Presentation Material
Abstract
First commercially introduced in 2013, Cisco Trust Anchor module(TAm) is a proprietary hardware security module that is used in a wide range of Cisco products, including enterprise routers, switches and firewalls. TAm is the foundational root of trust that underpins all other Cisco security and trustworthy computing mechanisms in such devices. We disclose two 0-day vulnerabilities and show a remotely exploitable attack chain that reliably bypasses Cisco Trust Anchor. We present an in-depth analysis of the TAm, from both theoretical and applied perspectives. We present a series of architectural and practical flaws of TAm, describe theoretical methods of attack against such flaws. Next, we enumerate limitations in current state-of-the-art offensive capabilities that made the design of TAm seem secure.
Using Cisco 1001-X series of Trust Anchor enabled routers as a demonstrative platform, we present a detailed analysis of a current implementation of TAm, including results obtained through hardware reverse engineering, Trust Anchor FPGA bitstream analysis, and the reverse engineering of numerous Cisco trustworthy computing mechanisms that depend on TAm. Finally, we present two 0-day vulnerabilities within Cisco IOS and TAm and demonstrate a remotely exploitable attack chain that results in persistent compromise of an up-to-date Cisco router. We discuss the implementation of our TAm bypass, which involves novel methods of reliably manipulating FPGA functionality through bitstream analysis and modification while circumventing the need to perform RTL reconstruction. The use of our methods of manipulation creates numerous possibilities in the exploitation of embedded systems that use FPGAs. While this presentation focuses on the use of our FPGA manipulation techniques in the context of Cisco Trust Anchor, we briefly discuss other uses of our bitstream modification techniques.
AI Generated Summary
This talk details a three-year research project reverse-engineering the secure boot process of Cisco routers, focusing on a hardware “trust anchor” implemented using a Xilinx Spartan-6 FPGA. The primary objective was to manipulate the FPGA’s configuration bitstream to bypass secure boot and run custom firmware on devices like the ASR 1001-X.
Key findings revealed that the FPGA, not the main x86 processor, enforced a strict 100-second reset cycle upon detecting any modification to the validated boot chain (UEFI, ROMmon, Linux kernel). Through electromagnetic emanation analysis, researchers identified the FPGA’s activity pattern and its role in reading bitstreams from SPI flash. A critical discovery was a Cisco patent describing secure boot via an external immutable device, confirming the FPGA’s function as the root of trust.
The core technical contribution is a novel attack methodology that does not require full bitstream reversal. Instead, researchers developed techniques to deterministically manipulate specific FPGA I/O pins connected to the reset logic. By physically intercepting and controlling these pins, they could disable the FPGA’s reset enforcement without understanding the entire proprietary bitstream format. This was facilitated by an automated testing framework (“Brian”) and low-cost hardware modifications, such as using a resistor to hold a reset pin high.
The vulnerability, named with emojis (tengu cat), affected hundreds of Cisco device models. Its practical implication extends far beyond networking equipment; any system using similar SRAM-based FPGAs for secure boot or critical control functions is potentially vulnerable. The work demonstrates that FPGA-based hardware roots of trust can be compromised through targeted pin manipulation, challenging the assumption of their immutability and highlighting a significant attack surface in embedded and infrastructure systems. Tools and libraries from the research were released publicly.