diff --git a/BitKeeper/etc/collapsed b/BitKeeper/etc/collapsed
index 8b90deae622..c008b9f18fc 100644
--- a/BitKeeper/etc/collapsed
+++ b/BitKeeper/etc/collapsed
@@ -1 +1,2 @@
 44d03f27qNdqJmARzBoP3Is_cN5e0w
+44ec850ac2k4y2Omgr92GiWPBAVKGQ
diff --git a/mysql-test/r/grant.result b/mysql-test/r/grant.result
index 3f3325354ee..e755822c490 100644
--- a/mysql-test/r/grant.result
+++ b/mysql-test/r/grant.result
@@ -867,3 +867,27 @@ insert into mysql.user select * from t2;
 flush privileges;
 drop table t2;
 drop table t1;
+GRANT CREATE ON mysqltest.* TO 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+GRANT CREATE ON mysqltest.* TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+REVOKE CREATE ON mysqltest.* FROM 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+REVOKE CREATE ON mysqltest.* FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+GRANT CREATE ON t1 TO 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+GRANT CREATE ON t1 TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+REVOKE CREATE ON t1 FROM 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+REVOKE CREATE ON t1 FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+GRANT EXECUTE ON PROCEDURE p1 TO 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+GRANT EXECUTE ON PROCEDURE p1 TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+REVOKE EXECUTE ON PROCEDURE p1 FROM 1234567890abcdefGHIKL@localhost;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+REVOKE EXECUTE ON PROCEDURE t1 FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
diff --git a/mysql-test/r/sp.result b/mysql-test/r/sp.result
index 7440ee16007..d4c77bc47e5 100644
--- a/mysql-test/r/sp.result
+++ b/mysql-test/r/sp.result
@@ -5374,4 +5374,17 @@ a
 1
 use test|
 drop table t3|
+DROP PROCEDURE IF EXISTS bug16899_p1|
+DROP FUNCTION IF EXISTS bug16899_f1|
+CREATE DEFINER=1234567890abcdefGHIKL@localhost PROCEDURE bug16899_p1()
+BEGIN
+SET @a = 1;
+END|
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+FUNCTION bug16899_f1() RETURNS INT
+BEGIN
+RETURN 1;
+END|
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
 drop table t1,t2;
diff --git a/mysql-test/r/trigger.result b/mysql-test/r/trigger.result
index f3e797d2344..b41dd66c390 100644
--- a/mysql-test/r/trigger.result
+++ b/mysql-test/r/trigger.result
@@ -1089,4 +1089,17 @@ begin
 set @a:= 1;
 end|
 ERROR HY000: Triggers can not be created on system tables
+use test|
+DROP TABLE IF EXISTS t1;
+DROP TABLE IF EXISTS t2;
+CREATE TABLE t1(c INT);
+CREATE TABLE t2(c INT);
+CREATE DEFINER=1234567890abcdefGHIKL@localhost
+TRIGGER t1_bi BEFORE INSERT ON t1 FOR EACH ROW SET @a = 1;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+TRIGGER t2_bi BEFORE INSERT ON t2 FOR EACH ROW SET @a = 2;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+DROP TABLE t1;
+DROP TABLE t2;
 End of 5.0 tests
diff --git a/mysql-test/r/view.result b/mysql-test/r/view.result
index 534065a33b6..c10f7d49157 100644
--- a/mysql-test/r/view.result
+++ b/mysql-test/r/view.result
@@ -2850,3 +2850,14 @@ Tables_in_test
 t1
 DROP TABLE t1;
 DROP VIEW IF EXISTS v1;
+DROP TABLE IF EXISTS t1;
+DROP VIEW IF EXISTS v1;
+DROP VIEW IF EXISTS v2;
+CREATE TABLE t1(a INT, b INT);
+CREATE DEFINER=1234567890abcdefGHIKL@localhost
+VIEW v1 AS SELECT a FROM t1;
+ERROR HY000: String '1234567890abcdefGHIKL' is too long for user name (should be no longer than 16)
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+VIEW v2 AS SELECT b FROM t1;
+ERROR HY000: String '1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY' is too long for host name (should be no longer than 60)
+DROP TABLE t1;
diff --git a/mysql-test/t/grant.test b/mysql-test/t/grant.test
index a9d52f559ca..94b63389771 100644
--- a/mysql-test/t/grant.test
+++ b/mysql-test/t/grant.test
@@ -681,3 +681,52 @@ drop table t2;
 drop table t1;
 
