NX-OS is vulnerable. But it has been vulnerable to a great many exploits for a long time. I think its the remotely exploitable things, like the CDP attack I mention below that really are serious threats to our customers.
If you need access into the box to begin with, and can just execute commands or root the box, I personally don't find that to be a big deal, as if you have access to the physical box its just a hard disk anyways, so its game over.
For details on how to just access the filesystem once you have access to the disk image see my posts:
http://www.feeny.org/deconstructing-nx-os-part-1-exploding-kickstart/
http://www.feeny.org/deconstructing-nx-os-part-2-exploding-the-system-image/
I discovered at least three different ways to root NX-OS, I filed PSIRTs and they went ignored. The only way to actually get a manufacturer to patch something unfortunately, is to actually post the exploit. This counter-intuitive behavior was realized years ago, and has accelerated security patches ever since. My first exploit I shared with a a computer security firm that I was cooperating with at the time, they did a presentation on the vulnerability, without revealing enough code to fully exploit.
The last several NX-OS exploits I have found go unacknowledged. I have not tested this on the latest versions of NX-OS. I will not post the exploit, even internally, as it would just upset Cisco. Would be nice however if when you email a manufacturer an exploit if they actually acknowledged it, posted a PSIRT and then an appropriate patch.
Here is an example of the ease I have had in rooting NX-OS, which runs a modified "secure" version of Linux called MonteVista which is based on Hard Hat Linux:
I have so far been able to file 3 PSIRTs on NX-OS vulnerabilities, most of which really aren't security risks, but just show insecurity in the protection of the UNIX OS (which itself is a secure version of Linux "Hard Hat Linux"). One PSIRT involved oversized CDP packets and the ability to take down a production system, others were more or less UNIX root exploits.
The latest NX-OS looks like it fixed almost all of these, so I had to go looking deeper, and it seems there is no end to the amount one can discover.
Here is an example, enabled by the use of bash at the CLI, ability to write to the filesystem, ability to manipulate the environment variables, and then most importantly ability to execute a certain binary that does not fully qualify its pathnames, and thus the combination of events leads to a successful manipulation of the passwd file.
The exploit below is useless to anyone that would not know the very specific binary I am exploiting and means of execution.
[snip]