Cross-Post: Exploiting Windows 2008 Group Policy Preferences – Expanded

Cross posting some work of a friend of mine that I was helping with, I say “helping” in the lightest form of the word (I had a domain controller ready to test, he didn’t).

Meatballs (over at: http://rewtdance.blogspot.com) has been doing some work attempting to put together a metasploit module to decrypt passwords found within the sysvol folder on win2k8 domains.

However rather than just settle for the disclosed “local users and groups are vulnerable…” he dug a little deeper after realising that datasources and other such things that have user credentials associated with them were also stored in the same manner.

What follows is a snippet from his blog, visit his site for the full article.

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This follows on from the disclosure http://esec-pentest.sogeti.com/exploiting-windows-2008-group-policy-preferences which discussed how Group Policy Preferences can be used to create Local Users on machines and the resulting passwords easily decrypted. (Expect a metasploit post module to gather these details soon…)

Browsing the MSDN documentation I noticed that there were many other preferences that could be set that, and delving further they also allow a password to be stored. For example Services.xml specifies services to run on end machines, and can specify a specific user and password for that service to run under.

Whilst these preferences may not be used as commonly as local users preferences (to set local administrator passwords), they may lead to current valid domain credentials rather than just local users accounts – for example specifying a domain user to connect to a network share in Drives.xml… (read more)

The finished result when run against my little 2k8 test domain.

Gpp

Adding Google Authenticator 2-Factor Auth to an OpenVZ VPS

…running Centos 6.2

After seeing a great little tutorial courtesy of @hak5darren I decided to implement this on my VPS box to provide a little extra security while removing the need for private keys.

Granted I can still use private keys at home but it’d be nice to have access to my box when I maybe don’t have access to my private key or using it via an internet cafe.

Step One: Make sure your time is set correctly

Everything we will end up doing is to set up a time sensitive token. If your server is not running on the same time as your mobile phone/authenticator then you’re going to find yourself unable to log into your machine.

In OpenVZ you are unable to use NTP to sync up time or manage timeservers and so you’re stuck to what your physical host is using. Providing your provider is decent enough and is ensuring his physical hosts are kept up to date you shouldn’t have a problem.

If you follow these next steps and your time is still out of sync, you’ve got a problem that I can’t solve. Contact your provider and get him to sort out NTP on his physical hosts.

As you can’t use NTP yourself to set the time, you have to set the correct “offset”. You do this by:

  1. rm /etc/localtime && ln -s /usr/share/zoneinfo/Europe/London /etc/localtime && date

Make sure you set the timezone appropriately for your area. The output should be in sync with your local time now.

Step Two: Get hold of the Google Authenticator PAM

The Pluggable Authentication Module (PAM for short) is available from: http://google-authenticator.googlecode.com

wget and extract the source tarball:

  1. wget http://google-authenticator.googlecode.com/files/libpam-google-authenticator-1.0-source.tar.bz2 && tar jxvf libpam-google-authenticator-1.0-source.tar.bz2
  2. Ensure you have the development tools installed: sudo yum groupinstall “Development Tools”
  3. Ensure you have the pam development libraries installed: sudo yum install pam-devel
  4. cd libpam-google-authenticator-1.0
  5. sudo make install

With no errors you should now have two files contained within /lib64/security and /usr/local/bin

  • pam_google_authenticator.so
  • google-authenticator

Step Three: Lets start configuring it

First the system authentication…

Copy the following line into your /etc/pam.d/sshd configuration file.

  • auth    required                        pam_google_authenticator.so

You can add the above to the system-auth file if you wish but expect problems if you’re running X. I only have a single service I want to protect so I’ve added it as per Darren’s instructions to the service’s configuration file.

Note that each stack (auth, account, session, etc…) is executed in line order. So if you place the above line after the line within the auth stack that deals with “password”, it’ll request a password first, if you place it above the password line, it’ll request a verification code first.

Any references to “include” means that it defers that point of the stack to another file under /etc/pam.d/ so if you want to fine tune where the google authenticator module is called, you’ll need to follow the stack.

Once you’ve added the above line edit your /etc/ssh/sshd_config file to make use of challenge_response authentication.

  • Ensure “usepam” is set to yes
  • Ensure “ChallengeResponse Authentication” is set to yes – This is the “Something I have” factor
  • Enable Password Authentication – This would be the second “something I know” factor.

Another thing to note is make sure that your login grace time is set to something sufficiently large enough for you to type in both the authenticator code and your password. I had mine set to 10s to allow me to type in a keyphrase for my private key, but kept finding my login timing out after submitting a verification code but before I could finish typing my password when using 2FA.

With all the above done you’re now ready for step 4.

Step 4: Configuring your user account

As your normal user enter the following command:

  • google-authenticator

The following exchange will take place. Answers are down to your preferences except for where I’ve marked it in bold.