 
+#
+# Test for BUG#16899: Possible buffer overflow in handling of DEFINER-clause.
+#
+# These checks are intended to ensure that appropriate errors are risen when
+# illegal user name or hostname is specified in user-clause of GRANT/REVOKE
+# statements.
+#
+
+# Working with database-level privileges.
+
+--error ER_WRONG_STRING_LENGTH
+GRANT CREATE ON mysqltest.* TO 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+GRANT CREATE ON mysqltest.* TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE CREATE ON mysqltest.* FROM 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE CREATE ON mysqltest.* FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+
+# Working with table-level privileges.
+
+--error ER_WRONG_STRING_LENGTH
+GRANT CREATE ON t1 TO 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+GRANT CREATE ON t1 TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE CREATE ON t1 FROM 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE CREATE ON t1 FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+
+# Working with routine-level privileges.
+
+--error ER_WRONG_STRING_LENGTH
+GRANT EXECUTE ON PROCEDURE p1 TO 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+GRANT EXECUTE ON PROCEDURE p1 TO some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE EXECUTE ON PROCEDURE p1 FROM 1234567890abcdefGHIKL@localhost;
+
+--error ER_WRONG_STRING_LENGTH
+REVOKE EXECUTE ON PROCEDURE t1 FROM some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY;
diff --git a/mysql-test/t/sp.test b/mysql-test/t/sp.test
index f48a210c7eb..bf255bf631a 100644
--- a/mysql-test/t/sp.test
+++ b/mysql-test/t/sp.test
@@ -6286,6 +6286,32 @@ select * from (select 1 as a) as t1 natural join (select * from test.t3) as t2|
 use test|
 drop table t3|
 
+
+#
+# Test for BUG#16899: Possible buffer overflow in handling of DEFINER-clause.
+#
+
+# Prepare.
+
+--disable_warnings
+DROP PROCEDURE IF EXISTS bug16899_p1|
+DROP FUNCTION IF EXISTS bug16899_f1|
+--enable_warnings
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=1234567890abcdefGHIKL@localhost PROCEDURE bug16899_p1()
+BEGIN
+  SET @a = 1;
+END|
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+  FUNCTION bug16899_f1() RETURNS INT
+BEGIN
+  RETURN 1;
+END|
+
+
 #
 # BUG#NNNN: New bug synopsis
 #
diff --git a/mysql-test/t/trigger.test b/mysql-test/t/trigger.test
index 95e8eaae83e..7e20b61c1e4 100644
--- a/mysql-test/t/trigger.test
+++ b/mysql-test/t/trigger.test
@@ -1301,6 +1301,36 @@ create trigger wont_work after update on event for each row
 begin
  set @a:= 1;
 end|
+use test|
 delimiter ;|
 
