Archive for the ‘Tech’ Category

Recreating a Docker container breakout with nsjail

March 1, 2018 Leave a comment

Way back in June 2014, Docker published a blog post detailing a security flaw that allowed an attacker to break out of a Docker container and access files on the host file-system. This meant that a user running code within a Docker container could read sensitive files in privileged locations on the host, such as /etc/shadow.

This flaw existed only in Docker versions earlier than 1.0.

A PoC exploit called shocker was made available, and we will use it today, along with nsjail, to demonstrate this vulnerability.

Jen Andre has written a thorough analysis of how this exploit works.

Today’s containers are built using important Linux components such as kernel namespaces, root capabilities, control groups (cgroups), seccomp system-call filtering, pivot_root, and on occasion containers will also use Mandatory Access Control implementations such as AppArmor or SELinux.

General outline:

  1. I recreated this on Ubuntu 18.04 beta (kernel v4.15.0-10)
  2. Download and install nsjail
  3. Download and compile the shocker exploit
    • execute touch /tmp/.dockerinit on your host system (shocker will look for this file)
    • compile shocker: gcc shocker.c -o /tmp/shocker
  4. OPTIONAL: Download amicontained and copy it to /tmp
  5. Launch nsjail
    • sudo nsjail --chroot /tmp -R /bin -R /usr/bin -R /lib64 -R /lib/x86_64-linux-gnu/ --cap CAP_DAC_READ_SEARCH --disable_clone_newuser -- /bin/bash -i
  6. Optionally run /amicontained (from within the nsjail container) and review the output
  7. From within the jailed bash shell, run the exploit: /shocker
  8. Done. You should now see the contents of the host’s /etc/shadow file

Steps 1-4 should be straightforward, but lets talk about the nsjail command:

sudo nsjail \
  --chroot /tmp \
  -R /bin \
  -R /usr/bin \
  -R /lib64 \
  -R /lib/x86_64-linux-gnu \
  --disable_clone_newuser \
  -- /bin/bash -i
  • --chroot /tmp simply sets the root directory of the container as the host’s /tmp directory
  • The -R switches instruct nsjail to mount the specified directories within the container as read-only. These are necessary for running /bin/bash and utilities such as id inside the container
  • --cap CAP_DAC_READ_SEARCH instructs nsjail to enable this capability for our container, and is crucial for this exploit to work. The documentation for this capability says:

CAP_DAC_READ_SEARCH – Bypass file read permission checks and directory read and execute permission checks; invoke open_by_handle_at(2); use the linkat(2) AT_EMPTY_PATH flag to create a link to a file referred to by a file descriptor.

  • --disable_clone_newuser disables the User namespace. Without disabling the User namespace, the shocker PoC exploit simply won’t work. So what does a User namespace do? see below:
    • For containers whose processes must run as the root user within the container, you can re-map this user to a less-privileged user on the host. The mapped user is assigned a range of UIDs which function within the namespace as normal UIDs from 0 to 65536, but have no privileges on the host machine itself. reference
    • Without container support for the User namespace, it means that if you are root in the container, you are root on the host. The User namespace changes that and allows us to let the container user run as root within the container only, while being mapped to a lower privileged UID on the host.
  • /bin/bash -i – run an interactive bash shell within the container
Categories: Tech

PsExec Software Deployer

December 26, 2016 Leave a comment

About 5 years ago I wrote a GUI wrapper around PsExec to assist me with using psexec to:

  1. Copy and execute software installations on a number of remote computers
  2. Run a command on a remote system and retrieve its output
  3. Log all activity and retrieved output in a grid view that can be exported to XLS, HTML, XML, or plain text.

When I needed to run complex scripts that depended on a number of files, I would create a self-extracting EXE (using this tool) and use this PsExec Software Deployer to copy the file and run it on remote systems.

I wrote it just for my own use, but I think others might get some use out of it as well so I am open-sourcing it. I wrote it in Delphi and as such it has no external dependencies and is a single EXE (although requires psexec.exe to be downloaded).

This tool is written to be multi-threaded, so multiple deployments can occur concurrently, which greatly sped up my ability to deploy software or retrieve data from machines.

The source is available here:

If you just want the binary, it is here:



Here’s an example using the tool’s ability to run a command on a remote system and capture its output:


Double-clicking on the cell the red arrow is pointing to will display the full text from the command. Also, you can simply save the grid to a file (HTML, XLS, XML, or plain text).


Categories: Tech

Bitwise operations with Python

September 14, 2015 Leave a comment is a Python script to perform bitwise operations on files (like XOR, ROL/ROR, …). You

specify the bitwise operation to perform as a Python expression, and pass it as a command-line argument. malware malware.decoded ‘byte ^ 0x10’

Direct download:
Categories: Python

Python pip and vcvarsall on Windows

April 9, 2015 Leave a comment

Linking to this SO answer and re-posting it here, as I seem to search for it every few weeks or so:

For Windows installations:

While running for package installations, Python 2.7 searches for an installed Visual Studio 2008. You can trick Python to use a newer Visual Studio by setting the correct path in VS90COMNTOOLSenvironment variable before calling

Execute the following command based on the version of Visual Studio installed:

  • Visual Studio 2010 (VS10): SET VS90COMNTOOLS=%VS100COMNTOOLS%
  • Visual Studio 2012 (VS11): SET VS90COMNTOOLS=%VS110COMNTOOLS%
  • Visual Studio 2013 (VS12): SET VS90COMNTOOLS=%VS120COMNTOOLS%
Categories: Programming, Python, Tech, Windows

FSExploitMe and Exploit-Exercises

March 17, 2015 Leave a comment

If you are interested in learning about ActiveX exploitation, security researcher/consultant/professor Brad Antoniewicz has created FSExploitMe for just that purpose.

You’ll want a copy of Internet Explorer 8 to get the most out of it, but fortunately you can get a VM running IE8 for *free* from Microsoft.

And if you are interested in getting deeper into Linux exploitation, has pre-built VM’s with capture-the-flag style levels and challenges for each level. It also has challenges for beginners to expert and is designed to teach:

[…]about a variety of computer security issues such as privilege escalation, vulnerability analysis, exploit development, debugging, reverse engineering, and general cyber security issues.

Categories: Disassembly, Linux, Tech, Windows

Visualizing Garbage Collection

February 16, 2015 Leave a comment

Visual guide to understanding garbage collection algorithms!

Categories: Programming, Tech

Data::Dumper formatting

October 8, 2014 Leave a comment

I look this up about every couple weeks, so I’m posting it here for posterity. In order to nicely format Data::Dumper output…

I almost always set

$Data::Dumper::Indent = 1;
$Data::Dumper::Sortkeys = 1;

with Data::Dumper. The first statement makes the output more compact and much more readable when your data structure is several levels deep. The second statement makes it easier to scan the output and quickly find the keys you are most interested in.

If the data structure contains binary data or embedded tabs/newlines, also consider

$Data::Dumper::Useqq = 1;

which will output a suitable readable representation for that data.

Much more in the perldoc.

Categories: Perl, Tech