Posted by CrashOverron on 01/30/2014 21:26:01
What is SQL Injection
SQL injection is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another.
0x00 - Intro
All the information contained in the article is from personal experience, if I don't go over something that you currently do or have seen in SQL injections, its because I do not use it; not saying I'm right just that's how it is. As you should already know, extracting database information from a server without administration approval is illegal and I cannot be held accountable for any malicious actions executed after reading this acticle.
0x01 - What is MySQL
"SQL" stands for "Structed Query Language," which simply allows users to send queries to the server database. There are different types of SQL such as MSSQL, which is Microsoft's version of the language and also has some different commands as well as syntax.
0x02 - Finding SQL Injections
Before jumping into this topic I want to explain to you about comments in MySQL. There are three variations to a comment in this language:
As you should already know a comment just blocks out a section so it will not be executed through the query. Typically, anytime you see a page from a website that takes in a paramater such as:
(not saying injections are narrowed down to only id parameters but they are quite common) you may want to test the page for a vulnerability. The simplest way I know of to check for a vulnerability is to add:
to the end of the url and see if the contents of the page change, even the slightest bit, if they don't then add
(it doesnt have to be 1=1 or 1=0 just something that returns true for the first statement and false for the second) and see if it changes after the second. If the contents change after the second query then you have a vulnerability.
0x03 - Gathering Information
To make your job or life a little easier you should look around the site some to gather information on what you are trying to retreive. For instance, if the site has a user registration look at the source code for the page and take note of the field names they use (most developers are lazy and use the same names for simplicity); you can also look around the site for more vulnerabilities. Alright so once you have found some good information to look forward to, its time to find out how many columns are being selected from the database from the original query. This is an important step because if number of columns you "select" and the number from the original are not identical, the injection does not work! To find out the number of column you simply add "order by x" on the end of your vulnerable url replacing "x"
with a increasing number until you get an error
the number of columns being selected is the value of x before the error.
0x04 - The Injection
I suppose this is where some people get confused. In MySQL in order to combine two query statements you can use the keyword "union", you can also include the keyword "all" which will dislay all results (default property of union is to remove duplicate results from display). After your "union all" you also need to inlcude the keyword "select" since we are going to want to select database information and display it on the screen so far you should be looking at something similar to:
Continueing the injection like the previous example will work fine, but it will also display all the original results as well as our new results, typically to bypass this I, as well as most of the other people exploiting sql injections, relace the id value, in the case of our example it would be 4, with one of the following:
or any result that would not be in the database, this way the original select query will not result anything but our new injected select query will display. In SQL each column being selected must be seperated by a comma(,) so if your vulnerable site is selecting 4 columns with the original statement (which was found earlier when we were gathering information using the "order by") you would just concatinate those on your injection; I like to set each column to a different numeric value that way i can keep track of which columns are actually being displayed on the screen. So far, if everything has been going ok, you should have an injection url looking something like:
If not then go back and keep reading it until you figure it out. The last part of our injection setup is the telling the query which table to "select" the information from; we do this with the keyword "from table"...pretty self explanitory right? So for example, we have a vulnerable site that has 4 columns being selected and we want to look at the "users" table we can have a set up such as:
Easy enough so far, now is where it gets a little more difficult, but not too much.
0x05 - Tables and Columns
Depending on the version of MySQL the administrators are running on the server, finding table and column names can be very easy or somewhat irritating. There is an easy way to figure out what version is running on the server, can you guess? If you did not guess version(), why the hell not, its like one of the easiest and self explanitory things ever! Anyways, replace one of the columns in your injection that displays on the screen with the function call version() and this will tell you which typically its either 4.x.x or 5.x.x. If they are running some form of version 4 then you're basically on your own when it comes to figuring out table and column names (i'll post some examples of common names later); though if version 5 is implemented then your life is easy. As of version 5.1 of MySQL the developers began to automatically include a master database on the server called INFORMATION_SCHEMA. Within information_schema there are tables that give information about all the tables, columns, users, etc on the entire sql server (to find more about the structure of information_schema and the table/column names visit http://dev.mysql.com/doc/refman/5.0/en/information-schema.html).
Once you figure out a table name and some column names within that table you want to look at just place them into our injection setup from before; suppose we have a site that has a "users" table and columns "user" and "pass" and the second and third columns are displayed onto the screen, we could view these by an injection such as:
This example will display both the user and pass onto the screen in the given positions, though what happens if only one column is selected or displayed? In MySQL there is function called concat() which simply concatinates fields together so to simplify our privious example we could have:
"0x3A" is just a colon(:) in hexidecimal, simply to seperate the two fields for my own viewing.
0x06 - Narrowing down the Selection
Typically when performing a SQL injection there are multiple results you want to look at or possibly just one individual. There are a couple of ways to narrow down your selection first way is to use the "where" keyword is just takes a logical parameter such as "where id=1" which would look in the id column in the table and find which row is equal to 1. The next way to to use the "limit" keyword; this way is a little more useful since you do not need to know an additional column name to increment through the selections limit takes two parameters, where to start the selection and how many to select. So in order to select only the very first "user" from the table "users" using the "limit" keyword you could have:
to look at the rest of the users individually you just increment the 0 up until you get an error. In order to look at all the results in a single swipe you can use the function group_concat() which works very similarly to concat() except it displays all the results for the given column(s) seperated by a comma(,) (the comma is just the default, you can change it by using the "separator" keyword and indicate
a symbol to use).
