AWS Security Blog

AWS Security Profile: Tom Scholl, VP and Distinguished Engineer, AWS

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In the AWS Security Profile series, we feature the people who work in Amazon Web Services (AWS) Security and help keep our customers safe and secure. This interview is with Tom Scholl, VP and Distinguished Engineer for AWS.

What do you do in your current role and how long have you been at AWS?

I’m currently a vice president and distinguished engineer in the infrastructure organization at AWS. My role includes working on the AWS global network backbone, as well as focusing on denial-of-service detection and mitigation systems. I’ve been with AWS for over 12 years.

What initially got you interested in networking and how did that lead you to the anti-abuse and distributed denial of service (DDoS) space?

My interest in large global network infrastructure started when I was a teenager in the 1990s. I remember reading a magazine at the time that cataloged all the large IP transit providers on the internet, complete with network topology maps and sizes of links. It inspired me to want to work on the engineering teams that supported that. Over time, I was fortunate enough to move from working at a small ISP to a telecom carrier where I was able to work on their POP and backbone designs. It was there that I learned about the internet peering ecosystem and started collaborating with network operators from around the globe.

For the last 20-plus years, DDoS was always something I had to deal with to some extent. Namely, from the networking lens of preventing network congestion through traffic-engineering and capacity planning, as well as supporting the integration of DDoS traffic scrubbers into network infrastructure.

About three years ago, I became especially intrigued by the network abuse and DDoS space after using AWS network telemetry to observe the size of malicious events in the wild. I started to be interested in how mitigation could be improved, and how to break down the problem into smaller pieces to better understand the true sources of attack traffic. Instead of merely being an observer, I wanted to be a part of the solution and make it better. This required me to immerse myself into the domain, both from the perspective of learning the technical details and by getting hands-on and understanding the DDoS industry and environment as a whole. Part of how I did this was by engaging with my peers in the industry at other companies and absorbing years of knowledge from them.

How do you explain your job to your non-technical friends and family?

I try to explain both areas that I work on. First, that I help build the global network infrastructure that connects AWS and its customers to the rest of the world. I explain that for a home user to reach popular destinations hosted on AWS, data has to traverse a series of networks and physical cables that are interconnected so that the user’s home computer or mobile phone can send packets to another part of the world in less than a second. All that requires coordination with external networks, which have their own practices and policies on how they handle traffic. AWS has to navigate that complexity and build and operate our infrastructure with customer availability and security in mind. Second, when it comes to DDoS and network abuse, I explain that there are bad actors on the internet that use DDoS to cause impairment for a variety of reasons. It could be someone wanting to disrupt online gaming, video conferencing, or regular business operations for any given website or company. I work to prevent those events from causing any sort of impairment and trace back the source to disrupt that infrastructure launching them to prevent it from being effective in the future.

Recently, you were awarded the J.D. Falk Award by the Messaging Malware Mobile Anti-Abuse Working Group (M3AAWG) for IP Spoofing Mitigation. Congratulations! Please tell us more about the efforts that led to this.

Basically, there are three main types of DDoS attacks we observe: botnet-based unicast floods, spoofed amplification/reflection attacks, and proxy-driven HTTP request floods. The amplification/reflection aspect is interesting because it requires DDoS infrastructure providers to acquire compute resources behind providers that permit IP spoofing. IP spoofing itself has a long history on the internet, with a request for comment/best current practice (RFC/BCP) first written back in 2000 recommending that providers prevent this from occurring. However, adoption of this practice is still spotty on the internet.

At NANOG76, there was a proposal that these sorts of spoofed attacks could be traced by network operators in the path of the pre-amplification/reflection traffic (before it bounced off the reflectors). I personally started getting involved in this effort about two years ago. AWS operates a large global network and has network telemetry data that would help me identify pre-amplification/reflection traffic entering our network. This would allow me to triangulate the source network generating this. I then started engaging various networks directly that we connect to and provided them timestamps, spoofed source IP addresses, and specific protocols and ports involved with the traffic, hoping they could use their network telemetry to identify the customer generating it. From there, they’d engage with their customer to get the source shutdown or, failing that, implement packet filters on their customer to prevent spoofing.