[[email protected] libpam-google-authenticator-1.0]$ google-authenticator
Do you want authentication tokens to be time-based (y/n) y

https://www.google.com/chart?BLAHBLAHBLAHBLAH….
Your new secret key is: AAAAAAAAAAAAAAAAAAAAAA
Your verification code is 111111111111111111111
Your emergency scratch codes are:
   111
   222
   333
   444
   555
Do you want me to update your “/home/user/.google_authenticator” file (y/n) y

Do you want to disallow multiple uses of the same authentication token?
This restricts you to one login about every 30s, but it increases your chances to notice or even prevent man-in-the-middle attacks (y/n)

By default, tokens are good for 30 seconds and in order to compensate for possible time-skew between the client and the server, we allow an extra token before and after the current time. If you experience problems with poor time synchronization, you can increase the window from its default size of 1:30min to about 4min.

Do you want to do so (y/n)

If the computer that you are logging into isn’t hardened against brute-force login attempts, you can enable rate-limiting for the authentication module. By default, this limits attackers to no more than 3 login attempts every 30s.

Do you want to enable rate-limiting (y/n)

Navigate to the URL given to you within the output.

  • Using the Google Authenticator app on your phone, add an account and select “scan barcode”
  • Position camera to view the QR code presented to you at the above URL

The Google Authenticator Application will automatically add an account with a value which will update every 30 seconds or so.

MAKE SURE YOU COPY ALL OF THE INFORMATION ABOVE RELATING TO SCRATCH CODES AND SECRET KEYS – IF YOU LOSE YOUR AUTHENTICATOR, YOU WILL BE ROYALLY SHAFTED WITHOUT THESE. Especially if it is your only method of authentication to your server.

Now restart your sshd server using:

  • sudo service sshd restart

or

  • sudo /etc/init.d/sshd restart

And before you log out of your current session. Open up a secondary putty window and attempt to login using your google verification code and a password (obviously having set one for your user).

Step Five: …now you’re auth’n with google

Providing the last step worked, you’re all sorted. Google authenticator is working for you.

2fa

Otherwise, undo the above changes and give up as I have no idea what went wrong and if you logout you’ll likely lock yourself out of your machine.

Mini Rant – Security that makes no sense…

In my line of work we encourage encrypted communications and securing sensitive data especially when it comes to PII.

However it’s increasingly common to see systems put into place that are obviously only there to mitigate litigation aspects should anything go wrong.

Take American Express for example:

An email from them asking you to send a copy of your passport/driving licence/etc… to confirm your identity suggests that you may reply via email however:

Please note that the internet can be insecure. You must use a secure encryption method when sending personal data and/or documentation to us via email to safeguard your personal data

Great… you encourage your customers to encrypt their personal data.

So I’ll just send over a truecrypt volume encrypted with twofish-aes-serpent shall I?

Or perhaps a PGP encrypted volume, whats your public key?

What about just an AES256 encrypted zip?

Okay so that’s point 1.

  • They’ve suggested that it is on the customers own head to protect their data. However they have not listed the accepted formats of encryption that they use.

So we’re assuming because they’ve not provided us with a public key they don’t want pgp or gpg encryption. They want something simple that doesn’t require too much infrastructure in place so we’ll go with the AES256 encrypted zip, which providing they have winzip/7zip/*ziprarace client means they can enter in a password and decrypt the contents.

Great, so how do I get the password to you?

AMEX are right, internet communication via email is all in the clear, so if someone was in the middle of my traffic (i’m on a corporate network, chances are they’re monitoring it at least so files could be logged or archived in an antivirus mail gateway for example) they could intercept the cleartext data and have my passport details.

So I encrypt it and send it via email, attacker or corporate network now only has an encrypted zip file.

How do AMEX suggest I send a password to them? I call their customer service desk, expecting them to give me a number to SMS it to or a voice service that instead reads me a password when I dial the number and enter my reference code?

Oh you just send it via email. I think you’re meant to send it all together

…I explain my concerns..

Erm, I’ve never been asked that before I guess I could give you another email address to send it to

Point 2:

  • Sending encrypted data along with the password in the same email is as good as sending cleartext data.
  • Sending encrypted data along with the password via the same mechanism is as good as sending cleartext data.

So despite all of AMEX’s good advice above “You must use a secure encryption method…” actually there is no way to use a secure encryption method to keep your data safe when dealing with them.

Extra Note:Along the same lines, as I mention above I regularly get asked to encrypt reports that are deemed commercially sensitive. So I email out the encrypted zip file, and they request that I SMS them the password.

2 minutes later, their blackberry chirps… twice*.

*Did you guess what just happened?

Their blackberry received both the encrypted zip and the plain text password. Loss/Theft of the blackberry once again could result in the loss of commercially sensitive data.

NFTF: Useful urls for malware investigations

Figured I’d keep a copy of this on here for the next time I need to do malware investigation. 