0x07 - Obstacles
Excluding the fact that version 4 in general is an obstacle, there are a few different things web developers can do to try and make sql injections a little more difficult. The most common of these annoyances would be magic_quotes; basically magic quotes disallows any type of quotation marks and breaks it by adding a back-slash(\), which of course is going to mess up your injection. To get around this there is the nice little function char(); char() takes ascii values and generates the corresponding character value, thus eliminating the need for a quote. Example time...say we want to look at the "pass" column FROM the table "users" but only WHERE the "user" column is only equal to "admin" and the site only selects one column from the original query, easy enough right? we learned this earlier
curve ball! the developers have enabled magic_quotes therefore your "admin" will not work properly...i know its sad. To fix it we simply take the ascii values of each character (http://crashoverron.t35.com/ascii.php) so now we get
TA-DA! injection fixed. Also another safety feature they try to block us with is regular expressions to search our input, but often times they have their expressions set to such narrow possibilities that you can bypass them by simply changing the case, the comment symbol, or replacing spaces with "+" (SQL is not case sensitive, it also sees "+" as a space filler much like a space).
0x08 - Additional opportunities
Although I said before version 4 was a pain in the ass, I have also noticed a nice feature common to version 4 vulnerable sites I have come across in my adventures; this feature would be the function load_file(), not saying the function is exclusive to version 4 but from my experience it is most commonly enabled for current users by developers for some reason in this version. load_file() acts just as file_get_contents() from PHP in that it returns the contents of the file into a string format. If enabled this allows for more than just SQL styles hacks on the server, it now allows for LFI vulnerabilities as well. Although, load_file() needs to have the exact full path to the file you are trying to open, for example: /home/CrashOverron/Desktop/file, and if input as a literal string then it must be encased in quotes, which brings back the issue of magic_quotes but as before just use the char() function. The next interesting feature that is hardly ever possible, but I have seen happen, is the use of the "INTO OUTFILE" keywords. This is the exact opposite of load_file(), in order to use either of these features the current user that MySQL is running as must have the FILE privilege on the server. Again, the full path is needed for the output file, which cannot be an existing file, though unlike load_file() the char() function does not fix magic_quotes. Time for an example of both, here is the situation: vulnerable site has 1 column selected also has a "users" table.
load_file with magic_quotes:
http://www.site.com/vulnerable.php?id=-1 union all select load_file(char(47,101,116,99,47,112,97,115,115,119,100))--
0x09 - Blind SQL Injection
Blind SQL injection occurs when the original select query obtains column information but does not display it onto the screen. In order to continue through a blind sql injection you must basically brute-force any value you want to know. There are a few functions we can use in conjuction with each other that make this quite easy yet tedious, those would be the mid() and the ascii() functions. mid() is MySQL's substring function and ascii() does the exact opposite of char() it takes a character and exchanges it with the corresponding ascii numeric value. Doing this allows us to determine the range each of our desired value is in on the ascii chart, thus narrowing each down until we find a match. Example situation; we have found a site that is vulnerable to blind sql injection and we want to figure out which user MySQL is currently running as, our injection sequence could look something like:
(this will tell us if the first letter in the user is above/below "a" then we can change the 97 to a different value until we find the
character to the first letter)
(just repeat as before and keep incrementing through the letters and you will eventually have the current user)
0x10 - Login Bypass
Ok, I left this for towards the end because it is not really very common anymore but I will through it in because I suppose you may run across it some day (I have only ran across this vulnerability once in real world). The concept behind the SQL login bypass is quite simple; in order to execute the exploit you input a username into the user field then in the password field of the form
this just ends the current password field and includes the logical OR with a constant true statement. A simple MySQL login script could look like:
$user = $_POST['user'];
$pass = $_POST['pass'];
$ref = $_SERVER['HTTP_REFERER'];
if((!$user) or (!$pass))
$conn = @mysql_connect("localhost", "root", "blah") or die("Could not connect");
$rs = @mysql_select_db("db", $conn) or die("db error");
$sql = "SELECT * FROM users WHERE user=\"$user\" AND pass=\"$pass\"";
$rs = mysql_query($sql, $conn) or die("query error");
$num = mysql_numrows($rs);
if($num != 0)
so if we input the user "admin" and "" or 1=1--" as the password the query sent to the server is going to look like this:
"SELECT * FROM users WHERE user="admin" AND pass="" or 1=1--"
so the server is going to select row where the "user" equals "admin" and disregard if the "pass" is correct because it is asking if the pass OR 1=1 are true, since 1=1 is always true you bypass the pass section.
0x11 - Useful Keywords/Functions
UNION ALL SELECT
Written by CrashOverron
Edited by Override on 01/30/2014 21:36:19