Companies relying on Microsoft BitLocker to encrypt the drives of their employees' computers should install the latest Windows patches immediately. A researcher disclosed a trivial Windows authentication bypass, fixed earlier this week, that puts data on BitLocker-encrypted drives at risk.
Ian Haken, a researcher with software security testing firm Synopsys, demonstrated the attack Friday at the Black Hat Europe security conference in Amsterdam. The issue affects Windows computers that are part of a domain, a common configuration on enterprise networks.
When domain-based authentication is used on Windows, the user's password is checked against a computer that serves as domain controller. However, in situations when, for example, a laptop is taken outside of the network and the domain controller cannot be reached, authentication relies on a local credentials cache on the machine.
In order to prevent an attacker from connecting a stolen, lost or unattended laptop to a different network and creating a spoofed domain controller that accepts another password to unlock it, the authentication protocol also verifies that the machine itself is registered on the domain controller using a separate machine password.
This additional check doesn't happen when the controller cannot be reached, because the protocol developers assumed that the attacker can't change the user password stored in the local cache. However, Haken figured out a way to do it -- and it only takes a few seconds if automated.
First, the attacker sets up a mock domain controller with the same name as the one the laptop is supposed to connect to. He then creates the same user account on the controller as on the laptop and creates a password for it with a creation date far in the past.
When authentication is attempted with the attacker's password on the laptop, the domain controller will inform Windows that the password has expired and the user will automatically be prompted to change it. This happens before verifying that the machine is also registered on the controller.
At this point the attacker will have the ability to create a new password on the laptop, which will replace the original one in the local credentials cache.
Logging in while connected to the rogue domain controller would still fail, because the controller does not have the machine password. However, the attacker could disconnect the laptop from the network in order to force a fallback to local authentication, which will now succeed because only the user password is verified against the cache.
This is a logic flaw that has been in the authentication protocol since Windows 2000, the researcher said. However, physical access did not used to be part of the Windows threat model, because in such a situation an attacker could boot from an alternative source, like a live Linux CD to access to the data anyway.
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