Skip to main content

DoS with LIKE query

I was assessing an application. The application was properly sanitizing all the characters which have special meaning for SQL Injection attack. So SQL Injection was not possible in the application. But then again I came across few search modules in the application where it was taking input of part numbers to proceed.
I entered single quote(') and the application was perfectly filtering it returning "Parts can't be found". Then out of curiosity I entered '%' character and observed the response. Now the application stuck into the loop of the search continuously searching.The two things I deduced from it:
1. The application was using LIKE query to search matching terms.
2. This can be used to perform a DoS by overloading the database.

The % and _ qualifier is often overlooked by developers to filter as its not so devastating as other characters. They are used for matching 0 or more characters and single character respectively.

$searhterm = mysql_real_escape_string(“%anything”); // still %anything
mysql_query(“SELECT * FROM messages WHERE subject LIKE ‘{$searchterm}%’”);

The intention of the query above is to search the contents matching user specified $searchterm.
In normal conditions the query will execute fast. But when entered a term with a leading % quantifier the query takes too long to perform as it can't find the index. It progressively goes slower as amount of data in table grows.
Same is the case with _ (underscore).
Although these are valid inputs, we need to filter them out. in PHP there is a function which actually excludes the terms specified.
Use addcslashes() for escaping the above characters:

$searchterm = addcslashes(mysql_real_escape_string(“%anything_”), “%_”); // $searchtearm == \%something\_
mysql_query(“SELECT * FROM messages WHERE subject LIKE ‘{$sub}%’”);

Here, the input is processed by the database’s prescribed escape function and is then filtered
through addcslashes() to escape all occurrences of % and _.

In case of my application it can be determined by the fact that entering % one or more times was causing the whole application to be non responsive for a longer time

Reference: http://dev.mysql.com/

Comments

Popular posts from this blog

Ardilla- New tool for finding SQL Injection and XSS

Three Researchers -- MIT's Adam Kiezun , Stanford's Philip Guo , and Syracuse University's Karthick Jayaraman -- has developed a new tool ' Ardilla ' that automatically finds and exploits SQL injection and cross-site scripting vulnerabilities in Web applications. It creates inputs that pinpoint bugs in Web applications and then generates SQL injection and XSS attacks. But for now Ardilla is for PHP -based Web app only. The researchers say Ardilla found 68 never-before found vulnerabilities in five different PHP applications using the tool -- 23 SQL injection and 45 XSS flaws. More information is awaited. For their attack generation techniques refer to their document at: http://www.cs.washington.edu/homes/mernst/pubs/create-attacks-tr054.pdf

Combining power of Fiddler with Burp

Both are pretty powerful tools when it comes to intercept and modify http communications. But at some point of time, they become even more powerful combo if tied with each other. They complement each other. In a recent pentest I came across a similar situation where in Burp was not able to intercept a specific kind of traffic and Fiddler came to rescue. The application was designed to upload video. The initial communication was straight forward, I mean logging into application, filling up the video details etc. And all these were easily captured by Burp except the point where you hit the Upload Video and it connects to a different server and surprisingly it was not captured by Burp, not sure why, even after repeated attempts. So, I fired Fiddler to see if the it sees this request. But it's a;ways to play with requests using Burp due to it's various functionalities like, Intruder, Repeaters etc. But it was necessary to capture this request in Burp. So the below steps can be

File Upload through Null Byte Injection

Sometimes, during file upload we come across situation wherein there would be check on the file extension at the client side as well as server side too. If the application does allow only .jpeg extension to be uploaded, the client side java script checks for the extension of the file before passing the request. We all know that how easily this can be defeated. Some applications, checks for the extension at the server side also. That's not easy to bypass. However there are some ways with which it still can be bypassed. Most of server side scripts are written in high level languages such as Php, Java etc who still use some C/C++ libraries to read the file name and contents. That leads to the problem. In C/C++ a line ends with /00 or which is called Null Byte. So whenever the interpreter sees a null byte at the end of the a string, it stops reading thinking it has reached at the end of the string. This can be used for the bypass. It works for many servers, specially php servers. T