+
+#
+# Test for BUG#16899: Possible buffer overflow in handling of DEFINER-clause.
+#
+
+# Prepare.
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+DROP TABLE IF EXISTS t2;
+--enable_warnings
+
+CREATE TABLE t1(c INT);
+CREATE TABLE t2(c INT);
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=1234567890abcdefGHIKL@localhost
+  TRIGGER t1_bi BEFORE INSERT ON t1 FOR EACH ROW SET @a = 1;
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+  TRIGGER t2_bi BEFORE INSERT ON t2 FOR EACH ROW SET @a = 2;
+
+# Cleanup.
+
+DROP TABLE t1;
+DROP TABLE t2;
+
+
 --echo End of 5.0 tests
diff --git a/mysql-test/t/view.test b/mysql-test/t/view.test
index 5cb85ca6c9b..3e0129dcb7d 100644
--- a/mysql-test/t/view.test
+++ b/mysql-test/t/view.test
@@ -2718,3 +2718,30 @@ DROP TABLE t1;
 --disable_warnings
 DROP VIEW IF EXISTS v1;
 --enable_warnings
+
+
+#
+# Test for BUG#16899: Possible buffer overflow in handling of DEFINER-clause.
+#
+
+# Prepare.
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+DROP VIEW IF EXISTS v1;
+DROP VIEW IF EXISTS v2;
+--enable_warnings
+
+CREATE TABLE t1(a INT, b INT);
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=1234567890abcdefGHIKL@localhost
+  VIEW v1 AS SELECT a FROM t1;
+
+--error ER_WRONG_STRING_LENGTH
+CREATE DEFINER=some_user_name@1234567890abcdefghij1234567890abcdefghij1234567890abcdefghijQWERTY
+  VIEW v2 AS SELECT b FROM t1;
+
+# Cleanup.
+
+DROP TABLE t1;
diff --git a/sql/mysql_priv.h b/sql/mysql_priv.h
index 0834af46d38..3ec9dd718e8 100644
--- a/sql/mysql_priv.h
+++ b/sql/mysql_priv.h
@@ -557,6 +557,7 @@ void get_default_definer(THD *thd, LEX_USER *definer);
 LEX_USER *create_default_definer(THD *thd);
 LEX_USER *create_definer(THD *thd, LEX_STRING *user_name, LEX_STRING *host_name);
 LEX_USER *get_current_user(THD *thd, LEX_USER *user);
+bool check_string_length(LEX_STRING *str, const char *err_msg, uint max_length);
 
 enum enum_mysql_completiontype {
   ROLLBACK_RELEASE=-2, ROLLBACK=1,  ROLLBACK_AND_CHAIN=7,
diff --git a/sql/share/errmsg.txt b/sql/share/errmsg.txt
index 5c967ba19bd..b3cb33090ce 100644
--- a/sql/share/errmsg.txt
+++ b/sql/share/errmsg.txt
@@ -5623,3 +5623,9 @@ ER_NO_TRIGGERS_ON_SYSTEM_SCHEMA
 	eng "Triggers can not be created on system tables"
 ER_REMOVED_SPACES
         eng "Leading spaces are removed from name '%s'"
+ER_USERNAME
+	eng "user name"
+ER_HOSTNAME
+	eng "host name"
+ER_WRONG_STRING_LENGTH
+	eng "String '%-.70s' is too long for %s (should be no longer than %d)"
diff --git a/sql/sql_acl.cc b/sql/sql_acl.cc
index ae5ea210a47..0c7b8626c93 100644
--- a/sql/sql_acl.cc
+++ b/sql/sql_acl.cc
@@ -2900,14 +2900,6 @@ bool mysql_table_grant(THD *thd, TABLE_LIST *table_list,
       result= TRUE;
       continue;
     }  
-    if (Str->host.length > HOSTNAME_LENGTH ||
-	Str->user.length > USERNAME_LENGTH)
-    {
-      my_message(ER_GRANT_WRONG_HOST_OR_USER, ER(ER_GRANT_WRONG_HOST_OR_USER),
-                 MYF(0));
-      result= TRUE;
-      continue;
-    }
     /* Create user if needed */
     error=replace_user_table(thd, tables[0].table, *Str,
 			     0, revoke_grant, create_new_users,
@@ -3112,15 +3104,6 @@ bool mysql_routine_grant(THD *thd, TABLE_LIST *table_list, bool is_proc,
       result= TRUE;
       continue;
     }  
-    if (Str->host.length > HOSTNAME_LENGTH ||
-	Str->user.length > USERNAME_LENGTH)
-    {
-      if (!no_error)
-	my_message(ER_GRANT_WRONG_HOST_OR_USER, ER(ER_GRANT_WRONG_HOST_OR_USER),
-                   MYF(0));
-      result= TRUE;
-      continue;
-    }
     /* Create user if needed */
     error=replace_user_table(thd, tables[0].table, *Str,
 			     0, revoke_grant, create_new_users,
@@ -3246,14 +3229,6 @@ bool mysql_grant(THD *thd, const char *db, List <LEX_USER> &list,
       result= TRUE;
       continue;
     }  
-    if (Str->host.length > HOSTNAME_LENGTH ||
-	Str->user.length > USERNAME_LENGTH)
-    {
-      my_message(ER_GRANT_WRONG_HOST_OR_USER, ER(ER_GRANT_WRONG_HOST_OR_USER),
-                 MYF(0));
-      result= -1;
-      continue;
-    }
     if (replace_user_table(thd, tables[0].table, *Str,
                            (!db ? rights : 0), revoke_grant, create_new_users,
                            test(thd->variables.sql_mode &
@@ -4144,14 +4119,6 @@ bool mysql_show_grants(THD *thd,LEX_USER *lex_user)
     DBUG_RETURN(TRUE);
   }
 
