Last Update: Wed Jul 19 15:02:32 PDT 2006

RFuzz HTTP Destroyer

RFuzz is the start of a Ruby based HTTP thrasher, destroyer, fuzzer, and client based on the Mongrel project’s HTTP parser and the statistical analysis of being very mean to a web server.

At the moment is has a working and fairly extensive HTTP 1.1 client and some basic statistics math borrowed from the Mongrel project.

RubyForge Project

The project is hosted at:

Where you can file bugs and other things, as well as download gems manually.


The motivation for RFuzz comes from little scripts I’ve written during Mongrel development to "fuzz" or attack the Mongrel code.

RFuzz will simply use the built-in ultra-correct HTTP client and a Ruby DSL to let you write scripts that exploit servers, thrash them with random data, or simply run simple test suites.

It may also perform analysis of performance data and work as a simply load or pen testing tool. This is only a secondary goal though since there’s plenty of good tools for that.


You can install RFuzz by simply using RubyGems:

  sudo gem install rfuzz

It doesn’t support windows unless you have build tools that can compile modules against Ruby. No, you don’t get this with Ruby One Click.

RFuzz HTTP Client

It also comes from not being satisfied with the stock net/http library. While this library is good for high-level HTTP access to resources, it is much too abstract and protective to be used in a fuzzing tool.

In a tool such as RFuzz you need to have the following features in an HTTP client library:

  1. No protection from exceptions to analyze exactly what’s happening.
  2. Ability to "throttle" the client to simulate different kinds of request loads.
  3. No threading or additional overhead to test the impact of threads, but thread safe.
  4. Ability to encode the majority of the request as data elements for loading.
  5. Fast and exact HTTP parser to validate the server’s response is correct.
  6. Tracks cookies between requests to keep session data going.

RFuzz::HttpClient supports all of these features already, with cookies being the weakest right now.

Using The Client

The client is designed that you create an RFuzz::HttpClient object once with all the common parameters and the host you want to talk with, and then you call a series of methods on the client object that match the HTTP methods GET, POST, PUT, DELETE, and HEAD. You can add more methods if you like (see the documentation).

Here’s a simple example:

  require 'rfuzz/client'

  cl ="", 80, :query => {"q" => "zed shaw"})

  resp = cl.get("/search")
  => ["<html><head><meta HTTP-EQUIV=\"content-type\" CONTENT=\"text/html;
       charset=ISO-8859-1\"><title>zed shaw - Google Search</title><style><!--\n"]

  resp = cl.get("/search", :query => {"q" => "frank"})
  => ["<html><head><meta HTTP-EQUIV=\"content-type\" CONTENT=\"text/html;
      charset=ISO-8859-1\"><title>frank - Google Search</title><style><!--\n"]

Notice that we made a client that actually had a default :query to just search for my name (Zed Shaw) and then we only had to cl.get("/search"). In the second query though we just set :query to something else (a search for "frank") and it automatically overrides the parameters. This makes it possible to set common parameters, cookies, and headers in blocks of requests to reduce repetition.

Client Limitations

The client handles chunked encoding inside the parser but the code for it is still quite nasty. I’ll be attacking that and cleaning it up very soon. Even with this it’s able to efficiently parse chunked encodings without many problems (but could be better).

It can’t also parse cookies properly yet, so the above example kind of works, but the cookie isn’t returned right.

Randomness Generator

RFuzz features a RandomGenerator class that uses the ArcFour random number generation algorithm to generate lots of random garbage very fast in various formats. RFuzz will use this to send the garbage it needs to the application in an attempt to find forms that can’t handle nastiness, badly implemented servers, etc. It’s amazing how many bugs you actually can find by sending junk to an application.

The types of randomness you can generate are:

  • words — RFuzz includes a simple word list, but you can add your own.
  • base64 — Arrays of base64 encoded junk.
  • byte_array — Arrays of just junk.
  • uris — Arrays of URIs composed of words strung together with /.
  • ints — Random integers (with an allowed maximum).
  • floats — Random floats.
  • headers,queries — Hashes of key=value where the keys and values can be any of the above.

The ArcFour fuzzrnd random generator is in a C extension so it’s small and fast. A big advantage of fuzzrnd is that it generates the same stream of random bytes for the same input seeds. This lets you set a seed and then if you find an error replay the same attack but still have random data.

