Prevent your systems from being hijacked: A quick guide

Thwart 'passing-the-hash' attacks by following these steps

It's time to rethink some old, and now outdated, security truisms that enable a very scary kind of attack.

As computing power and programming prowess have increased, so have the means to make this attack simple, effective and lightning-quick, to the point where it's now a huge risk. Tools are freely available to let someone own your entire Active Directory infrastructure in a matter of minutes, without brute-force cracking or any other compute-intensive resources.

The attack is not new: It's known as a "passing-the-hash" attack, and it's been around for years. Those proficient in security matters know of the attack and the general principle behind it: Passwords are converted by Windows into a "hash" -- a fixed-size string of cryptographically transformed data -- every time they are created. This way, your passwords aren't sitting in plain text anywhere on your machine, nor are they transmitted in clear text when the authentication happens over the wire.

When authentication does need to happen against one resource or another, Windows hashes the password you provide and then submits that hash to the host. The host then compares it with the hash it has on record; if it's the same, you're granted access; if not, you're turned away, of course.

Security flaws

But what if you were able to access the hash database for privileged accounts -- for, say, an administrator -- and pass that hash directly to a resource? Remember, Windows never transmits the passwords themselves. The only computation that is done is matching the transmitted hash to a hash that's stored in a database of valid authentication data. If you connect and pass an administrative username and an administrator's hash, that's all you need to gain privileged access to a system. You don't need to know the password; you just need the hash.

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