SlideShare a Scribd company logo
Andy Brody      Greg Brockman    Siddarth Chandrasekaran
@alberge        @thegdb          @sidd
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      (__)
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Stripe makes it easy to start
accepting credit cards on the web
             today.

          Why a CTF?
Educational

Challenging
Fun!
Some
Numbers
Numbers: IPs at each level
Numbers: cumulative IPs / level
Numbers: cumulative IPs / level
Numbers: concurrent logins
CTF Security




Oh, UNIX has multiuser in its bones
 — this will be easy.
CTF Security


Support for anonymous users isn't great.
CTF Security




Services vulnerable to execution of
          arbitrary code!
Goal: per-user sandbox
Goal: per-user sandbox



- lightweight spin-up

- locked down environment

- blissful unawareness of other users
Implementation: chroot jail
Implementation: chroot jail

         User for each level

Debootstrap full install inside chroot

Separate filesystem for writable data

 No /proc, no setuid binaries in /bin
       Limited nodes in /dev
Implementation: chroot enforcement



chroot by user group with ssh

chroot with suPHP
Implementation: R/O FS


Great for security — even root can't modify
 without remounting.

Terrible for maintenance: can't make
 changes on the fly.
Implementation: R/O FS


Next time:

Mount the filesystem R/W elsewhere.
Bind mount it R/O inside the chroot.
Reality: imperfect isolation
Isolation: fork bombs

perl -e 'fork while fork'
Isolation: fork bombs
Causes


- script kiddies
- people trying to brute force level06
- process exhaustion from lots of users
Isolation: fork bombs
Mitigation


- cgroups
- ulimits
- killall -STOP …; killall -KILL …
      - by tty   - by pgid or sid
      - by user + process name
      - send CONT to innocent bystanders
Isolation: others
- disk exhaustion
- memory exhaustion
- greedy I/O
- level05 server
     Didn't want setuid for python
     Arbitrary code execution
     Cron job to kill & restart
Next time


make user accounts!


let built-in user isolation do the work


control level access with groups, setgid
Cloud supported
Cloud supported

Completely isolated from the rest of our servers


Outbound traffic open during spin-up, but firewalled off
 in production


Spin up capacity to handle unexpected load
Questions?

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