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A USB device is all it takes to steal credentials from locked PCs

Lucian Constantin | Sept. 9, 2016
Attackers can use rogue USB-to-Ethernet adapters to steal credentials from locked Windows, and possibly OS X, computers

Most users lock their computer screens when they temporarily step away from them. While this seems like a good security measure, it isn't good enough, a researcher demonstrated this week.

Rob Fuller, principal security engineer at R5 Industries, found out that all it takes to copy an OS account password hash from a locked Windows computer is to plug in a special USB device for a few seconds. The hash can later be cracked or used directly in some network attacks.

For his attack, Fuller used a flash-drive-size computer called USB Armory that costs $155, but the same attack can be pulled off with cheaper devices, like the Hak5 LAN Turtle, which costs $50.

The device needs to masquerade as an USB-to-Ethernet LAN adapter in such a way that it becomes the primary network interface on the target computer. This shouldn't be difficult because: 1) operating systems automatically start installing newly connected USB devices, including Ethernet cards, even when they are in a locked state and 2) they automatically configure wired or fast Ethernet cards as the default gateways.

For example, if an attacker plugs in a rogue USB-to-Gigabit-Ethernet adapter into a locked Windows laptop that normally uses a wireless connection, the adapter will get installed and will become the preferred network interface.

Furthermore, when a new network card gets installed, the OS configures it to automatically detect the network settings through the Dynamic Host Configuration Protocol (DHCP). This means that an attacker can have a rogue computer at the other end of the Ethernet cable that acts as a DHCP server. USB Armory is a computer on a stick that's powered via USB and can run Linux, so no separate machine is required.

Once an attacker controls a target computer's network settings via DHCP, he also controls DNS (Domain Name System) responses, can configure a rogue internet proxy through the WPAD (Web Proxy Autodiscovery) protocol and more. He essentially gains a privileged man-in-the-middle position that can be used to intercept and tamper with the computer's network traffic.

According to Fuller, computers in a locked state still generate network traffic, allowing for the account name and hashed password to be extracted. The time it takes for a rogue USB device to capture credentials from a system using this attack is around 13 seconds, he said.

He tested the attack successfully on Windows and OS X. However, he's still working on confirming if OS X is vulnerable by default or if it was his Mac's particular configuration that was vulnerable.

"First off, this is dead simple and shouldn’t work, but it does," the researcher said in a blog post. "Also, there is no possible way that I’m the first one who has identified this, but here it is."

 

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