NTLM Challenge Response is 100% Broken (Yes, this is still relevant)

Updated again: CloudCracker has dropped off the web. A new successor has arisen. Meet Crack.sh. 

Updated as it seems a lot of folks don't read. The reason this attack is interesting is that I steal or phish the hash in the attack WITHOUT physical or administrative access. Yes, I am aware of the 30 amazing tools that "already did that" using local admin access. Also here for why this is still matters.

First off, I can’t say I broke the NTLM handshake; the march of time did it.  Apparently, I am just the one who bothered to put it together.

There have been numerous whitepapers, hacker conference sessions, and blog posts dedicated to the weaknesses of NTLM (and LM) authentication.  However, the weaknesses described in previously published works were theoretical, or required stealing hashes using admin rights. This means the host was already compromised, thus the exploits themselves are a bit boring and redundant.  One couldn’t just phish for the hashes or MITM the hashes; due to the challenge response mechanism.   The best case for getting to the hash or password from outside the host was to do a MITM attack (or a phish) and substitute a chosen challenge.  This only worked if the victim was willing to negotiate NTLM without the Session Security Flag. This would then allow an attacker to build rainbow tables to get the hash or password. Rather, the attacker probably already had tables built for the chosen challenge.  This scenario is a pretty high bar to reach.

To frame the conversation, there are actually 4 handshakes in the NTLM suite that move up in security and complexity. They are LM, NTLM, NTLM with session security, and NTLMv2.  As of now, only NTLMv2 stands as secure.  There is no way, other than encrypting your link with say IPSec, to secure the 3 weak handshakes.  The good news is, all Microsoft OSs already have a registry key that can control the handshakes options.  MS will release a KB and Advisory on 1/8 on this and the information can be found below.

Now, thanks to Moxie Marlinspike’s Cloudcracker, an attacker can skip the pre-chosen challenge and brute force the challenge response to get the NTLM hash.  If the victim is running XP, the situation is even worse, as Cloudcracker will return the LM hash which can always be broken overnight to derive the user’s password .

Why Does this Matter Now?

Before I get to the exploit details, let me clarify how relevant these technologies still are in today’s world. You might think that with all the papers and presentations, no one would be using NTLM...or, God forbid, LM. NTLMv2 has been around for quite some time. Surely, everyone is using it. Right?

Wrong!

According to the last data from the W3 Schools, 21% of computers are running XP, while NetMarketShare claims it is 39%.  Unless someone has hardened these machines (no MS patches do this), these machines are sending LM and NTLM responses!  While these lists leave out server OSs, 2003 Server still sends NTLM responses by default.  Yes, every MS OS since NT 4.0 SP4 has supported NTLMv2, but NTLM and LM were not excluded by default until Vista.  

But wait, there’s more! It is also very common for companies that have heterogeneous environments to use Active Directory Group Policy to keep the settings weak, usually out of fear of breaking Samba connectivity.  Sure, Samba has supported NTLMv2 for a long time, but most IT folks tend to think “Why beef up security if you might break something? No one is claiming to have broken NTLM.”

Well, here it is: I’VE BROKEN NTLM.

Now, get to fixin’.

More on fixin’ later.  (I can’t take credit for breaking NTLM. I’m no math whiz. I just happen to specialize in applied crypto, and I looked in the right place at the right time.)

The Attack

When I read a summary of Moxie’s MS-CHAPv2 crack, I saw that the big deal was not that the implementation had some crazy flaw, it was that Moxie had affordably built a system that can brute force the DES keys that make up the heart of the challenge response mechanism.  In less than 24 hours, given a known 64 bit plaintext (challenge) and a ciphertext (response), Cloudcracker can return the key to you.  

This made me wonder what else was broken, given affordable DES brute forcing now exists.  

I didn’t dig a lot deeper into the attack at the time, as I was researching NTLM so I could write a blog post on Pass the Hash Attacks.  I know this is well covered territory, but I never found a paper that covered all my questions, so I figured I’d do it myself.  Much of my research was done by reading the protocol details on Eric Glass’s exhaustive page on the topic. I’ve found no better source for understanding the protocols. 

That’s until I stumbled across this: (Bold added for emphasis.)

The NTLM response is calculated as follows:

The MD4 message-digest algorithm (described in RFC 1320) is applied to the Unicode mixed-case password. This results in a 16-byte value - the NTLM hash.

The 16-byte NTLM hash is null-padded to 21 bytes.

This value is split into three 7-byte thirds.

These values are used to create three DES keys (one from each 7-byte third).

Each of these keys is used to DES-encrypt the challenge from the Type 2 message (resulting in three 8-byte ciphertext values).

These three ciphertext values are concatenated to form a 24-byte value. This is the NTLM response.

Note that only the calculation of the hash value differs from the LM scheme; the response calculation is the same.

