Greg Brockman

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System Design: Stripe Capture the Flag

We launched Stripe CTF 2.0 on Wednesday. Thus far we’ve had 14,000 signups, and over 500 people have captured the flag. Designing an architecture to handle this many users, all running their potentially malicious code on the level servers, was definitely a challenge, but it was also a ton of fun.

Our first and foremost design goal was pretty simple: don’t let people root the machines. It’s horrendously difficult to keep a shared Linux login machine secure, so the best you can do is apply all of the security countermeasures you can think of. Each level server is configured in the same way (maintaining multiple configurations is a great way to end up with an oversight due to increased complexity). All user-facing services run in a chroot with only /home, /tmp, /var/tmp, and /var/log writeable. This is implemented by mounting a filesystem (created using debootstrap) at /var/chroot and...

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