Tuesday, January 15, 2013

NTLM hasn’t been relevant for like 12 years... and other lies.

A surprising number of foolish Slashdotters have pointed out that my latest work, breaking the NTLM and LM handshakes and phishing for users’ NT hashes, is totally irrelevant and has been for 12ish years.  

As a fan of debate, I’ll start with points that are interesting but have no real bearing on the topic.
  • Slashdotters are clearly not qualified to make this assessment.  Their Appeal to Authority fails.
  • Microsoft wouldn’t issue an advisory and Fix-It if it weren’t relevant.  My Appeal to Authority is better than theirs.  ;-)
  • The existence of a newer protocol, Kerberos, does not make NTLM simply disappear.
  • Wikipedia actually doesn’t say NTLM is long dead.  Wikipedia as an appeal to authority is a joke.  I link to it regularly, not for its completeness, but because it is written for a layman audience.  It is a great place to start if you don’t know something.
  • I’m not a Linux fanboy looking to disgrace MS.  I’m a long time MCSE and even gave MS some props in my post.  
  • Finally, my work is what it is, probably the last nail, of hundreds, in the coffin.  I make no claim to it being inspired by God.

Now to the relevant details...  When MS introduced Active Directory in Windows 2000, they implemented Kerberos 5 as the default authentication protocol FOR DOMAIN ACCOUNTS. This is a pretty important requirement. If a machine is not domain joined, or the account is not a domain account, Kerberos is not an option. The upside here is that when machines are in workgroups, it is much less likely that the accounts will have any sort of value off of the host.  However, this does not stop the SSPI from trying to authenticate using your cached account credentials when accessing resources that are not on the host.  This means a workgroup host could still be vulnerable to my “Send the Hash” attack if not properly configured.

Even for machines that are domain joined, while Kerberos is the default, NTLM is used in several situations:
  • If the service is not Kerberos enabled (Kerberized).  Maybe it runs on an NT server?
  • The service/server does not have a Service Principal Name (SPN) registered.
  • The service/server has duplicate SPNs registered.   
  • When accessing the system by IP rather than name.
  • Improperly built clusters.
  • 3rd party system implemented incorrectly.
  • When accessing data across forests, using an older domain type trust.
  • When the client can’t access a KDC/DC, such as when it is outside the firewall.
  • When the KDC/DC is behind NAT.

Before getting in depth with a couple of these cases, I’ll make a generalization, “Kerberos is very hard to get right, except under simple conditions.”

Outside the Firewall or Behind NAT
MS’s implementation of Kerberos requires that the clients, servers, and KDC/DCs all be on the same routed network with AD integrated DNS or DDNS that allows the DCs to register SRV records. The clients must be able to find and access the KDCs to get Kerberos tickets.  I am not going to cover all the details of Kerberos, but this is a key difference.  With NTLM, the server you want to access does the job of finding a DC and getting the DC to validate the challenge/response after your client has done its handshake.  The resource server passes the challenge and response to the DC over RPC using packet privacy and gets back a pass/fail and a list of group memberships which it uses to build the user’s access token. This is super simple and easy when the client is outside your firewall.  You only need to open one port, the application's port.  

If you intend to make Windows Kerberos work across NAT or behind a firewall, prepare for pain.  Each Windows client has a component called the dcLocator.  Its exact operations vary slightly from version to version of Windows.  You might think you just need to open up TCP88 to a KDC and you’re set.  You might get a pony in the mail too.  

