Getting Started in Offensive Security

Please note that this post, like all of those on my blog, represents only my views, and not those of my employer. Nothing in here implies official hiring policy or requirements.

I’m not going to pretend that this article is unique or has magic bullets to get you into the offensive security space. I also won’t pretend to speak for others in that space or in other areas of information security. It’s a big field, and it turns out that a lot of us have opinions about it. Mubix maintains a list of posts like this so you can see everyone’s opinions. I highly recommend the post “So You Want to Work in Security” by Parisa Tabriz for a view that’s not specific to offensive security. (Though there’s a lot of cross-over.)

My personal area of interest – some would even say expertise – is offensive application security, which includes activities like black box application testing, reverse engineering (but not, generally, malware reversing), penetration testing, and red teaming. I also do whitebox code review and various other things, but mostly I attack things using the same tools and techniques that an illicit attacker would. Of course, I do this in the interest of securing those systems and learning from the experience to help engineer stronger and more robust systems.

I do a lot of work with recruiting and outreach in our company, so I’ve had the chance to talk to many people about what I think makes a good offensive security engineer. After a few dozen times and much reflection, I decided to write out my thoughts on getting started. Don’t believe this is all you need, but it should help you get started.

A Strong Sense of Curiousity and a Desire to Learn

This isn’t a field or a speciality that you get into after a few courses and can stop there. To be successful, you’ll have to constantly keep learning. To keep learning like that, you have to want to keep learning. I spend a lot of my weekends and evenings playing with technology because I want to understand how it works (and consequently, how I can break it). There’s a lot of ways to learn things that are relevant to this field:

Everyone has a different learning style, you’ll have to learn what works for you. I learn best by doing (hands-on) and somewhat by reading. Videos are just inspiration for me to look more into something. Twitter and Reddit are the starting grounds to find all the other resources.

I see an innate passion for this field in most of the successful people I know. Many of us would do this even if we weren’t paid (and do some of it in our spare time anyway). You don’t have to spend every waking moment working, but you do have to keep moving forward or get left behind.

Understanding the Underlying System

To identify, understand, and exploit security vulnerabilities, you have to understand the underlying system. I’ve seen “penetration testers” who don’t know that paths on Linux/Unix systems start with and use / as the path separator. Watching someone try to exploit a potential LFI with \etc\passwd is just painful. (Hint: it doesn’t work.)

If you’re attacking web applications, you should at least have some understanding of:

For non-web networked applications:

You don’t have to know everything about every layer, but each item you don’t know is either something you’ll potentially miss, or something that will cost you time. You’ll learn more as you develop your skills, but there’s some fundamentals that will help you get started:

You don’t have to have a full CS degree (but it certainly wouldn’t hurt), but if you don’t understand how developers do their work, you’ll have a much harder time looking for and exploiting vulnerabilities. Many of the best penetration testers and security researchers have had experience as network administrators, systems administrators, or developers – this experience is incredibly useful in understanding the underlying systems.

The CIA Triad

To understand security at all, you should understand the CIA triad. This has nothing to do with the American intelligence agency, but everything to do with 3 pillars of information security: Confidentiality, Integrity, and Availability.

Confidentiality refers to allowing only authorized access to data. For example, preventing access to someone else’s email falls into confidentiality. This idea has strong parallels to the notion of privacy. Encryption is often used (and misused) in the pursuit of confidentiality. Heartbleed is an example of a well-known bug affecting confidentiality.

Integrity refers to allowing only authorized changes to state. This can be the state of data (avoiding file tampering), the state of execution (avoiding remote code execution), or some combination. Most of the “exciting” vulnerabilities in information security impact integrity. GHOST is an example of a well-known bug affecting integrity.

Availability is, perhaps, the easiest concept to understand. This refers to the ability of a service to be access by legitimate users when they want to access it. (And probably also as the speed they’d like.)

These 3 concepts are the main areas of concern for security engineers.

Understanding Vulnerabilities

There are many ways to categorize vulnerabilities, so I won’t try to list them all, but find some and understand how they work. The OWASP Top 10 is a good start for web vulnerabilities. The Modern Binary Exploitation course from RPISEC is a good choice for understanding “Binary Exploitation”.

It’s really valuable to distinguish a bug from a vulnerability. Most vulnerabilities are bugs, most bugs are not vulnerabilities. Bugs are accidentally-introduced misbehavior in software. Vulnerabilities are ways to gain access to a higher (or different) privilege level in an unintended fashion. Generally, a bug must violate one of the 3 pillars of the CIA triad to be classified as a vulnerability. (Though this is often subjective, see [systemd bug].)

Doing Security

At some point, it stops being about what you know and starts being about what you can do. Knowing things is useful in being able to do, but merely reciting facts is not very useful in actual offensive security. Getting hands-on experience is critical, and this is one field where you need to be careful how to do it. Please remember that, however you choose to practice, you should stay legal and observe all applicable laws.

There’s a number of different options here that build relevant skills:

Of these, lab work is the most relevant to me, but also the one requiring the most time investment to setup. Typically, a lab will involve setting up one or more practices machines with known-vulnerable software (though feel free to progress to finding unknown issues). I’ll have a follow-up post with information on building an offensive security practice lab.

Bug bounties are a good option, but to a beginner, they’ll be very daunting because much of the low-hanging fruit will be gone, and there should be no known vulnerabilities to practice on. Getting into bug bounties without any prior experience at all is likely to only teach frustration and anger.


There are some suggested resources for getting started in Offensive Security. I’ll try to maintain them if I receive suggestions from other members of the community.

Web Resources (Reading/Watching)



Lab Resources

I’ll have a follow-up about building a lab soon, but there’s some things worth looking at here:


This isn’t an especially easy field to get started in, but it’s the challenge that keeps most of us into it. I know I need to constantly be pushing the edge of my capabilities and of technology for it to stay satisfying. Good luck, and maybe you’ll soon be the author of one of the great resources in our community.

If you have other tips/resources that you think should have been included here, drop me a line or reach me on Twitter.