		 mysql_sqlstate(mysql), &ds_res);
+  else
+    handle_no_error(command);
 
   dynstr_free(&ds_user);
   dynstr_free(&ds_passwd);

=== added file 'mysql-test/r/change_user_notembedded.result'
--- a/mysql-test/r/change_user_notembedded.result	1970-01-01 00:00:00 +0000
+++ b/mysql-test/r/change_user_notembedded.result	2013-01-24 23:17:39 +0000
@@ -0,0 +1,5 @@
+ERROR 28000: Access denied for user 'foo'@'localhost' (using password: NO)
+ERROR 28000: Access denied for user 'foo'@'localhost' (using password: NO)
+ERROR 28000: Access denied for user 'foo'@'localhost' (using password: NO)
+ERROR 08S01: Unknown command
+ERROR 08S01: Unknown command

=== modified file 'mysql-test/r/mysqltest.result'
--- a/mysql-test/r/mysqltest.result	2011-11-24 16:48:58 +0000
+++ b/mysql-test/r/mysqltest.result	2013-01-24 23:17:39 +0000
@@ -847,9 +847,9 @@
 b	varchar(255)	YES		NULL	
 c	datetime	YES		NULL	
 drop table t1;
-mysqltest: At line 1: change user failed: Unknown database 'inexistent'
-mysqltest: At line 1: change user failed: Access denied for user 'inexistent'@'localhost' (using password: NO)
-mysqltest: At line 1: change user failed: Access denied for user 'root'@'localhost' (using password: YES)
+mysqltest: At line 1: query 'change_user root,,inexistent' failed: 1049: Unknown database 'inexistent'
+mysqltest: At line 1: query 'change_user inexistent,,test' failed: 1045: Access denied for user 'inexistent'@'localhost' (using password: NO)
+mysqltest: At line 1: query 'change_user root,inexistent,test' failed: 1045: Access denied for user 'root'@'localhost' (using password: YES)
 REPLACED_FILE1.txt
 file1.txt
 file2.txt

=== added file 'mysql-test/t/change_user_notembedded.test'
--- a/mysql-test/t/change_user_notembedded.test	1970-01-01 00:00:00 +0000
+++ b/mysql-test/t/change_user_notembedded.test	2013-01-24 23:17:39 +0000
@@ -0,0 +1,24 @@
+source include/not_embedded.inc;
+
+#
+# MDEV-3915 COM_CHANGE_USER allows fast password brute-forcing
+#
+# only three failed change_user per connection.
+# successful change_user do NOT reset the counter
+#
+connect (test,localhost,root,,);
+connection test;
+--error 1045
+change_user foo,bar;
+--error 1045
+change_user foo;
+change_user;
+--error 1045
+change_user foo,bar;
+--error 1047
+change_user foo,bar;
+--error 1047
+change_user;
+disconnect test;
+connection default;
+

=== modified file 'sql/sql_class.cc'
--- a/sql/sql_class.cc	2012-08-22 14:13:54 +0000
+++ b/sql/sql_class.cc	2013-01-24 23:17:39 +0000
@@ -675,6 +675,7 @@
    warning_info(&main_warning_info),
    stmt_da(&main_da),
    global_disable_checkpoint(0),
+   failed_com_change_user(0),
    is_fatal_error(0),
    transaction_rollback_request(0),
    is_fatal_sub_stmt_error(0),

=== modified file 'sql/sql_class.h'
--- a/sql/sql_class.h	2013-01-21 23:23:40 +0000
+++ b/sql/sql_class.h	2013-01-24 23:17:39 +0000
@@ -1865,6 +1865,7 @@
   bool	     no_errors;
   uint8      password;
 
+  uint8      failed_com_change_user;
   /**
     Set to TRUE if execution of the current compound statement
     can not continue. In particular, disables activation of

=== modified file 'sql/sql_parse.cc'
--- a/sql/sql_parse.cc	2012-08-22 14:13:54 +0000
+++ b/sql/sql_parse.cc	2013-01-24 23:17:39 +0000
@@ -1157,7 +1158,17 @@
     thd->security_ctx->user= 0;
     thd->user_connect= 0;
 
