Privileges in MySQL and MariaDB: The Weed Of Crime Bears Bitter Fruit
Let’s look at how well MySQL and MariaDB support privileges (part of “access control” in standard terms), compared to other DBMSs, and consequences thereof.
Count the Privileges
I go to the DBMS manuals (here and starting here and here) and I count the privileges. This is like judging a town by the number of traffic lights it claims to have, but I’m trying to get an indicator for how granular the DBMS’s “authorization” is.
Number of privileges listed in the manuals MySQL/MariaDB Oracle 12c DB2 9.7 SQL Server 2014 31 240 52 124
Pretty small number in the first column, eh? There are historical reasons that MySQL was reluctant to add new privileges, illustrated by Bug#43730.
What is the effect of having a limited number of privileges? Sometimes the same privilege has to be used for two different things. For example, the SUPER privilege is good for “CHANGE MASTER TO, KILL, PURGE BINARY LOGS, SET GLOBAL, and mysqladmin debug command”, while the PROCESS privilege is what you need for SHOW PROCESSLIST — but also for selecting from information_schema.innodb_sys_tables.
Why is this a flaw? If administrators want to allow access to a goose, they are forced to allow access to a gander as well — even when the gander is none of the grantee’s business. As an example that affected us: to make Ocelot’s stored-procedure debugger work, we have to be able to set values in a single global variable, which is impossible without the SUPER privilege, therefore to allow people to use the debugger you have to allow them to purge binary logs too.
The SQL standard mentions 9 privileges: INSERT UPDATE DELETE SELECT REFERENCES USAGE UNDER TRIGGER EXECUTE. MySQL and MariaDB do a fair job of handling them:
INSERT: perfect support, including column-level grants.
UPDATE: perfect support, including column-level grants.
DELETE: perfect support.
SELECT: perfect support, including column-level grants.
REFERENCES: perfect support, not including column-level grants. I think this is not well known yet. Deep in the caverns of the manual are the words: “The REFERENCES privilege is unused before MySQL 5.7.6. As of 5.7.6, creation of a foreign key constraint requires the REFERENCES privilege for the parent table.” Kudos to MySQL. The MariaDB manual, on the other hand, still says the REFERENCES privilege is “unused”. For some background about this new feature, click the high-level architecture tab in the old worklog task Foreign keys: reference privilege.
USAGE: no support. In standard SQL, USAGE is for access to domains or UDTs or sequence generators or transliterations or character sets or collations. In MySQL/MariaDB, USAGE is the minimum privilege — you can log in, that’s all. So USAGE is unsupported, but unimportant.
UNDER: no relevance. This is for optional UDT features.
TRIGGER: perfect support.
EXECUTE: perfect support.
Looking at the down side, MySQL and MariaDB don’t allow for the standard GRANT OPTION. Yes, they have a GRANT OPTION privilege, but that’s not standard — what’s needed (and what’s supported by the other serious DBMSs) is an option to grant a particular privilege, not a privilege to grant any privileges.
The objection about having hundreds of possible privileges is: it’s hard to keep track of them, or even remember what they are. This should be a solved problem: allow a package of privileges, in other words support CREATE ROLE. This time the kudos go to MariaDB which has allowed roles for over two years. But what if you have MySQL and it’s tedious to grant multiple times?
It’s still simple. You either make a script which contains a bunch of GRANT statements, or you create a stored procedure. Certainly I’d recommend a stored procedure, because it will be “inside the database”, and therefore subject to tracking. Scripts are a tad more dicey security-wise, since changing or deleting files is a process outside the DBMS’s control.
After all, doing grants via an insecure mechanism would kinda mess up the idea of using grants for extra security.
There is a standard and reasonable way to get at metadata: you can see the information_schema table, but you won’t see rows for database objects that you don’t have access to.
MySQL and MariaDB follow this plan, but there is a major exception: InnoDB. Consider INNODB_SYS_TABLES, which has information about other tables. Of course this table should not exist at all (the sensible place is information_schema.TABLES), but the more troubling fact is that the relevant privilege is not “whether you have access to the other tables”, but — wow — the PROCESS privilege. And to top it off, in MySQL (though not MariaDB) instead of an empty table you get an error message.
Statement: select * from information_schema.innodb_sys_tables;
Response from MySQL 5.7: ERROR 1227 (42000): Access denied; you need (at least one of) the PROCESS privilege(s) for this operation
We Live In A Just World
Therefore, here is how I can crash my MySQL 5.7 server. Provided, ironically, that I do not have any privileges on any database objects. In other words, I’ve logged in as a user who has been granted the minimum:
GRANT USAGE ON *.* to ‘peter’@’localhost’;
The important prerequisites are: MySQL 5.7.9 compiled from source, a new installation, and an unprivileged user. It doesn’t seem to happen under any other circumstances. So this is not a vulnerability alert. I like to show it, though, as an illustration of the punishment that awaits violators of the precepts of privileges.
As I indicated, I’ve logged in, and the database is empty. Now I say:
SELECT A.NAME, B.NAME FROM INFORMATION_SCHEMA.INNODB_SYS_TABLES A LEFT JOIN INFORMATION_SCHEMA.INNODB_SYS_TABLES B ON A.SPACE = B.SPACE;
On the client I see
ERROR 2013 (HY000): Lost connection to MySQL server during query
On the server I see
mysqld: /home/pgulutzan/Downloads/mysql-5.7.9/sql/sql_error.cc:444: void Diagnostics_area::set_error_status(uint, const char*, const char*): Assertion `! is_set() || m_can_overwrite_status’ failed.
18:37:12 UTC – mysqld got signal 6 ;
As this is a crash and something is definitely wrong, the information collection process might fail.
Those who live in glass houses
I don’t think it would be fair to end this without confessing: us too.
For example, the ocelotgui GUI client for MySQL and MariaDB can crash if I ask it not to send /* comments */ to the server, and there is a very long comment at the end of a statement after the semicolon. We are all sinners.
However, that bug, and a few minor ones, have been found during alpha tests. I’m still hopeful that we’ll go beta within a few weeks, and invite anyone to try and find an embarrassing problem before that happens. The readme and the download are on github at https://github.com/ocelot-inc/ocelotgui.