In order to fully actualize the end result, we will build out the main three files required, in order that they are used to circumvent the intended boot process and finally allow for the takeover of the target device.įollowing on from Part One and Part Two of this series we will further utilize buildroot to assist in the building of:Ģ. This allows an attack to “ live off the land” without the need to implement foreign toolsets. In essence, we will look to bypass the hardened firmware image and supplant one of our own.Ī practical use case for this type of attack is surprisingly common, as most vendors and/or companies utilize this type of approach for remote upgrades and hence the vector itself can typically fly under the radar using tools and mechanisms that are frequently available. The specific vector that we have chosen for this blog, is the utilization of the TFTP protocol to assist in circumventing the intended boot process. Casting your mind back to Part Two of the series, we remember that the U-Boot prompt has a vernacular/syntax of its own and it is this very syntax that we will be using to further our cause. Once access to the U-Boot prompt is gained we are then ready to forge further attacks. We used the U-Boot example in our case to help illustrate a typical case scenario. With the above in place, we are ready to kick off the proceedings! Breaking it downĪs we described in Part One and Two of the series, it is possible to interrupt the bootloader process in order to gain access to an intermediary stage within the boot process itself. ![]() Preload/Prebuilt Kernel, Filesystem, and Device Tree files A network segment that an attacker can co-opt, one of which is shared with the target deviceĤ. In order for this particular attack vector to be fully realized and enacted, a specific state of play is required to be in place.ģ. One main drawback of TFTP is the lack of retransmission in the event that packets are lost over a network segment. Based on this efficiency it is typically considered the file transfer protocol of choice. It is used for the transferring of files without the overhead one would expect with the use of a TCP-based protocol. This protocol uses UDP, meaning that it is stateless and typically runs over port 69. Its main usage is to retrieve or transmit a file between compatible devices. TFTP or Trivial File Transfer Protocol is a protocol whose use is quite ubiquitous due to its ease of use and great utility. We will look at how to first set up the attack device and then ultimately try our hand at gaining root access to our target device. ![]() ![]() As our attack vector, we will be looking at using TFTP to load a kernel and filesystem of our own onto the affected target. Once landed within the Das U-Boot prompt, an attacker is able to surge forward to ultimately take over the device that underlies it. In this post, we will be completing the loop on our three-part series by describing a specific attack vector that is available upon successful bypass of the bootloader process.
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