Linux Privilege Escalation with Setuid and Nmap

I recently completed a CTF ‘boot to root’ style virtual machine from vulnhub.com and encountered an interesting privilege escalation technique that I thought I would share. Vulnhub is a lot of fun check it out for practice, I’ve recently had a lot of fun with online and offline virtual machine CTF’s and continue to learn a lot. To cut a long story short, I ended up landing a limited user shell on a Linux web server through a php script back to meterpreter. I dropped into a shell from meterpreter, the user had limited access. I was essentially logged in as the web server deamon user id. I started off going through the usual Linux enumeration, gathering some basic information, getting my bearings. A few tasks later on, I started to look at how I could escalate my privileges. I started  looking at files in the file system that could be executed as root. I was specifically looking for executable files where the setuid parameter was marked and where the owner was root. This essentially means when the program is executed it is executed in the permission of the owner of the file (where the EUID, the Effective User ID is root), in this case root. We would look for these types of file with the below find command:

Breaking the above command down:

find – the find command đŸ˜‰
/ – this is where we are looking in the file system, ie in the root.
-user root – looking for files with the owner of root.
-perm -4000 – looking for where the permissions of file are set to 4000
– exec ls -la – executing the commands ls -la. So we can see additional info.

In the list of files returned I came across the below:

I was able to abuse the above by executing nmap in the context of root. Nmap is set this way as it needs to run in the context of root to be able to perform its many functions. This version of Nmap has an interactive mode, where we are able to execute commands (you can already guess where this is going!). This can be called with ‘nmap –interactive’ which then gives us a special prompt ‘nmap>’. With this prompt we can then execute commands with an ! in front. Even drop into a shell with an EUID as root.

As we can see from the id output the ‘UID’ ie the ‘user id’ is ‘robot’ however the ‘EUID’ ie the ‘effective user id’ is root. Yay!

Well hopes this small snippet of info helps. Happy New Year to all!

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