Fracker

Fracker is a suite of tools that allows to easily trace and analyze PHP function calls, its goal is to assist the researcher during manual security assessments of PHP applications.

It consists of:

• a PHP extension that needs to be installed in the environment of the target web application which sends tracing information to the listener application;

• a listener application that runs locally and is in charge of receiving the tracing information from the PHP extension and performing some analysis in order to show some meaningful data to the user.

Setup

Install the PHP extension, either by using the deploy script or manually, then install the listener application locally.

Otherwise for a quick test, jump straight to the demo.

Demo

1. Install the listener application.

2. Spin a Docker container serving the PHP demo application:

   docker run -d -p 80:80 -v "$PWD/demo/:/var/www/html/" --name hello-fracker php:8.2-apache

3. Check that the demo PHP application works as expected:

   curl http://localhost/?expression=7*7

   The result is: 49
   

4. Deploy Fracker to the container using the deploy script:

   ./scripts/deploy.sh hello-fracker

5. Start Fracker in another terminal, then repeat the above curl command:

   fracker

   +++ │ Listening on 0.0.0.0:6666
   +++ │
   001 │ GET localhost/?expression=7*7
   001 │ {main}() /var/www/html/index.php +0
   001 │ »  is_safe(expression="7*7") /var/www/html/index.php +16
   001 │ »  »  preg_match(pattern="/^[0-9+.\\-*\\/() ]+/", subject="7*7") /var/www/html/index.php +12
   001 │ »  calculate(expression="7*7") /var/www/html/index.php +17
   001 │ »  »  eval("return 7*7;") /var/www/html/index.php +5
   +++ │
   +++ │ Shutting down...
   

6. Press Ctrl-C to exit Fracker, then run it again with --help, and experiment with other options too, for example, pass the -v option to show the return values too!

7. Finally stop and remove the container with:

   docker rm -f hello-fracker

Architecture

Every PHP request or command line invocation triggers a TCP connection with the listener. The protocol is merely a stream of newline-terminated JSON objects from the PHP extension to the listener, such objects contain information about the current request, the calls performed, and the return values.

This decoupling allows the users to implement their own tools. Raw JSON objects can be inspected by dumping the stream content to standard output, for example (assuming the default settings):

socat tcp-listen:6666,fork,reuseaddr - | jq

PHP extension

The PHP extension is a modification of Xdebug, hence the installation process is fairly the same, so is the troubleshooting.

The most convenient way to use Fracker is probably to deploy it to the Docker container where the web server resides using the provided deploy script, use the manual setup for a more versatile solution.

Deploy script

This script should work out-of-the-box with Debian-like distributions running Apache:

./scripts/deploy.sh <container> [<port> [<host>]]

It configures the PHP module to connect to specified host on the specified port (defaults to the host running Docker and the default port).

Manual setup

Install the PHP development files and other dependencies. For example, on a Debian-like distribution:

apt-get install php8.2-dev libjson-c-dev pkg-config

Then move into the ./ext/ directory and just run make to fetch Xdebug, apply the patch, and build Fracker.

To check that everything is working fine, start the listener application then run PHP like this:

php -d "zend_extension=$PWD/xdebug/modules/xdebug.so" -r 'var_dump("Hello Fracker!");'

Finally, install the PHP extension the usual way. Briefly:

1. copy ./ext/xdebug/modules/xdebug.so to the PHP extension directory (e.g., php-config --extension-dir);

2. place zend_extension=xdebug, along with any other optional custom settings, in some INI file that gets parsed by PHP in the target environment (SAPI) used by the application;

3. if needed, reload the web server.

At this point the source repository is no more needed.

Settings

The default INI settings should work just fine in most cases, the following aspects can be configured.

Address of the listener application

By default, the PHP extension will try to connect to 127.0.0.1 on the port 6666. This can be changed with:

xdebug.trace_fracker_host = 10.10.10.10
xdebug.trace_fracker_port = 1234

Trace only certain requests

By default, every request will be traced. It is possible to switch to an on-demand behaviour with:

xdebug.start_with_request = trigger
xdebug.trigger_value = FRACKER

In this way, only those requests having XDEBUG_TRACE=FRACKER in their GET, POST, or cookies parameters will be traced by Fracker.

Listener application

The provided listener application is a Node.js package, it is commonly installed locally, but it can resides anywhere, provided that it can be reached by the PHP extension.

Installation

Install the dependencies with:

npm install -C ./app/

Then run Fracker locally with ./app/bin/fracker.js.

Optionally, install the executable globally by creating a symlink to this folder with:

npm install -g ./app/

After that, the fracker command will be globally available.

Uninstall with:

rm -fr ./app/node_modules/
npm uninstall -g fracker

Usage and configuration

Run fracker --help to obtain the full usage.

For convenience some configuration files listing some classes of interesting PHP functions are provided along with this repository. Use them like:

fracker ./app/configs/file-* # ...

License

This product relies on Xdebug, unless explicitly stated otherwise, for the PHP extension itself, the copyright is retained by the original authors.

The listener application instead is released under a different license.