Challenge - Hello World

This functions as a basic example, of how to create a challenge for OCTP. This example will take base in a Docker based challenge, whereas other challenge types can be seen in coming soon.

Creating the Dockerfile

The most important aspects of this, is understanding the different components and how they function together.

A full reference of how a Dockerfile works, can be found here.

# What we base our challenge on, could be Debian, Ubuntu, etc.
# But using Alpine is good, since its footprint (size) is around 5.5MB
FROM alpine:3.9

# Can be omitted, basically just tells who made it
# and who maintains the Dockerfile
LABEL maintainer="eyJhb <[email protected]>"

# Current version of the challenge
# specified as MAYOR.MINOR(.PATCH)
ENV VERSION 1.0

# This will be used, to generate the 
# flag when the Docker container is started.
# This also tells that it supports dynamic flags.
# The param can be omitted, if there is not a possibility
# to change the flag at start
ENV FLAG CTF{1337HelloToYou!}

# Tell whichs ports needs to be exposed,
# before this challenge is sovleable (80/tcp).
# Will be used for mapping to another port.
EXPOSE 80

# Install nginx and make two directories.
RUN \
    apk add --no-cache nginx && \
    mkdir -p /run/nginx/ && \
    mkdir -p /var/www/html/

# Copy everything into the root of our container.
# Keep in mind! The structure of src follows that of
# the root of the container (e.g. /etc).
COPY src /

# Make this our entrypoint (what is run when the container starts)
ENTRYPOINT ["./init.sh"]

Reading the above Dockerfile, it should be fairly self explanatory how to make your own. The most important params, that will be used by OCTP are the following:

• VERSION - tells which version of the challenge this is
• FLAG - tells whether or not dynamic changing the flag is supported
• EXPOSE - which ports needs to be exposed, for this challenge to work (should almost always be at least one)

FLAG environment

If the contestants might be able to get a shell or read environment variables, then it is crucial that the FLAG environment gets safely deleted. This can be done using the following.

echo $FLAG > /flag.txt
if [ ! -z $FLAG ]; then
    unset FLAG
    exec sh /init.sh
fi 

Make sure that this is at the beginning of the file, right after #!/bin/sh, as any process creating in the init.sh script, will have the FLAG environment set. But what happens it that we put the flag where we want it, unset the FLAG environment variables and spawn a new shell which replaces our old shell, which calls the same init.sh file. By doing this, the FLAG environment is gone. If this is not there, the flag can still be read from /proc/1/environ.

Naming of challenges (subject to change)

All challenges should be prefixed with chal-, then a maximum of two tags e.g. chal-web-sqli., followed by a delimiter . and then lowercase name as e.g. chal-web-sqli.last-man-standing. Remember only the following chars are allowed for naming a challenge [a-z0-9.-].

Please file a issue if there are any questions or problems!