Cracking a Windows 7 Password

Windows 7 stores passwords using NTLM, an unsalted hash vulnerable to rainbow table attacks in addition to brute-force/dictionary attacks.

Dumping the hash

Boot into another drive with access to the target drive. Everything needed to dump the target drive resides in \Windows\System32\config, so copy this directory and move to a machine of your choice.

Dumping a Windows 7 hash is more involved than on El Capitan1, but ophcrack automates it:

$ sudo apt-get install ophcrack
$ ophcrack

You can provide ophcrack the config directory from the target machine and it will dump each user's password hash in plaintext (I have not found a way to dump the hash from the ophcrack command-line interface). With the hash in cleartext, we can now use the attack of our choice.

Rainbow Table Attack

Because the NTLM hash is unsalted, it is vulnerable to rainbow table attacks. The authors of ophcrack have made publicly available multiple terabytes of rainbow tables for use with various editions of Windows. These rainbow tables can be downloaded from their webpage2, extracted, and used:

$ mkdir tables
$ unzip -d tables/vista_free
$ ophcrack -g -d ./tables -t vista_free -f hash.txt

Dictionary Attack

The hash can also be attacked with a dictionary of possible passwords (or by brute-force) using password recovery tools such as John the Ripper or Hashcat1.


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