 

http://www.urlvoid.com – checks URL’s against lots of blacklists, emergingthreats, malwaredomainlist and zeustracker/etc…

http://www.ipvoid.com – Same as above but for IP addresses

http://support.clean-mx.de – Searches above databases and records logs of abuse claims. Useful as it can sometime give you extra URI’s for a host to comb your logs for. Also usefully gives you the date that its crawler last was able to pull down the malicious binary.

http://hosts-file.net/ – Provides an assessment according to the type of nastiness a domain or IP is associated with.

http://www.malwaredomains.com – Provides a blacklist DNS entry head to the downloads page, open the text file version and CTRL-F to search. Will give reason for blocking (i.e. listed in emergingthreats.net) along with the date.

http://www.malwaredomainlist.com – Provides a listing of hosts and ip’s known to be associated with malware.

http://www.emergingthreats.net – Provides snort rules configured to detect malicious traffic/hosts.

http://www.robtex.com – Advanced DNS lookups, links hosts to nameservers, can give aliases and associated subdomains as well as any shared hosts.

http://www.network-tools.com – Basic network tools, whois, dns, traceroute, etc… useful for performing checks NOT from your own ip 🙂

http://www.zeustracker.com – lists zeus C&C nodes

http://www.spyeyetracker.com – lists spyeye C&C nodes

 

Be careful if you use any of these tools on the affected network as often it will contain the hostname or IP you’re looking for in the request parameters which means they’ll flag you up as an infected laptop without looking at the actual URL you’re browsing to (happened to me previously).

*10/06/12 – Correction courtesy of Steven B: I originally had “hp-hosts.com” listed instead of hosts-file.net, thanks for the heads up.

NFTF: Alternative Data Streams – bits and pieces.

To those not familiar with the world of NTFS. It offers a feature known as Alternate Data Streams which can allow a user to create hidden content attached to a file.

Typically generated using echo or type it normally requires a command prompt to get to generate these files or view the files.

However an alternative method in XP and 2K/2K3 series of operating systems was to add data to the summary properties of a text document created in notepad as it turns out this data was held within an ADS associated with the original text file.

What happens if like me last week you find yourself on a system with a tight group policy forbidding command line access and an execution arbiter that worked from a whitelist of very very few programs?

You get creative.

I know:

type hideme.txt > public.txt:hideme.txt

will generate an ADS.

I also know that typing

notepad c:pathtopublic.txt:hideme.txt

will let me edit the contents of hideme.txt which would not ordinarily be accessible by any other means.

Unfortunately opening a file in notepad and throwing public.txt:hideme.txt as a filename within the save as box will not work as windows dislikes the colon.

But what else runs console commands?

Batch files – nope not in this case, execution arbiter stops batch files running.

What about shortcuts?

Bang on.

Right click “Create New -> Shortcut”

Enter in “notepad” without quotes as the target, and complete the wizard with defaults.

Right click the created shortcut change the target field to show

%windir%/system32/notepad.exe "c:pathtopublic.txt:hideme.txt"

Save the changes and double click the shortcut.

Pow! You’re now editing an ADS attached to the public.txt file that you had available earlier, ADS created and without additional tools you’re free to hide data away from an administrators prying eyes on a system that gave you no access to a command prompt, stopped you running Batch files and more…

What Next

So with that juicy thing done what else could I do? What about exporting sensitive company data? Maybe the customer contact list for a company or medical records or financial details?

Hmm okay so I’m going to have to get it off the system some how, but the company is smart and doesn’t allow the use of USB drives so I can’t use an NTFS formatted USB drive to export data (on non NTFS file systems the ADS is dropped as it’s not supported).

What about CD? Well I did say on non NTFS file systems the ADS disappears. It’s true for CDs ISO9660 and UDF formats don’t support alternate data streams so you’re stuck again.

Except, what if you change the file?

What if you zip it? then burn the zip?

Well sad to say using WINZIP v14+ and the default compressed folders function in windows, I believe you’re out of luck, both tools appeared to just drop the ADS content on the floor.

Using winrar however to create the zip… I’ve shown that it maintains the ADS across filesystems, now my test was using a local FAT32 formatted partition and an NTFS one, I didn’t actually burn it to CD-ROM so it may not be the case but it’s certainly looking promising.

If it is the case, having the ability to covertly export and import information using ADS suddenly becomes a big issue.

I plan on looking into it a bit more as it could have just been a series of flukes that worked for me but it was definitely promising.

My initial thoughts for this are: uuencoded zip file (ASCII friendly so will play nice as ADS content) containing lots of juicy personal information that shouldn’t be leaked. Add to a benign text file expected to leave the building. Winrar zip the lot, burn to CD… get home and do the reverse.

Ba doom boom! You’ve just circumvented the whole lot of data controls put in place to protect a companies data.