Cracking LM Hashes with Ophcrack – No GUI

Believe it or not, despite the fact it is 2016 I am still finding LanManager (LM) hashes on internal networks during penetration tests.

Although in my experience it is becoming more frequent that LM hashing has been disabled, and the hashes I am finding are for accounts that have not had their password changed since that time and therefore still have the password stored in this weakly protected format.

The LM hash format is weak because the maximum password length it can support is 14, password is uppercased, split into two 7 character chunks and then hashed separately. (Note this is not really accurate, but it is sufficient for this post. See here for an accurate description of the LM ‘hashing’ scheme.)

If you find (or are informed) that you have LM password hash storage, you should prevent Windows from storing a LM hash and change all account passwords the number of times required by the password history account option to completely purge the previous LM hashes.

I often use John the Ripper to crack a wide variety of hashes, however the weaknesses in the LM hash format have allowed Rainbow Tables (aka Lookup Tables) to be created which allow rapid recovery of the plain text password. Ophcrack an industry favourite tool to crack LM hashes using rainbow tables, I prefer to use it without the GUI in order to decrease the amount of resources it requires – in fact I have recently started running it on a server I have built for password cracking which does not have a GUI environment so command line usage is a must.

Since I pretty much always use the same options for Ophcrack I have created a simple bash function to which I can pass the pwdump file containing the hashes I need to crack. It is not pretty, but I have decided to share it in the hope that it will be of some use to others and my future self.

ophcracklm () {
 log=$(echo $1.log)
 outfile=$(echo $1.cracked)
 session=$(echo $1.ophcracklm_session)
 (set -x; ophcrack -g -v -u -n 7 -l $log -o $outfile -S $session -d /path/to/ophcrack_tables/ -t xp_free:xp_special -f $1)
}

This bash function will create log, output file, and session file names based on the hash file name passed on the command line, enable debugging mode in a sub shell of bash, and run ophcrack with the following options:

-g disable GUI
-v verbose output
-u display statistics when cracking ends
-n number of threads (I have this set to 7 for my machine, you may need to change it to suit)
-l log all output to the file name created based on the input file name
-o output cracked hashes, in the pwdump format, to the file name created based on the input file name
-S save progress of the search to the file name created based on the input filename
-d base directory containing the tables
-t tables to use separated by colons
-f the file to load the hashes from (I am passing the second command argument, the first contains the script name, the second is the first parameter)

Note that I am using bash’s debug output in order to echo the command that will be executed, and I am doing this in a subshell because it is automatically reverted.

---

As always, if you have any questions, comments or suggestions please feel free to get in touch.