Attack on Windows authentication mechanism
At the recent BlackHat Europe conference (November 10 – 13, Amsterdam) a security researcher called Ian Haken presented a very interesting, simple yet powerful attack allowing to bypass Windows (Kerberos) authentication on machines being part of a Domain.
The attack in itself allows someone – having physical access to the Windows workstation or laptop – to log into the system by resetting the password of a domain account. For that the attacker will just need to setup a rogue Domain Controller and configure it to declare the target account as expired. Indeed Haken identified a loophole in the mechanism which allows an attacker to force the local update of the cached Domain credentials despite the Kerberos KDC failing to prove it’s identity to the target workstation.
The whole attack, whose details are well explained in Ian Haken’s paper, can be setup using common open-source software and very simple configuration steps.
By itself, this would not be a significant breakthrough as other means of achieving similar results are widely known (e.g. by booting the machine on a live system and tampering with the system). This attack however becomes very interesting when applied to systems using Microsoft Bitlocker for full-disk encryption and configured for using the machine’s TPM (without PIN or USB key) in order to avoid explicit pre-boot authentication.
In this context, the encryption keys are automatically retrieved by the Windows bootloader from the TPM without any user (or attacker) input in order to provide a transparent Windows boot. In this scenario, the whole security of the (encrypted) data on the system falls back on the Windows login mechanism as any user being capable of logging into the system would (transparently) have access to all or part of the local data (depending on the user privileges).
Obviously in this context, Haken’s attacks turns out into a way of bypassing Bitlocker encryption in order to have access to the (encrypted) system and data.
A security bulletin (and corresponding patch) has been issued by Microsoft : MS15-122.
According to Microsoft, “the update addresses the bypass by adding an additional authentication check that will run prior to a password change.”
Microsoft’s security bulletin is rated as Important which does not correspond to the highest severity level. However, as the attack targets a common Bitlocker configuration (actually Microsoft does not generally recommend the use of pre-boot authentication) this attacks appears as very easy to implement against stolen or lost laptops with potentially critical consequences in terms of data confidentiality.
For that reason SCRT heavily recommends this patch to be applied as quickly as possible on affected systems, taking care not to neglect mobile systems that may be used by employees “on the field” or “on the road” and that may not be subject to frequent system updates (e.g. because they are often out of reach from standard company’s infrastructure and update workflow).