BSides SF CTF Author Writeup: Flagsrv

Flagsrv was a 300 point web challenge in this year’s BSidesSF CTF. The description was a simple one:

We’ve built a service for the sole purpose of serving up flags!

The account you want is named ‘flag’.


BSides SF CTF Author Writeup: Cloud2Clown

The Challenge

Sometimes you see marketing materials that use the word cloud to the point that it starts to lose all meaning. This service allows you to fix that with clowns instead of clouds. Note: there are 2 flags, they should be clearly labeled.


Understanding Shellcode: The Reverse Shell

A recent conversation with a coworker inspired me to start putting together a series of blog posts to examine what it is that shellcode does. In the first installment, I’ll dissect the basic reverse shell.

First, a couple of reminders: shellcode is the machine code that is injected into the flow of a program as the result of an exploit. It generally must be position independent as you can’t usually control where it will be loaded in memory. A reverse shell initiates a TCP connection from the compromised host back to a host under the control of the attacker. It then launches a shell with which the attacker can interact.


Course Review: Adversarial Attacks and Hunt Teaming

At DerbyCon 8, I had the opportunity to take the “Adversarial Attacks and Hunt Teaming” presented by Ben Ten and Larry Spohn from TrustedSec. I went into the course hoping to get a refresher on the latest techniques for Windows domains (I do mostly Linux, IoT & Web Apps at work) as well as to get a better understanding of how hunt teaming is done. (As a Red Teamer, I feel understanding the work done by the blue team is critical to better success and reducing detection.)


Course Review: Software Defined Radio with HackRF

Over the past two days, I had the opportunity to attend Michael Ossman’s course “Software Defined Radio with HackRF” at Toorcon XX. This is a course I’ve wanted to take for several years, and I’m extremely happy that I finally had the chance. I wanted to write up a short review for others considering taking the course.

Course Material

The material in the course focuses predominantly on the basics of Software Defined Radio and Digital Signal Processing. This includes the math necessary to understand how the DSP handles the signal. The math is presented in a practical, rather than academic, way. It’s not a math class, but a review of the necessary basics, mostly of complex mathematics and a bit of trigonometry. (My high school teachers are now vindicated. I did use that math again.) You don’t need the math background coming in, but you do need to be prepared to think about math during the class. Extracting meaningful information from the ether is, it turns out, an exercise in mathematics.

There’s a lot of discussions of frequencies, frequency mixers, and how frequency, amplitude, and phase are related. Also, despite more than 20 years as an amateur radio operator, I finally understand dB properly. It’s possible to understand reasonably without having to do logarithms:

  • +3db = x2
  • +10db = x10
  • -3db = 1/2
  • -10db = 1/10

In terms of DSP, he demonstrated extracting signals of interest, clock recovery, and other techniques necessary for understanding digital signals. It really just scratches the surface, but is enough to get a basic signal understood.

From a security point of view, there was only a single system that we “attacked” in the class. I was hoping for a little bit more of this, but given the detail in the other content, I am not disappointed.

Mike pointed out that the course primarily focuses on getting signals from the air to a digital series of 0 an 1 bits, and then leaves the remainder to tools like python for adding meaning and interpretation of the bits. While I understand this (and, admittedly, at that point it’s similar to decoding an unknown network protocol), I would still like to have gone into more detail.

Course Style

At the very beginning of the course, Mike makes it clear that no two classes he teaches are exactly the same. He adapts the course to the experience and background of each class, and that was very evident from our small group this week. With such a small class, it became more like a guided conversation than a formal class.

Overall, the course was very interactive, with lots of student questions, as well as “Socratic Method” questions from the instructor. This was punctuated with a number of hands-on exercises. One of the best parts of the hands-on exercises is that Mike provides a flash drive with a preconfigured Ubuntu Linux installation containing all the tools that are needed for the course. This allows students to boot into a working environment, rather than having to play around with tool installation or virtual machine settings. (We were, in fact, warned that VMs often do not play well with SDR, because the USB forwarding has overhead resulting in lost samples.)

Mike made heavy use of the poster pad in the room, diagramming waveforms and information about the processes involved in the SDR architecture and the DSP done in the computer. This works well because he customizes the diagrams to explain each part and answer student questions. It also feels much more engaging than just pointing at slides. In fact, the only thing displayed on the projector is Mike’s live screen from his laptop, displaying things like the work he’s doing in GNURadio Companion and other pieces of software.

If you have devices you’re interested in studying, you should bring them along with you. If time permits, Mike tries to work these devices into the analysis during the course.

Tools Used

Additional Resources

Opinions & Conclusion

This was a great class that I really enjoyed. However, I really wish there had been more emphasis on how you decode and interpret the unknown signals, such as discussion of common packet types over RF, any tools for signals analysis that could be built either in Python or in GNURadio. Perhaps he (or someone) could offer an advanced class that focuses on the signal analysis, interpretation, and “spoofing” portions of the problem of attacking RF-based systems.

If you’re interested in doing assessments of physical devices, or into radio at all, I highly recommend this course. Mike obviously really knows the material, and getting a HackRF One is a pretty nice bonus. Watching the videos on his website will help you prepare for the math, but will also result int a good portion of the content being duplicated in the course. I’m not disappointed that I did that, and I still feel that I more than made good use of the time in the course, but it is something to be aware of.