Initially, only a few networks were capable of doing this well. This meant I had to spend a fair amount of energy in educating various networks around the globe on what spoofed traffic is, how to use their network telemetry to find it, and how to handle it. This was the most complicated and challenging part because this wasn’t on the radar of many networks out there. Up to this time, frontline network operations and abuse teams at various networks, including some very large ones, were not proficient in dealing with this.

The education I did included a variety of engagements, including sharing drawings with the day-in-the-life of a spoofed packet in a reflection attack, providing instructions on how to use their network telemetry tools, connecting them with their network telemetry vendors to help them, and even going so far as using other more exotic methods to identify which of their customers were spoofing and pointing out who they needed to analyze more deeply. In the end, it’s about getting other networks to be responsive and take action and, in the best cases, find spoofing on their own and act upon it.

Incredible! How did it feel accepting the award at the M3AAWG General Meeting in Brooklyn?

It was an honor to accept it and see some acknowledgement for the behind-the-scenes work that goes on to make the internet a better place.

What’s next for you in your work to suppress IP spoofing?

Continue tracing exercises and engaging with external providers. In particular, some of the network providers that experience challenges in dealing with spoofing and how we can improve their operations. Also, determining more effective ways to educate the hosting providers where IP spoofing is a common issue and making them implement proper default controls to not allow this behavior. Another aspect is being a force multiplier to enable others to spread the word and be part of the education process.

Looking ahead, what are some of your other goals for improving users’ online experiences and security?

Continually focusing on improving our DDoS defense strategies and working with customers to build tailored solutions that address some of their unique requirements. Across AWS, we have many services that are architected in different ways, so a key part of this is how do we raise the bar from a DDoS defense perspective across each of them. AWS customers also have their own unique architecture and protocols that can require developing new solutions to address their specific needs. On the disruption front, we will continue to focus on disrupting DDoS-as-a-service provider infrastructure beyond disrupting spoofing to disrupting botnets and the infrastructure associated with HTTP request floods.

With HTTP request floods being much more popular than byte-heavy and packet-heavy threat methods, it’s important to highlight the risks open proxies on the internet pose. Some of this emphasizes why there need to be some defaults in software packages to prevent misuse, in addition to network operators proactively identifying open proxies and taking appropriate action. Hosting providers should also recognize when their customer resources are communicating with large fleets of proxies and consider taking appropriate mitigations.

What are the most critical skills you would advise people need to be successful in network security?

I’m a huge proponent of being hands-on and diving into problems to truly understand how things are operating. Putting yourself outside your comfort zone, diving deep into the data to understand something, and translating that into outcomes and actions is something I highly encourage. After you immerse yourself in a particular domain, you can be much more effective at developing strategies and rapid prototyping to move forward. You can make incremental progress with small actions. You don’t have to wait for the perfect and complete solution to make some progress. I also encourage collaboration with others because there is incredible value in seeking out diverse opinions. There are resources out there to engage with, provided you’re willing to put in the work to learn and determine how you want to give back. The best people I’ve worked with don’t do it for public attention, blog posts, or social media status. They work in the background and don’t expect anything in return. They do it because of their desire to protect their customers and, where possible, the internet at large.

Lastly, if you had to pick an industry outside of security for your career, what would you be doing?

I’m over the maximum age allowed to start as an air traffic controller, so I suppose an air transport pilot or a locomotive engineer would be pretty neat.

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Tom Scholl

Tom Scholl

Tom is Vice President and Distinguished Engineer at AWS.

Amanda Mahoney

Amanda Mahoney

Amanda is the public relations lead for the AWS portfolio of security, identity, and compliance services. She joined AWS in September 2022.