LFI to DB and backdoor

I am decribing a high level, theortical way of converting LFI to DB compromise and creating a backdoor.
A long time ago, came across an application which was vulnerable to LFI (Local File Inclusion).

The LFI found was different from traditional one, since the filenmames were not being passed in any parameter, so was hard to detect also. A file called default.cgi contained code that which looks for a file in the directory and include it for some sort of processing, that was a business requirement, if not found, displayed 'File not found' error. So in order to exploit it one need to append a file name just after default.cgi?.  This way, could view any file from app server to OS level. Many sensitive files are easily accessible even from other folders too. /etc/passwd was accessible but /etc/shadow was not present, unfortunately. 

After browsing through almost all the files present at the server, came across a few interesting files (conn.cgi) which contained the logic of connecting to DB. 

The file also mentioned name of other files which needs to be downloaded in order to connect the file. It describes  about an encrypted db-password file which has to be decrypted. In order to decrypt the file, we needed to download a binary file from the server. The file also describes what command to be run in order to get the un-encrypted password file.

We accessed both the files, and saved the responses locally. Now we had to run the command mentioned. After a few hours of efforts we finally got the decrypted db password file, which contained passwords and other necessary details required for connecting the DB. Finally, using a client we were able to connect to the DB. 

Next steps were to insert a backdoor code in the table and write to a file using OUTFILE command, and save to one of the paths on the server so that we can fetch the files from the internet and run the command. Unfortunately, it didn't work for us. Were facing issue with using the OUTFILE command on the clients we had. But theoretically, it's possible, maybe with some other DB type. 

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