By Rafael Ballestas | February 08, 2018
is a simple language that
can be used for software documentation and testing.
It can be thought of as a tool for
communication between stakeholders and developers which
helps minimize misunderstandings and regressions
through precision in the definition of use-case scenarios.
Gherkin can be used for more than just
software specification and documentation.
In fact, it could be used to
specify any kind of procedure,
from scientific experiments to
how to sew a button.
Here we will focus on how we can use the Gherkin language to document attack vectors, or the way a vulnerability in a system can be found and exploited.
Gherkin is natural language
describing a procedure.
Gherkin supports more than 60 of those.
What gives structure to it are,
at the core level,
are used to specify an event and
its expected outcome. For example:
When I plug my phone into the AC outlet Then it starts charging
Such combinations are
the building blocks of a
The complete file is called a 'feature',
since they are normally used to
describe a single capability
of a piece of software.
But a given feature can have many use cases:
Gherkin calls them 'scenarios'.
In turn, a scenario is made up of
a (preferably small) number of steps.
Feature: Make coffee Coffee should brew after mixing the ingredients Background: Given a coffee pot And coffee beans And water Scenario: Make coffee from scratch When I grind the coffee Then I can put it in the pot When I put the coffee and water in the pot And I turn it on Then coffee should brew
As seen above,
can be used to replace
so as not to have too many of the latter.
Other keywords works as you expect:
what their natural language counterparts do.
Given is for pre-conditions and
Background is used
to several scenarios.
Also, as seen above,
you can write in plain language
after a section beginning.
This is useful to explain
what is not expressed by steps and scenarios.
We can also use comments (with
but only sparingly,
since we’re trying to be
as explicit as possible
about how the feature should work.
Now that we have the basics of
Gherkin under our belts,
let’s see how we can use it
to explain how we can attack and
exploit a vulnerability in a system.
Consider the following website from
a very buggy web app.
bWAPPDNS lookup site
It has a very simple function:
give it a valid
it prints details about the domain name,
such as its
We can write this up in
Gherkin as follows:
Scenario: Normal use case Given I am at the page bWAPP/commandi.php When I type a valid URL Then the IP address of that URL is printed When I type any text that is not a URL Then there is no output
Very simple, yet concrete. It specifies the expected behavior of the site in detail and, more importantly, without ambiguities. Also, this helps avoid support calls, since the feature file works as a kind of manual and troubleshooting guide.
However, we’re here to break that app, so let’s document that as well.
how can the app get all those details?
Actually, the output looks
like that of an
If user input is not properly validated,
we could take advantage of that to
execute some other commands in the server.
If, as we expect,
the server executes a
user_input as argument,
it’s as if we did this at a terminal:
$ command user_input
UNIX-like operating systems,
we can use “;” to execute one command after another.
For example, we can say
$ echo "first line"; echo "second line" first line second line
and so we have executed two commands in one go.
Thus, if we append
; another_command arguments
in the input,
the command executed by the app
would become this:
$ command user_input; another_command arguments
Let’s see if that works with
the simple command
ls -aR /, which
lists all files Recursively in /,
i.e., all files in the server:
UNIXcommand injection in the same site
It does! If we can ``inject'' that command, a malicious user could definitely inject more harmful commands.
We can document this entire procedure (minus the explanations, which are for the reader’s benefit) like this:
Scenario: Dynamic detection and exploitation When I type ";ls -aR /" in the field Then all files in / are listed recursively
We have given this scenario the epithet ``dynamic'' since we found and exploited this vulnerability by 'dynamically' interacting with the app. This is in contrast with 'static' detection, where we find and exploit the bug by looking into the app’s code, which is what we’ll do next.
The website is written in
but as long as you can understand any code,
you’re good to go.
Let’s look at the file
The lines that execute the command go like this:
$input = $_POST["target"] echo shell_exec("nslookup " . $input);
So we were right!
PHP asks the server’s shell to
run the command
$input is not validated or
changed at all.
That’s why we could exploit
the vulnerability the way we did.
A simple input validation or sanitization
can prevent this
from happening to your app.
bWAPP had added just three lines
to clean the control operators
$input = str_replace("&", "", $data); $input = str_replace(";", "", $input); $input = str_replace("|", "", $input);
this vulnerability would have been a lot harder to exploit, or just wouldn’t exist.
Combining all the steps above,
we get the full documentation
for this vulnerability
Feature: Detect and exploit vulnerability OS Command Injection From the bWAPP application From the site localhost:80/bWAPP/commandi.php Background: Given I am running Manjaro GNU/Linux kernel 4.9.77-1-MANJARO And I am browsing in Firefox 57.0.4 And I am runing bWAPP from docker container raesene/bwapp Given a PHP site with an input that says "DNS lookup: ____ -> Lookup" Scenario: Normal use case Given I am at the page bWAPP/commandi.php When I type a valid URL Then the IP address of that URL is printed When I type any text that is not a URL Then there is no output Scenario: Static detection When I look in the code for commandi.php Then I see the called function invokes shell_exec("nslookup ") """ $input = $_POST["target"] echo shell_exec("nslookup " . $input); """ Scenario Outline: Dynamic detection and exploitation When I append ;<command> to "www.nsa.gov" in the input Then the <output> is rendered in the browser Examples: | <command> | <output> | | ls -lR / | all files in / are listed recursively | | grep -r password / | looks for all ocurrences of "password" in / | | rm -rf | nothing | | vi | other pages don't load! server & docker crashed |
We wrap long lines
Python-like docstrings (
This feature includes the keywords
Scenario Outline which is like
having variables in scenarios
in order to avoid repetition.
We use it to show
the output produced by
different commands or trials.
If you’re interested,
see Cucumber docs
for a more thorough,
but still nice and short,
As we’ve seen,
was not exactly designed
with hacking documentation in mind,
we can still make it into a useful tool
for this purpose.
It enables us to write
unambiguous, reproducible and — given the right configuration and environment — executable documentation that
also simplifies testing.
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