kCTF VRP Setup

kCTF is a Kubernetes-based infrastructure for CTF competitions

kCTF VRP Setup

We invite you to demonstrate an exploit against our kCTF demo cluster based on the challenges presented on this page. Successful demonstrations are eligible for rewards between 5,000 to 10,000 USD as defined in https://g.co/vrp.

kCTF is an open source infrastructure for CTF competitions. You can find details on how it works in the kCTF documentation, but in short, it’s running on a hardened Kubernetes cluster with the following security features:

At present, we’re interested in two attack scenarios against this infrastructure:

  1. Breaking out of the nsjail sandbox as it would allow solving challenges in unintended ways.
  2. Breaking the isolation that Kubernetes provides and accessing the flags of other challenges.

For this purpose, we set up two kCTF challenges with secret flags: “kctf” and “full-chain”. You can demonstrate a working exploit by leaking the flags of either of these. You can find the code for the challenges here.

drawing showing the location of the flags

kctf challenge

The “kctf” challenge is the only entry point to the cluster. You can connect to it via:

socat FILE:`tty`,raw,echo=0 TCP:kctf.vrp.ctfcompetition.com:1337

It will ask you to solve a proof-of-work and then gives you access to a bash running in a setup similar to the kCTF pwn template challenge. The only difference is that the flag is not accessible inside of the nsjail sandbox and you will need to break out of the chroot in order to read it.

full-chain challenge

The “full-chain” challenge is a challenge that runs a while sleep loop and doesn’t have any exposed ports. In order to get access to the flag, you will need to break out of the “kctf” challenge and break the pod isolation of the cluster.


The flags are stored in Kubernetes secrets and mounted to the filesystem of the two challenges at “/flag/flag”. They are of the format:


As you can see, the flags include a timestamp and are rotated frequently. You can send us the flag here before you are ready to disclose the exploit as we can use it to resolve the timing in case of an exploit collision (we will reward whoever was the first to obtain and record the submission of a flag). Otherwise, once the vulnerability is fixed, please contact us at g.co/vulnz.


We want to encourage the community to help research vulnerabilities such as those found by Syzkaller, but which are still unfixed since they have not been shown to be exploitable. As such:

In case of questions or suggestions, you can reach us at kctf@google.com