That’s right, to reverse the challenge response, I just need to brute force two DES iterations that have 56 bit keys and one that only uses 2 bytes of the 56 bit key space for the last crack.  Moxie shows it like this, for those who are more visual.

I immediately went to Moxie’s posts on the topic to figure out how to use Cloudcracker to brute force out my hashes.  That’s when I discovered that MS-CHAPv2 uses the EXACT same math as the LM and NTLM challenge response.  Moxie was kind enough to point me to the line in his code that demonstrates how one can submit the challenge and response to Cloudcracker to get back the LM or NTLM hash.

print "CloudCracker Submission = $99$%s" % base64.b64encode("%s%s%s%s" % (plaintext, c1, c2, k3[0:2]))

Just concatenate your challenge, and the first two thirds of your response, and k3 as base64 and Bob’s your uncle.  The only thing that threw me was, why is k3 only 2 bytes?  To save on processing power, I assume, you must brute force the final key before sending it to Cloudcraker, which just appends it to the first two recovered keys and sends it back.  As the last key is the one that has 5 bytes that are always all 00, this is easy.

Using Eric Glasses example, I sent in $99$ASNFZ4mrze8lqYwcMegYR0ZrKbLfRoDz2Ag=  to Cloudcracker.  This is the challenge 0123456789ABCDEF, low response 25A98C1C31E81847, mid response 466B29B2DF4680F3, and D808, the first two bytes of the final third of the hash, which I brute forced locally.  If you decide to follow along with me, make sure to look at Glass’s parity adjusting of the keys.  DES keys are really 64 bit, not 56, but as the right bit of each byte is a parity bit, it can’t count toward the total entropy or key space.  Or, you can just use his provided Java code.

Then I just had to wait until I got an email with this:

CloudCracker has successfully completed its attack against your CHAPv2 handshake. The NT hash for the handshake is included below, and can be plugged back into the 'chapcrack' tool to decrypt a packet capture, or to authenticate to the server:

cd06ca7c7e10c99b1d33b7485a2ed808                           

This run took 68799 seconds. Thank you for using cloudcracker.com, this concludes your job.

For those who missed it, I just got the NT hash from the challenge and response, which are easily observed on the wire. I just got the hash WITHOUT compromising the host first.  To use this in an attack I just need the right MITM foothold or a phishing email.

To: All Employees

From: HR Communications

Subject: Updates to the Employee Handbook

Body: Human Resources has completed a significant rewrite and update to the Employee Handbook.  While some of the changes are minor, it is worth a look for all employees.  Employees with aging parents will likely be excited to see the increase in paid time off for emergency care of elder dependents.  The guidelines for company events where alcoholic beverages are provided have also been updated. 

Finally, with the passing of Washington Initiative 502, we are publishing the new guidelines for Marijuana in the work place.     

The handbook can be found here:                                                                          

\\hrFiles.ru\HRFiles\EmployeeManualv3.docx 

Best Regards,

Human Resources

I call this attack “Request the Hash”.  Sorry for the pun, but who wouldn’t immediately click the link to find out their companies new pot policy???

The Good News

First of all, I commend MS for changing the default to NLTMv2, as of Vista, and for leaving us the option if we just can’t live without LM or NTLM.  At this point though, I think it is incumbent on MS to push the issue at the local security policy level, letting domain administrators override common sense as needed.

There might be some testing and some interoperability failures for enterprises, but the good news is the settings for LM, NTLM, and NTLMv2 have been around a long time. The reg key HKLM\SYSTEM\CurrentControlSet\Control\Lsa\LmCompatibilityLevel allows you to choose the level of fallback your system will allow.  Set this to 3 or greater for workstations and you are probably set. This situation is a bit more complex as almost all windows computers are not just clients, but also servers, offering you the admin$ share and c$, etc.  The same setting has slightly different bearing on which versions the machine will accept when acting as a server, say when doing remote administration of your desktops. This great article, by Jesper Johansson, covers both the client and server aspect. 

Personally, I am setting all my systems to 5.  If you allow systems to be set below 5, you may be masking the fact that clients are still performing NTLM or LM handshakes. 

Level	Group Policy Name	Sends	Accepts	Prohibits Sending
0	Send LM and NTLM Responses	LM, NTLM, NTLMv2 Session Security is negotiated	LM, NTLM, NTLMv2	NTLMv2 Session Security (on Windows 2000 below SRP1, Windows NT 4.0, and Windows 9x)
1	Send LM and NTLM—use NTLMv2 session security if negotiated	LM, NTLM NTLMv2 Session Security is negotiated	LM, NTLM, NTLMv2	NTLMv2
2	Send NTLM response only	NTLM, NTLMv2 Session Security is negotiated	LM, NTLM, NTLMv2	LM and NTLMv2
3	Send NTLMv2 response only	NTLMv2 Session Security is always used	LM, NTLM, NTLMv2	LM and NTLM
4	Send NTLMv2 response only/refuse LM	NTLMv2 Session Security	NTLM, NTLMv2	LM
5	Send NTLMv2 response only/refuse LM and NTLM	NTLMv2, Session Security	NTLMv2	LM and NTLM

You may ask, “Who cares if my server accepts LM or NTLM?”  If not all your clients are managed, your server could be unwittingly used to compromise an account used by a client with weak settings.

