危险的Alter User权限
Alter user 是Oracle数据库系统中十分powerful的权限，可以修改一般用户的属性例如默认表空间、profile等，甚至于可以修改SYS用户的密码。所以该权限是十分危险的， 在赋权时要十分小心。
针对现有用户已经拥有ALTER USER权限的，又不能revoke回来的，可以考虑使用如下触发器的方法，禁止任何拥有Alter user权限的用户修改SYSDBA的密码：
http://www.askmaclean.com/archives/alter-user-privs.html
SQL> grant alter user to alteruser;Grant succeeded.SQL> select * from DBA_SYS_PRIVS where grantee like 'ALTER%USER%';GRANTEE PRIVILEGE ADMIN_-------------------- ------------------------- ------ALTERUSER UNLIMITED TABLESPACE NOALTERUSER ALTER USER NOSQL> conn / as sysdbaConnected.SQL> CREATE or REPLACE TRIGGER prohibit_alter_SYSTEM_SYS_pass BEFORE ALTER on ALTERUSER.schema BEGIN IF ora_sysevent='ALTER' and ora_dict_obj_type = 'USER' and (ora_dict_obj_name = 'SYSTEM' or ora_dict_obj_name = 'SYS') THEN RAISE_APPLICATION_ERROR(-20003, 'You are not allowed to alter SYSTEM/SYS user.'); END IF; END; /Trigger created.
使用范例：
SQL> conn alteruser/oracleConnected.SQL> alter user system identified by manager;alter user system identified by manager *ERROR at line 1:ORA-00604: error occurred at recursive SQL level 1ORA-20003: You are not allowed to alter SYSTEM/SYS user.ORA-06512: at line 5SQL> alter user sys identified by manager; alter user sys identified by manager *ERROR at line 1:ORA-00604: error occurred at recursive SQL level 1ORA-20003: You are not allowed to alter SYSTEM/SYS user.ORA-06512: at line 5SQL>SQL> alter user dbsnmp identified by dbsnmp;User altered.