-    rc= acl_authenticate(thd, 0, packet_length);
+    /*
+      to limit COM_CHANGE_USER ability to brute-force passwords,
+      we only allow three unsuccessful COM_CHANGE_USER per connection.
+    */
+    if (thd->failed_com_change_user >= 3)
+    {
+      my_message(ER_UNKNOWN_COM_ERROR, ER(ER_UNKNOWN_COM_ERROR), MYF(0));
+      rc= 1;
+    }
+    else
+      rc= acl_authenticate(thd, 0, packet_length);
     MYSQL_AUDIT_NOTIFY_CONNECTION_CHANGE_USER(thd);
     if (rc)
     {
@@ -1170,6 +1183,8 @@
       thd->variables.collation_connection= save_collation_connection;
       thd->variables.character_set_results= save_character_set_results;
       thd->update_charset();
+      thd->failed_com_change_user++;
+      my_sleep(1000000);
     }
     else
     {

=== modified file 'tests/mysql_client_test.c'
--- a/tests/mysql_client_test.c	2013-01-10 12:54:04 +0000
+++ b/tests/mysql_client_test.c	2013-01-24 23:17:39 +0000
@@ -15386,6 +15386,7 @@
   const char *pw= "password";
   const char *db= "mysqltest_user_test_database";
   int rc;
+  MYSQL*       conn;
 
   DBUG_ENTER("test_change_user");
   myheader("test_change_user");
@@ -15429,149 +15430,173 @@
   rc= mysql_query(mysql, buff);
   myquery(rc);
 
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
 
   /* Try some combinations */
-  rc= mysql_change_user(mysql, NULL, NULL, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-
-  rc= mysql_change_user(mysql, "", NULL, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", "", NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", "", "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, NULL, "", "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-
-  rc= mysql_change_user(mysql, NULL, NULL, "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", NULL, "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, NULL, "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, "", "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, "", NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, NULL, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, "", db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, NULL, db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_pw, pw, db);
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_pw, pw, NULL);
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_pw, pw, "");
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_no_pw, pw, db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_no_pw, pw, "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_no_pw, pw, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, user_no_pw, "", NULL);
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_no_pw, "", "");
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_no_pw, "", db);
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user_no_pw, NULL, db);
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, "", pw, db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", pw, "");
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", pw, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, NULL, pw, NULL);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, NULL, NULL, db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, NULL, "", db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
-
-  rc= mysql_change_user(mysql, "", "", db);
-  DIE_UNLESS(rc);
-  if (! opt_silent)
-    printf("Got error (as expected): %s\n", mysql_error(mysql));
+  rc= mysql_change_user(conn, NULL, NULL, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+
+  rc= mysql_change_user(conn, "", NULL, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, "", "", NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, "", "", "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, NULL, "", "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+
+  rc= mysql_change_user(conn, NULL, NULL, "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, "", NULL, "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_pw, NULL, "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_pw, "", "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, user_pw, "", NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_pw, NULL, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_pw, "", db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, user_pw, NULL, db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_pw, pw, db);
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_pw, pw, NULL);
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_pw, pw, "");
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_no_pw, pw, db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_no_pw, pw, "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, user_no_pw, pw, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, user_no_pw, "", NULL);
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_no_pw, "", "");
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_no_pw, "", db);
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user_no_pw, NULL, db);
+  myquery(rc);
+
+  rc= mysql_change_user(conn, "", pw, db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, "", pw, "");
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, "", pw, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, NULL, pw, NULL);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, NULL, NULL, db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, NULL, "", db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
+
+  rc= mysql_change_user(conn, "", "", db);
+  DIE_UNLESS(rc);
+  if (! opt_silent)
+    printf("Got error (as expected): %s\n", mysql_error(conn));
 
   /* Cleanup the environment */
 
-  mysql_change_user(mysql, opt_user, opt_password, current_db);
+  mysql_change_user(conn, opt_user, opt_password, current_db);
+
+  mysql_close(conn);
 
   sprintf(buff, "drop database %s", db);
   rc= mysql_query(mysql, buff);
@@ -16234,29 +16259,35 @@
   static char db[NAME_CHAR_LEN+1];
   static char query[LARGE_BUFFER_SIZE*2];
 #endif
+  MYSQL* conn;
 
   DBUG_ENTER("test_bug31669");
   myheader("test_bug31669");
 
-  rc= mysql_change_user(mysql, NULL, NULL, NULL);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, NULL, NULL, NULL);
   DIE_UNLESS(rc);
 
