System Overlord

A blog about security engineering, research, and general hacking.

Useful ARM References

I started playing the excellent IOARM wargame on netgarage. No, don’t be expecting spoilers, hints, or walk-throughs, I’m not that kind of guy. This is merely a list of interesting reading I’ve discovered to help me understand the ARM architecture and ARM assembly.

GOT and PLT for pwning.

So, during the recent 0CTF, one of my teammates was asking me about RELRO and the GOT and the PLT and all of the ELF sections involved. I realized that though I knew the general concepts, I didn’t know as much as I should, so I did some research to find out some more. This is documenting the research (and hoping it’s useful for others).

BSidesSF 2017

BSidesSF 2017 was, by far, the best yet. I’ve been to the last 5 or so, and had a blast at almost every one. This year, I was super busy – gave a talk, ran a workshop, and I was one of the organizers for the BSidesSF CTF. I’ve posted the summary and slides for my talk and I’ll update the video link once it gets posted.

I think it’s important to thank the BSidesSF organizers – they did a phenomenal job with an even bigger venue and I think everyone loved it. It was clearly a success, and I can only imagine how much work it takes to plan something like this.

It’s also important to note that our perennial venue, DNA Lounge, (except that one year we don’t talk about) is having some money problems. Apparently you can’t spend more than you bring in each year. This is the venue that, in addition to hosting BSidesSF, also hosts Cyberdelia. This is a venue that allows all kinds of independent art and events to thrive in one of the most expensive cities in the country. I encourage you to reach out and go to a show, buy some pizza, or just donate to their Patreon. If my encouragement is not enough, how about some from Razor and Blade?

Again, big thanks to BSidesSF and DNA Lounge for such a successful event!

SANS Holiday Hack Challenge 2016

This is my second time playing the SANS holiday hack challenge. It was a lot of fun, and probably took me about 8-10 hours over a period of 2-3 days, not including this writeup.

New Tool: sshdog

I recently needed an encrypted, authenticated remote bind shell due to a situation where, believe it or not, the egress policies were stricter than ingress! Ideally I could forward traffic and copy files over the link.
I was looking for a good tool and casually asked my coworkers if they had any ideas when one said “sounds like SSH.”

Well, shit. That does sound like SSH and I didn’t even realize it. (Tunnel vision, and the value of bouncing ideas off of others.) But I had a few more requirements in total:

  • Encrypted
  • Authenticated
  • Bind (not reverse)
  • Windows & Linux
  • No Admin/Installation required
  • Can be shipped preconfigured
  • No special runtime requirements

At this point, I began hunting for SSH servers that fit the bill, but found none. So I began to think about Paramiko, the SSH library for Python, but then I’d still need the Python runtime (though there are ways to build a binary out of a python script). I then recalled once seeing that Go has an ssh package. I looked at it, hoping it would be as straightforward as Paramiko (which can become a full SSH server or client in about 10 lines), but it’s not quite so. With the Go package, all of the crypto is handled for you, but you need to handle the incoming channels and requests yourself. Fortunately, the package provides code for marshaling and unmarshaling messages from the SSH wire format.

I decided that I would get a better performance and more predictable behavior without needing to package the Python runtime, plus I appreciated the stability Go would provide (fewer runtime errors), so I began developing. What I ended up with is sshdog, and I’m releasing it today.

sshdog supports:

  • Windows & Linux
  • Configure port, host key, authorized keys
  • Pubkey authentication (no passwords)
  • Port forwarding
  • SCP (but no SFTP support)

Additionally, it’s capable of being installed as a service on Windows, and daemonizing on Linux. It uses go.rice to embed configuration within the resulting binary and give you a single executable that runs the server.

Example Usage

% go build .
% ssh-keygen -t rsa -b 2048 -N '' -f config/ssh_host_rsa_key
% echo 2222 > config/port
% cp ~/.ssh/ config/authorized_keys
% rice append --exec sshdog
% ./sshdog
[DEBUG] Adding hostkey file: ssh_host_rsa_key
[DEBUG] Adding authorized_keys.
[DEBUG] Listening on :2222
[DEBUG] Waiting for shutdown.
[DEBUG] select...

Why sshdog?

The name is supposed to be a riff off netcat and similar tools, as well as an anagram for “Go SSHD”.

Please, give it a try and feel free to file bugs/pull requests on the Github project.