An example of using RandomGenerator is:

  g ="resources/words.txt").read.split("\n"))
  h = g.headers(2,4,type=:ints)
  => [{1398667391=>2615968266, 465122870=>2683411899, 2100652296=>4131806743,
     158954822=>2544978312}, {3126281447=>2247028995, 269763016=>1444943723,
     2401569363=>1661839605, 2811294153=>400252371}]

As you can see this produces 2 hashes consisting of 4 key=value pairs with integers in them. You can quickly replace type=:ints with type=:words and get:

  => [{"Europeanizes"=>"Byronize's", "royalization's"=>"Americanizer's",
     "celiorrhea"=>"unliteralized", "unvictimized"=>"doctrinize"},
     {"pouder"=>"unchloridized", "chattelize"=>"unmodernize",
     "uncrystallizability"=>"uncenter", "Egyptianization's"=>"ostracization's"}]

Using the included dictionary of words.

Fuzzing Sessions And Statistics

The main way that you’ll use RFuzz is to use the RFuzz::Session class to perform RFuzz runs and store the results in various .csv files for analysis later. RFuzz makes the stance that it shouldn’t be used for analyzing the data, but rather it should generate information that you can put through a better tool. Examples of such tools are R, gnuplot, ploticus, or a spreadsheet.

The Session class is initialized in a similar fashion to the HttpClient, except you can’t set the :notifier (it’s used to collect statistics about the requests). Once you have a Session object you call it’s Session#run method to do a run of a set of samples and then put your tests inside a block.

When a run is done it saves the results to two CSV files so you can analyze them.

Here’s a small sample of how Session is used:

  require 'rfuzz/session'
  include RFuzz
  s = :host => "localhost", :port => 3000 5, :save_as => ["runs.csv","counts.csv"] do |c,r|
    uris = r.uris(50,r.num(30))
    uris.each do |u|
      s.count_errors(:words) do
        resp = c.get(u)
        s.count resp.http_status

If you run this (having a server at localhost:3000) you’ll find two files in the current directory: runs.csv and counts.csv. These files might look like this:

  -- runs.csv --

  -- counts.csv --

You can then easily load these two files into any tool you want to analyze the results.

Counts vs. Samples vs. Runs

Something many people don’t do correctly which RFuzz tries to implicitly enforce is that doing just one run isn’t as useful as doing a set of runs. You might not be familiar with the terminology, so let’s cover that first.

  • count — Just a simple count of some variable during a run.
  • sample — A sample is the result of taking a measurement during a run.
  • run — This is a test that you perform and then collect counts and samples for.

In the above sample script, we are doing the following:

  • 5 runs.
  • That do GET requests for up to 50 randomly selected URIs.
  • Counting errors, HTTP status codes.
  • And gathers stats on the request timing (Session does this automatically).

If you were to structure this into a data structure it would like this:

    ["run", "name", "sum", "sumsq", "n", "mean", "sd", "min", "max"],
    [0, :request, 0.605363, 0.0149, 50.0, 0.0121, 0.0124, 0.00851, 0.095579],
    [1, :request, 0.520827, 0.0116, 50.0, 0.0104, 0.0112, 0.00189, 0.088004],

Taking a look at this, we have run 0, run 1, … and then each "row" has a set of satistics we’ve gathered on the HTTP request (shown as "name"). These statistics are actually generated from the random 50 URI requests we built with this set of code:

  uris = r.uris(50,r.num(30))

Which means that each row is the statistics collected as each request is made from the 50 randomly generated URIs. If I were to write this out it’d be:

  1. Generate 50 random URIs.
  2. Request URIs 1-50, record how long each one takes.
  3. Average (with standard deviation) the times for each request.
  4. Store this as one "run".
  5. Repeat until all the runs are done.

By doing this you cut down on the amount of information you need to analyze to figure out if a server is behaving correctly. Instead of wading through tons of data about each request, you just analyze the "meta-statistics" about the runs.

Sample Runs Reduce Error

The reason for doing a series of runs and analyzing their standard deviation (sd) and means is that it reduces the chance that one long run was just done at the wrong time or in the wrong situation. If you just ran a test once with the same settings every time you might not find out until later that there was some confounding element which made the test invalid.

Source Code

You can also view for the rcov generated coverage report which is also a decent source browser.