Thursday, August 27, 2009

Windows password filters

During my HAR talk, I briefly described the password filter mechanism. A password filter is a DLL that is listed in the registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Notification Packages.

Whenever a user changes his password, Windows passes the plaintext password to all the DLL’s listed there. The password filter can then determine whether the password meets its requirements (complexity, length etc.) and approve or disapprove the password. You can use this feature to implement your own password filter DLL and enforce your custom company password policy.

Of course, a password filter could also choose to do something else with the plaintext password. As I described in my previous blog post, Microsoft does this with RASSFM.DLL. An attacker could also create his own backdoor password filter that, for example, sends the password to a remote server.

The procedures you need to implement are quite well documented on MSDN. There is even a universal password filter available on Sourceforge. This allows you to pass the password to a custom script.

Wednesday, August 26, 2009

Passwords stored using reversible encryption: how it works (part 2)

In part one of this article, I described how the reversible encryption of Windows domain passwords works. In this part, we will look at the security of this mechanism.

To decrypt the password you need the following components:
- The encrypted password (G$RADIUSCHAP)
- The 16 byte random (G$RADIUSCHAPKEY)
- The global LSA secret (G$MSRADIUSCHAPKEY)
- A static key hardcoded in RASSFM.DLL

The hardest thing to get is that global LSA secret. This is stored in active directory and synchronized between domain controllers. To access this key, you need domain administrator privileges. An obvious risk here is that once someone gains domain administrator privileges, he won’t need to crack any passwords, but can simply decrypt them. Of course, if an attacker gains domain administrator privileges on your domain, you are already in big trouble anyway.
However, the other components are all semi-public information. The static key is hardcoded in RASSFM.DLL which comes with every Windows server, so is easy to get. The G$RADIUSCHAP and G$RADIUSCHAPKEY are stored in active directory in the userParameters structure. If you have a user account on a domain you can use AD Explorer to access the Active Directory database and read this information. Of course, to decrypt the password you will still need that LSA secret.

The encrypted version of the password can be interesting though; by looking at the encrypted password you can derive the length of the plaintext password. Two examples I used in my presentation:

Pwd1 encrypted: 0f53 8420 9418 05ce 01ad

Pwd12 encrypted: 5d69 9375 6f92 1b63 7728 439f

So, by looking at the encrypted passwords you should notice that the encrypted version of Pwd12 is two bytes longer than the encrypted version of Pwd1. So, although we cannot determine from just looking at the encrypted password that they are very similar, we can determine their length. What this means is, that as a domain user, you can determine the length of other people’s passwords, which could be quite interesting.

If you obtain the LSA secret somehow (maybe because you temporarily gain domain administrator privileges), from that point on you can decrypt passwords stored using reversible encryption. This could be used as a nice backdoor, just steal the LSA secret, enable reversible encryption (if it hasn’t been enabled yet) and you can grab the domain administrator password with just a normal user account.

Someone in the HAR 2009 audience had a very nice question: Is it possible to recreate the LSA secret if you’re afraid it has been stolen. Of course the better option is to recreate the entire domain, but this is not always an option. To recreate the LSA secret you need to write a program that sets the LSA secret to NULL. According to the documentation, Windows will delete the LSA secret. It will generate a new LSA secret when it needs to encrypt another password. Of course, Windows won’t be able to decrypt the passwords stored before that point anymore. However, it will still try to decrypt them using the new LSA secret, which will result in gibberish most of the time. If you’re really unlucky it could decrypt the first two bytes to NULL, which basically means the password is suddenly empty. So if you ever have to do this, resetting all passwords immediately is probably a good idea.

Tuesday, August 25, 2009

HAR 2009 talk references

I promised to put up some nice ‘further reading’ material for those who have seen my HAR talk. So here it is:

Disabling the LM hash
Free Rainbow Tables and download site

Passing the hash
The original post from 1997 by Paul Ashton
Core Pass-The-Hash toolkit
Tenable SMBshell

Token stealing
Luke Jennings research page
A tutorial on the tool by CG

You can see the video of my talk here or download it here or here.

Passwords stored using reversible encryption: how it works (part 1)

In case you missed my HAR2009 talk: in the second part I talked about a Windows feature called ‘Store passwords using reversible encryption’. When this is enabled (per user or for the entire domain), Windows stores the password encrypted, but in such a way that it can reverse the encryption and recover the plaintext password. This feature exists because some authentication protocols require the plaintext password to function correctly, the two most common examples are HTTP Digest Authentication and CHAP.

This feature is not enabled by default but I’ve seen it a couple of times in customer networks. As I couldn’t find any description of how this mechanism works or any tool to recover these passwords, I decided to investigate.

When you change your password on a domain that has reversible encryption enabled, a password filter called RASSFM.DLL is used to store the password using reversible encryption. The key that is used to do this is G$MSRADIUSCHAPKEY, which is stored as a global LSA secret. This key is decrypted using a static key (hardcoded in the DLL). The result of this operation is combined with a 16-byte random value (generated every time someone changes their password) and that key is used to encrypt a Unicode version of the password using the RC4 algorithm.

I found out these passwords are stored in Active Directory in a per-user structure called userParameters. If you use a tool such as AD Explorer you can look at this structure in an AD that has enabled this feature. When you look at this structure, it looks like a binary blob, with some human-readable parts in there. When you enable reversible encryption you will notice two readable strings: G$RADIUSCHAP and G$RADIUSCHAPKEY. The userParameters can also be used to store settings unrelated to reversible encryption, such as per-user Terminal Server settings.

Following the G$RADIUSCHAP part is the ascii-hex encoded encrypted password. The part following the G$RADIUSCHAPKEY name is the 16-byte random value.

So to decrypt this password we use the following steps:
- Take the G$MSRADIUSCHAPKEY Global LSA secret
- Decrypt it using the static key
- Parse the userParameters structure and extract the G$RADIUSCHAP and G$RADIUSCHAPKEY values
- Combine the value of G$RADIUSCHAPKEY (the 16-byte random) with the decrypted LSA secret to create an RC4 key
- Decrypt the value of G$RADIUSCHAP using that RC4 key

The result is a plaintext Unicode password. My tool 'Revdump' automates this procedure.

In part two of this article, I will look at the security of this mechanism.

Saturday, August 15, 2009

RevDump v0.2 release

This is my tool to dump password stored using reversible encryption. You can download it here. Enjoy your stay at HAR.