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7002 – Linux Security: Got Wurzel

02 Dec 2015
CTF: Hacking Lab
Link to challenge: https://www.hacking-lab.com
Date Completed: 02 December 2015

I log into the system to discover I am in a restricted shell.
Some testing reveals I can run commands like ls but cannot run cat etc.
I realise that I can use the /  character in arguments but not in the command name.
So I can’t call any commands by path.

Thus I can only execute commands in the local ./bin directory.
Lets see what is in the directory:

Okay so I can run these three commands. They behave as expected.

I look around the machine and notice a directory /home/restricted  which my user restricted1 also owns but this is simply a copy of the /home/restricted1  directory and thus a dead end.

I then realise that /home/restricted1/ping  is world writeable.
I can thus use tee and echo to write whatever I want to it.
Initially I wrote shell code to the program but this is an issue as it would not spawn a shell in POSIX mode but rather console mode.

Instead I use the flowing commands:

Then I run the ping program and a (regular) shell is spawned.
I am no longer in a restricted shell.

I set my ENV path so I don’t have to specify full paths to programs:

Now I need to escalate privileges from my regular shell.
I try to find world writeable files.

I run:

And keep the results in mind as I explore other things.

I check many things, one of them being the /etc/  directory for cron jobs.
I see that the  cron.minutely directory contains one program mtr that runs every minute but it is not world writeable.
I still check the file using:

It turns out that /usr/bin/mtr-check  is called as part of this cronjob.

I check my list from before to discover that this particular file happens to be world writeable!

So we can write any program and it will be executed by root as a cronjob.

We decide to write the following to the file (using echo and file redirection):

Essentially, we are copying /bin/sh  to our directory and setting the flag using chmod 4755 (so we setuid upon execution). So when we run rootbash, it should give us a root shell.

I go drink a cup of water (enough time for the minutely cronjob to run) and come back.
I then see my rootbash program!

I run it and a shell is spawned.

Then I read the file at /root/secret.txt  to capture the flag.

 

Security Questions:

1. Explain the security problem
The issue here is that I was able to break out of the restricted shell because the /home/restricted1/bin/ping  program was world writeable. If it was not, I wouldn’t be able to leave the restricted shell that way.

Now assume I someone managed to break out of the restricted shell. The second issue is that a cronjob executed by root called a program that was world writeable.

So the issues are simply permission issues.

2. Explain your attack, How you were able to get the /root/secret.txt file
Read above!
In summary, I spawned a shell using the world writeable ping program.
Then I spawned a root shell by executing a cronjob as root which put a copy of /bin/sh  in my directory with the s flag set. With the root shell, I read the secret file.

3. Add some proofing information to your solution (traces, exploit code, gold nugget)
All commands are above.

4. What do you recommend for protection?
Simply fix the permissions of the /home/restricted1/bin/ping  and /usr/bin/mtr-check  so they are not writeable!

 
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