I’ll blog on the exact details at some point, but the dcLocator first needs to find the KDC DNS SRV records in the _msdcs.domainname.org zone.  Right off the bat, this means that you need split DNS, as the answers inside your firewall will not be the same IP as outside. Once you have your external DNS zone and main DC records, the client will ping all the DCs and select the fastest to respond. If ICMP is blocked, nothing proceeds. The client sends a CLDAP query to the fastest DC. This is connectionless LDAP over 389 UDP.  This query is to ask which AD site the client is a member of.  This query is not answered in a traditional way, based on the filter.  Instead, AD uses the source IP to map the IP to an AD subnet which maps to an AD site, which the LDAP search response will contain.  If your client is behind NAT, then the source IP will likely be a SNAT address.  From this response, the dcLocator then does a second DNS SRV query to get the DCs that are in the AD site returned from the CLDAP query.  The dcLocator then pings each of those DCs and the first to respond is queried and if the response is satisfactory, then this becomes the default DC for a period of time.  This time can vary by OS version.  Now we are ready to do Kerberos. Some versions of Windows try UDP88 first and then when they get back the “response too large” they try TCP88 route. If UDP is blocked, these versions may not try TCP88 even if it is open. I will not be swearing to this in court as it has been over a year since I configured this type of scenario and I am writing this without a net.  

This means, that for Kerberos to work outside the firewall or behind NAT, you need to:
  • Setup Split DNS
  • Create at least one domain level SRV pointing to the external IP address
  • Create a site DNS SRV record for EVERY DC in the default site, pointing to the external IP address.
  • Open port 389 UDP
  • Open port 88 UDP
  • Open Port 88 TCP
  • Open ICMP
Kerberos Diagram

NTLM is a lot easier to use in both NAT and outside the firewall scenarios.

NTLM Diagram

Messed up SPN Scenarios
A Service Principal Name (SPN) maps an instance of a service running on a server to the account that it runs under.  Kerberos uses shared secrets, between each party and the KDC/DC, to allow for authentication and key exchange.  When a client wants to use a service, it asks the KDC for a ticket to the specific service/server combination.  The ticket is encrypted by the KDC so that only the service can open it.  If there is no SPN registered for the service, a ticket can’t be granted.  If two accounts have the SPN registered, the ticket cannot be granted.  If the service is running under the context of a different account, the ticket cannot be decrypted.  

Many web admins just allow their appPools to run as local system or network service.  This means they are running under the machine account, which is a domain account and has an SPN or at least can have one.  In order to make your cluster work with Kerberos, all appPools must run under the same domain account and usually an SPN must be manually created by an admin. MS has made some changes to IIS to make this easier, but in many cases it is out of the frying pan...

SQL server tries to register its SPN whenever it comes online.  If you are following best practices, and using a domain account to run SQL, then SQL will try to register the SPN attribute on that account.  This is a proper configuration, however most accounts so not have the SELF write SPN permission (ACE), and fail.  This means that Kerberos won't work and NTLM is negotiated.

Connection by IP Address

This fails as there is not an SPN set that uses the IP, such as HTTP/ Rather the SPN is HTTP/www.domain.com.  This can be overcome by registering the IP SPN, but does not scale well.  I’ve thought about building a simple tool to do this, but my list of to-dos is long.

In the end, the threat and attack models are very different for the enterprise and stand alone users.

Enterprise Exposure

The enterprise is likely to block many outgoing ports, making it less likely that “send the hash” attacks will succeed.  This however doesn’t mean there is no risk from NTLMv1.  With any type of foothold, the attack is VERY effective.  All it takes is one compromised host, one badge not checked, a network port activated outside of your secure areas, etc.
Enterprise users spend time in hotels, airports, hospitals, etc. These types of places are ripe for the picking. 

Workgroup Exposure

For workgroup members, the threats vary considerably.  The user may spend much of their time behind NAT, which may make it hard for an attacker who steals a hash to use it.  The user may spend time in coffee shops, etc, making them ripe for attack.


While Kerberos is more secure than NTLMv2, it is not really fair to say it is better.  They both have pros and cons.  NTLM is VERY commonly used today both by design and due to it being a fallback for failed Kerberos.  Depending on your systems’ settings, you may be sending LM and NTLMv1.  ALL VERSIONS of Windows still accept LM and NTLM by default, so you may still be allowing the issue. You may not be initiating dirty handshakes, but you will accept them if offered.
Finally, the fix is very simple. There is almost no downside to making the change. If due to some crazy twist of fate you have a system that only works with NTLMv1 or LM, that system will break.


