The University of Florida Student Infosec Team competed in the Leet More CTF 2010 yesterday. It was a 24-hour challenge-based event sort of like DEFCON quals. Ian and I made the team some ridiculous Team Kernel Sanders shirts at our hackerspace just before the competition started. The good colonel vs. Lenin: FIGHT!
Here’s a walkthrough/writeup of one of the challenges.
Injection 300: SQL injection with raw MD5 hashes
One challenge at yesterday’s CTF was a seemingly-impossible SQL injection worth 300 points. The point of the challenge was to submit a password to a PHP script that would be hashed with MD5 before being used in a query. At first glance, the challenge looked impossible. Here’s the code that was running on the game server:
The only injection point was the first
mysql_query(). Without the
complication of MD5, the vulnerable line of code would have looked like this:
$r = mysql_query("SELECT login FROM admins WHERE password = '" . $_GET['password'] . "'");
If the password
foobar were submitted to the script, this SQL statement would
be executed on the server:
SELECT login FROM admins WHERE password = 'foobar'
That would have been trivial to exploit. I could have submitted the password
OR 1 = 1; -- instead:
SELECT login FROM admins WHERE password = '' OR 1 = 1; -- '
…which would have returned all the rows from the
admins table and tricked
the script into granting me access to the page.
However, this challenge was much more difficult than that. Since PHP’s
function was encrypting the password first, this was what was being sent to the
SELECT login FROM admins WHERE password = '[output of md5 function]'
So how could I possibly inject SQL when MD5 would destroy whatever I supplied?
The trick: Raw MD5 hashes are dangerous in SQL
The trick in this challenge was that PHP’s
md5() function can return its
output in either hex or raw form. Here’s
md5()’s method signature:
string md5( string $str [, bool $raw_output = false] )
If the second argument to MD5 is
true, it will return ugly raw bits instead
of a nice hex string. Raw MD5 hashes are dangerous in SQL statements because
they can contain characters with special meaning to MySQL. The raw data could,
for example, contain quotes (
") that would allow SQL injection.
I used this fact to create a raw MD5 hash that contained SQL injection code.
But it might take years to calculate
In order to spend the least possible time brute forcing MD5 hashes, I tried to think of the shortest possible SQL injection. I came up with one only 6 characters long:
I quickly wrote a C program to see how fast I could brute force MD5. My netbook could compute about 500,000 MD5 hashes per second using libssl’s MD5 functions. My quick (and possibly wrong) math told me every hash had a 1 in 28 trillion chance of containing my desired 6-character injection string.
So that would only take 2 years at 500,000 hashes per second.
Optimizing: Shortening the injection string
If I could shorten my injection string by even one character, I would reduce the number of hash calculations by a factor of 256. After thinking about the problem for a while and playing around a lot with MySQL, I was able to shorten my injection to only 5 characters:
This would produce an SQL statement like this (assuming my injection happened
to fall in about the middle of the MD5 hash and pretending
xxxx is random
SELECT login FROM admins WHERE password = 'xxx'||'1xxxxxxxx'
|| is equivalent to
OR, and a string starting with a
1 is cast as an
integer when used as a boolean. Therefore, my injection would be equivalent to
SELECT login FROM admins WHERE password = 'xxx' OR 1
By Just removing a single character, that got me down to 2.3 days' worth of calculation. Still not fast enough, but getting closer.
Lopping off another character, and more improvements
Since any number from 1 to 9 would work in my injection, I could shorten my
injection string to just
'||' and then check to see if the injection string
were followed by a digit from 1 to 9 (a very cheap check). This would
simultaneously reduce my MD5 calculations by a factor of 256 and make it 9
times as likely that I’d find a usable injection string.
|| is the same as
OR, I could check for it too (2x
speedup) and all its case variations (16x speedup). Running my program on a remote dual-core desktop instead of my netbook got me another 10x speedup.
The final hash
After computing only 19 million MD5 hashes, my program found an answer:
content: 129581926211651571912466741651878684928 count: 18933549 hex: 06da5430449f8f6f23dfc1276f722738 raw: ?T0D??o#??'or'8.N=?
So I submitted the password
129581926211651571912466741651878684928 to the
PHP script, and it worked! I was able to see this table:
The last step of the challenge was to turn the MD5 hash into a password. I
could have used a brute forcer like John, but instead I just searched Google.
The password had been cracked by opencrack.hashkiller.com and was