-  rc= mysql_change_user(mysql, "", "", "");
+  rc= mysql_change_user(conn, "", "", "");
   DIE_UNLESS(rc);
 
   memset(buff, 'a', sizeof(buff));
 
-  rc= mysql_change_user(mysql, buff, buff, buff);
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
+  rc= mysql_change_user(conn, buff, buff, buff);
   DIE_UNLESS(rc);
 
-  rc = mysql_change_user(mysql, opt_user, opt_password, current_db);
+  rc = mysql_change_user(conn, opt_user, opt_password, current_db);
   DIE_UNLESS(!rc);
 
 #ifndef EMBEDDED_LIBRARY
   memset(db, 'a', sizeof(db));
   db[NAME_CHAR_LEN]= 0;
   strxmov(query, "CREATE DATABASE IF NOT EXISTS ", db, NullS);
-  rc= mysql_query(mysql, query);
+  rc= mysql_query(conn, query);
   myquery(rc);
 
   memset(user, 'b', sizeof(user));
@@ -16265,54 +16296,59 @@
   buff[LARGE_BUFFER_SIZE]= 0;
   strxmov(query, "GRANT ALL PRIVILEGES ON *.* TO '", user, "'@'%' IDENTIFIED BY "
                  "'", buff, "' WITH GRANT OPTION", NullS);
-  rc= mysql_query(mysql, query);
+  rc= mysql_query(conn, query);
   myquery(rc);
 
   strxmov(query, "GRANT ALL PRIVILEGES ON *.* TO '", user, "'@'localhost' IDENTIFIED BY "
                  "'", buff, "' WITH GRANT OPTION", NullS);
-  rc= mysql_query(mysql, query);
-  myquery(rc);
-
-  rc= mysql_query(mysql, "FLUSH PRIVILEGES");
-  myquery(rc);
-
-  rc= mysql_change_user(mysql, user, buff, db);
+  rc= mysql_query(conn, query);
+  myquery(rc);
+
+  rc= mysql_query(conn, "FLUSH PRIVILEGES");
+  myquery(rc);
+
+  rc= mysql_change_user(conn, user, buff, db);
   DIE_UNLESS(!rc);
 
   user[USERNAME_CHAR_LENGTH-1]= 'a';
-  rc= mysql_change_user(mysql, user, buff, db);
+  rc= mysql_change_user(conn, user, buff, db);
   DIE_UNLESS(rc);
 
   user[USERNAME_CHAR_LENGTH-1]= 'b';
   buff[LARGE_BUFFER_SIZE-1]= 'd';
-  rc= mysql_change_user(mysql, user, buff, db);
+  rc= mysql_change_user(conn, user, buff, db);
   DIE_UNLESS(rc);
 
   buff[LARGE_BUFFER_SIZE-1]= 'c';
   db[NAME_CHAR_LEN-1]= 'e';
-  rc= mysql_change_user(mysql, user, buff, db);
+  rc= mysql_change_user(conn, user, buff, db);
   DIE_UNLESS(rc);
 
+  mysql_close(conn);
+  conn= client_connect(0, MYSQL_PROTOCOL_TCP, 0);
+
   db[NAME_CHAR_LEN-1]= 'a';
-  rc= mysql_change_user(mysql, user, buff, db);
+  rc= mysql_change_user(conn, user, buff, db);
   DIE_UNLESS(!rc);
 
-  rc= mysql_change_user(mysql, user + 1, buff + 1, db + 1);
+  rc= mysql_change_user(conn, user + 1, buff + 1, db + 1);
   DIE_UNLESS(rc);
 
-  rc = mysql_change_user(mysql, opt_user, opt_password, current_db);
+  rc = mysql_change_user(conn, opt_user, opt_password, current_db);
   DIE_UNLESS(!rc);
 
   strxmov(query, "DROP DATABASE ", db, NullS);
-  rc= mysql_query(mysql, query);
+  rc= mysql_query(conn, query);
   myquery(rc);
 
   strxmov(query, "DELETE FROM mysql.user WHERE User='", user, "'", NullS);
-  rc= mysql_query(mysql, query);
+  rc= mysql_query(conn, query);
   myquery(rc);
-  DIE_UNLESS(mysql_affected_rows(mysql) == 2);
+  DIE_UNLESS(mysql_affected_rows(conn) == 2);
 #endif
 
+  mysql_close(conn);
+
   DBUG_VOID_RETURN;
 }