Thangrycat is caused by a series of hardware design flaws within Cisco's Trust Anchor module. First commercially introduced in 2013, Cisco Trust Anchor module (TAm) is a proprietary hardware security module used in a wide range of Cisco products, including enterprise routers, switches and firewalls. TAm is the root of trust that underpins all other Cisco security and trustworthy computing mechanisms in these devices. Thangrycat allows an attacker to make persistent modification to the Trust Anchor module via FPGA bitstream modification, thereby defeating the secure boot process and invalidating Cisco's chain of trust at its root. While the flaws are based in hardware, Thangrycat can be exploited remotely without any need for physical access. Since the flaws reside within the hardware design, it is unlikely that any software security patch will fully resolve the fundamental security vulnerability.
From a news article:
Thrangrycat is awful for two reasons. First, if a hacker exploits this weakness, they can do whatever they want to your routers. Second, the attack can happen remotely it's a software vulnerability. But the fix can only be applied at the hardware level. Like, physical router by physical router. In person. Yeesh.
That said, Thrangrycat only works once you have administrative access to the device. You need a two-step attack in order to get Thrangrycat working. Attack #1 gets you remote administrative access, Attack #2 is Thrangrycat. Attack #2 can't happen without Attack #1. Cisco can protect you from Attack #1 by sending out a software update. If your I.T. people have your systems well secured and are applying updates and patches consistently and you're not a regular target of nation-state actors, you're relatively safe from Attack #1, and therefore, pretty safe from Thrangrycat.
Unfortunately, Attack #1 is a garden variety vulnerability. Many systems don't even have administrative access configured correctly. There's opportunity for Thrangrycat to be exploited.
And from Boing Boing:
Thangrycat relies on attackers being able to run processes as the system's administrator, and Red Balloon, the security firm that disclosed the vulnerability, also revealed a defect that allows attackers to run code as admin.
It's tempting to dismiss the attack on the trusted computing module as a ho-hum flourish: after all, once an attacker has root on your system, all bets are off. But the promise of trusted computing is that computers will be able to detect and undo this kind of compromise, by using a separate, isolated computer to investigate and report on the state of the main system (Huang and Snowden call this an introspection engine). Once this system is compromised, it can be forced to give false reports on the state of the system: for example, it might report that its OS has been successfully updated to patch a vulnerability when really the update has just been thrown away.
As Charlie Warzel and Sarah Jeong discuss in the New York Times, this is an attack that can be executed remotely, but can only be detected by someone physically in the presence of the affected system (and only then after a very careful inspection, and there may still be no way to do anything about it apart from replacing the system or at least the compromised component).