I did a fresh Ubuntu install and verified that its SMB/CIFS client only sent NTLMv2 response, even when I attempted to downgrade using Cain.  I have not yet had a chance to test web browsers that support NTLM auth via SPNEGO.   That’s right; most web browsers perform NTLM auth in HTTP Headers when a trusted site requests it. 

A Reminder About the Downside of Doing Nothing

While web browsers only perform the NTLM auth for trusted sites by default, generally, Windows OSs seem to make no distinction between trusted and un-trusted zones for other protocols, such as CIFS/SMB, SQL/TDS, and RPC.  This means that a phishing attack with a file://server//share link could yield great results for an attacker.  Based on the rapid succession of tries and retries, it is safe to assume that the first 2 – 5 attempts by a machine are the OS, via SSPI, trying to auth with the victim’s cached creds.  This means the attacker does not need to see a successful auth to trust that the credentials are valid. Maybe you took my advice on Pass the Hash, and blocked all the protocols that use NTLM, at your perimeter.  This doesn’t stop one of your clients, say a laptop at home outside your perimeter from trying to do something that won’t try an NTLM handshake. There are just too many ways to fail, if you allow LM or NTLM in any context. OK, so the attacker has the hash, but they are outside the firewall… You’re safe, right?  No!  At this point, the attacker has a valid hash and is ready to use the account to get your data.  Now the attacker has two options:

·         If your system is sending LM responses, getting that hash makes for super light work in cracking the password.  If you are sending the better NTLM hash, you are still in hot water.  Cloudcracker also has a huge set of rainbow tables.  For another $20 I can submit the hash and likely get the password of an average user. There are surely plenty of ways inside your perimeter with just username and password. Moxie, if you are reading this, how about a one step operation (from the customer perspective) that pipes the recovered hash into the rainbow tables after it’s been cracked. Maybe a price a tad under $40 for the two cracks? 

·         BUT WAIT!!  Attackers don’t need your stinking password! If I have the hash, it is password equivalent! The first step of every auth involving NTLM and NTLMv2 is to convert the password to the NT hash.  Your weak settings just saved the attacker a step.  Once the hash has been stolen, the only ways to render it useless are to change the password or remove all of the channels by which it can be passed. (The latter feels infeasible). 

Protecting Yourself and Your Company

If you are a lone user, make sure all of your systems are set to LmCompatibilityLevel 3 or higher via local security policy, group policy, or via the registry key.

If you are a corporate type, make sure the GPO is set to force 3 or higher.  You may notice that levels 4 and 5 talk about domain controllers refusing different levels.  This is not a great protection and may even mask a problem.  While the DC blocking the LM auth passed by a client won’t let the user access the system, the damage is already done: The LM response was sent, and the attacker may have it.  Sure, it is nice for the attacker to see the successful auth so they know the crack is worth the time, but the rapid fails and retries will tell the attacker that the client OS had the creds cached and the hash that was used is almost certainly good, at a minimum on that client machine, if not the domain.

If you are running a Samba client, enter this line ‘client ntlmv2 auth’ into your smb.conf file.

Final Paranoid Thought

If you have any of the weak settings, CHANGE YOUR PASSWORD after fixing them!

I made this discovery when I realized DES was brute-forcible on a budget.  I immediately started thinking of a list of all of the processes that broke along with DES.  I’m sure this is exactly the list that every nations’ spy services started making when they acquired the computing power to brute force DES.  I assume that it has been affordable for most of them for 5 – 10 years now. 

Sorry, spies, if this closes a door for you.  I read a book that says another one will open.  ;-)

Appendix A: Proof of Concept Code

In order to validate my work, I built a simple tool proof the crypto and create the CloudCracker token.  First off, I am not a great or professional coder, so don’t laugh when you look at the code.  Second, Eric Glass’s code was a great help.  His is in Java, while mine is c#.  Last, I used Bouncy Castle’s c# crypto library, for two reasons.  The .NET libraries do not support MD4, because for hashing it is VERY weak and the .NET libraries block the use of weak DES keys.  NTLM does not account for weak keys.

My code is a fair bit redundant, checking and rechecking itself.  This is because each CloudCracker summation costs $20 and I didn’t want to have to send/spend several failed submissions during debugging.

The code can be found here.

The challenge and response data should be in hex, while the password should be ASCII.