-  if (lex_user->host.length > HOSTNAME_LENGTH ||
-      lex_user->user.length > USERNAME_LENGTH)
-  {
-    my_message(ER_GRANT_WRONG_HOST_OR_USER, ER(ER_GRANT_WRONG_HOST_OR_USER),
-               MYF(0));
-    DBUG_RETURN(TRUE);
-  }
-
   rw_rdlock(&LOCK_grant);
   VOID(pthread_mutex_lock(&acl_cache->lock));
 
diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index 061c29e16c3..f3ac1280983 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -7533,16 +7533,34 @@ LEX_USER *create_definer(THD *thd, LEX_STRING *user_name, LEX_STRING *host_name)
 
 LEX_USER *get_current_user(THD *thd, LEX_USER *user)
 {
-  LEX_USER *curr_user;
   if (!user->user.str)  // current_user
-  {
-    if (!(curr_user= (LEX_USER*) thd->alloc(sizeof(LEX_USER))))
-    {
-      my_error(ER_OUTOFMEMORY, MYF(0), sizeof(LEX_USER));
-      return 0;
-    }
-    get_default_definer(thd, curr_user);
-    return curr_user;
-  }
+    return create_default_definer(thd);
+
   return user;
 }
+
+
+/*
+  Check that length of a string does not exceed some limit.
+
+  SYNOPSIS
+    check_string_length()
+      str         string to be checked
+      err_msg     error message to be displayed if the string is too long
+      max_length  max length
+
+  RETURN
+    FALSE   the passed string is not longer than max_length
+    TRUE    the passed string is longer than max_length
+*/
+
+bool check_string_length(LEX_STRING *str, const char *err_msg,
+                         uint max_length)
+{
+  if (str->length <= max_length)
+    return FALSE;
+
+  my_error(ER_WRONG_STRING_LENGTH, MYF(0), str->str, err_msg, max_length);
+
+  return TRUE;
+}
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 1dbed6d3cdb..133b6e18fee 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -7511,6 +7511,9 @@ user:
 	  $$->user = $1;
 	  $$->host.str= (char *) "%";
 	  $$->host.length= 1;
+
+	  if (check_string_length(&$$->user, ER(ER_USERNAME), USERNAME_LENGTH))
+	    YYABORT;
 	}
 	| ident_or_text '@' ident_or_text
 	  {
@@ -7518,6 +7521,11 @@ user:
 	    if (!($$=(LEX_USER*) thd->alloc(sizeof(st_lex_user))))
 	      YYABORT;
 	    $$->user = $1; $$->host=$3;
+
+	    if (check_string_length(&$$->user, ER(ER_USERNAME), USERNAME_LENGTH) ||
+	        check_string_length(&$$->host, ER(ER_HOSTNAME),
+					       HOSTNAME_LENGTH))
+	      YYABORT;
 	  }
 	| CURRENT_USER optional_braces
 	{
@@ -8995,15 +9003,9 @@ definer:
            */
           YYTHD->lex->definer= 0;
 	}
-	| DEFINER_SYM EQ CURRENT_USER optional_braces
+	| DEFINER_SYM EQ user
 	{
-          if (! (YYTHD->lex->definer= create_default_definer(YYTHD)))
-            YYABORT;
-	}
-	| DEFINER_SYM EQ ident_or_text '@' ident_or_text
-	{
-          if (!(YYTHD->lex->definer= create_definer(YYTHD, &$3, &$5)))
-            YYABORT;
+	  YYTHD->lex->definer= get_current_user(YYTHD, $3);
 	}
 	;