Stripe CTF 2014, Level 2
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Level 2

Getting started

As in Level 0, you can run this level using test/harness and you can it submit for scoring using git push. Your goal is to modify the reverse proxy in shield to create a defensive proxy that mitigates floods of malicious traffic. When you submit your shield for scoring, we will run it using a copy of the network simulation code that is available in network_simulation/.

To run the provide code, you will need a Node.js installation. See:


The provided ./ will install the level's Node.js dependencies using the Node Package Manager (npm). Remember that this build script will be run on our scoring servers to build your submitted code, and you can modify it however you want.

Included files

  • ./shield and ./ You should modify these!
  • ./network_simulation/: a copy of the simulation code that we run on our servers. They are provided for your reference and for local simulations. We maintain our own copies of them for purposes of scoring. ** backend.js: the code for the fragile backend servers ** sword.js the code that simulates the malicious and legitimate traffic against the backends
  • ./network_simulation/lib/: some Node.js modules that are shared by the rest of the included code. Yours to modify (although you often won't need to).

The simulation and scoring

The layout of the simulated network

The network has three components: your proxy (shield), backends, and clients. The standard configuration, used for scoring and used by test/harness, is to have a backend listen on port 3001 and a second on 3002. Then shield connects to the two backends and listens on port 3000. Then sword.js is run, and it simulates a swarm of many clients (some legitimate and some malicious) connecting to the proxy. Although there are two backends in the scoring simulation, the stub code in shield does not perform any load balancing unless you modify it to do so.

Technical note: the simulation framework uses HEAD requests to check the upness of the shield and backends (it won't run until they're up). If you are doing a major re-write, you should preserve the current semantics around HEAD (see the sword.js source for more).

The scoring

In the simulated environment, there are a large number of legitimate clients making just a few requests each. There are also a small number of malicious clients making an enormous number of requests each. Think of these as mice and elephants: the goal of the level is to let the mice through while keeping the elephants out.

The scoring simulation runs for 20 seconds. During that period, you receive one point each time that you successfully proxy a response to a request that was made by a mouse. At the end of the 20 seconds, you lose points in proportion to the idleness of your backend boxes (i.e. if you don't have an opportunity to proxy a mouse request, it is better to proxy an elephant request than do nothing) [1]. The lowest official score is 0.01 (there are no negative scores).

When the requests are coming in they are not labeled as legitimate or malicious. However, the originating IP is identified by the 'X-Forwarded-For' header on each packet. You can determine, by watching the network whether an IP is malicious. A given IP is either always malicious or always legitimate

[1] The gory details of scoring follow: You get +1 point for proxying a response to a request that was made my a mouse. Then there's the issue of backend downtime. The backends working together at full capacity can process a theoretical maximum of N requests per simulation (the value of N depends on the simulation constants, which you can investigate in sword.js). If you instead handle M < N requests, you receive negative (N - M) / 8 points. It does not matter, for this part of the scoring, whether you are handling mice or elephants.