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. The high level concept is like:

1. Create a php file with any backdoor code. Numerous examples are out there on Internet.
2. Now this code PHP code needs to be merged into the .jpeg file. There are a few command line tools available on Kali, which can do it. Suppose the resultant file is Happy.jpeg. This file contains the php backdoor code also. But the we can not execute the php commands inside the Happy.jpeg file since it's and jpeg extension which the php interpreter will not understand.
3. Now here the Null byte injection works. Change the name of Happy.jpeg to Happy.phpA.jpeg. We can choose 'A', 'B' any letter. Since we all are more familiar with hex of  'A' as 41, we use that for simplicity.
4. Now upload the file. Capture the request in the Burp, switch to the Hex view. Locate the file name in the string view. See the corresponding Hex table and replace the 41 ('A') with 00 (which is null byte).
5. Now the resultant string becomes Happy.php(space) .jpeg. Now since php interpreter uses C libraries internally, it's going to stop reading the file name post Happy.php and the file will be saved as Happy.php.
6. Now thus file can be accessed depending on the path of the upload directory along with the domain name. Eg, and all the commands can be executed.


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