Tuesday, January 8, 2013

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

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 whitepapershacker 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?
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:
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:                                                                          
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. 
Group Policy Name
Prohibits Sending
Send LM and NTLM Responses
NTLMv2 Session Security is negotiated
Session Security (on Windows 2000 below SRP1, Windows NT 4.0, and Windows 9x)
Send LM and NTLM—use NTLMv2 session security if negotiated
NTLMv2 Session Security is negotiated
Send NTLM response only
NTLMv2 Session Security is negotiated
LM and NTLMv2
Send NTLMv2 response only
Session Security is always used
Send NTLMv2 response only/refuse LM
NTLMv2 Session Security
Send NTLMv2 response only/refuse LM and NTLM
Session Security

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 policygroup 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. 

Friday, January 4, 2013

Rehashing Pass the Hash

The first question one might ask is, “Really…  Why are you writing about this old news now?”  The answer is simple; even with the release of Window 8 and Server 2012, Pass the Hash (PTH) attacks are still incredibly simple and effective.  While from an academic standpoint passing the hash is simple to understand, it is a bit more complex from an attacker or defender’s stand point.  The reason for this post is that having read a huge number of papers and post on pass the hash, none of them gave me a clear picture.   

The reason pass the hash is so powerful is due to NTLM’s initial design flaw.  NTLM was essentially broken from the day it was released.  While UNIX started saving salted hashes in the 1970s, the designers of NTLM chose to use an unsalted hash.  Salting a hash is probably the most important design criterion when designing an authentication system.

A hash is a one way transformation of a variable input to a fixed length output.  A hash function must be deterministic, meaning that the same input will always generate the same output, no matter what platform it might be run on. Another key property of a good hash function is that even a small change in the input creates a significant change in the output.  The final vital property of a hash function is that it must be truly one way.  There must be no method allowing the input to be derived using only the output.

With these properties, and the benefit of hindsight, a hash is the perfect way to for one party to prove they know a password without the other party actually knowing it.   Storing reversible or god forbid plaintext passwords introduces a significant number or risks.  Only the ignorant do this.

If the hash can’t be reversed, then you might think there are no more risks and you can just hash those passwords and be on your merry way.  If we didn’t have a security model to think about, this might make sense.   The reason we bother to hash the passwords and not store them as plaintext on our authentication servers is that, on the off chance that the auth server is compromised, we want to minimize the value of the stolen identities.  Assuming you kick out the attacker and restore from backup, it would be nice to not have to force all of your users to change their passwords.  It would be nice to be sure that the breach wasn’t just a foothold to a larger attack. Additionally, as users tend to reuse passwords, we don’t want to put the users’ accounts at risk on other systems or sites.  This brings us to the risk created by just storing the password hash with no salt.  If one has access to the hash and wants to derive the input (password), all that needs to be done is test passwords until the outputs (hashes) match.  This Brute force method is always a theoretical option when trying to defeat a cryptographic function.  Trying all possible passwords is time consuming, which works against the attacker.  Unfortunately, with super fast processors and huge amounts of cheap storage, it is now possible to pre-compute and store huge numbers of hashes and the corresponding passwords.  In the space of less than 1 TB, you can store tables that cover 96% of 1 to 9 character passwords containing upper case, lower case, and numbers.  By storing just the password hash we make it completely worth an attacker’s time to create and save the pre-computes values.  Even better, you can just pay when you need to use someone else’s’ tables.

Simply salting the hash wastes all that work the attacker did to pre-compute the hashes.  Salting a hash is simply adding random or pseudo-random data to the input before computing the hash.  The salt is saved and later used when verifying the password.  When verifying the password, the salt is looked up, re-applied to the password that is being tested, and the data hashed.  If the result on file and the real-time computed result match, the password was correct. By applying the random data, we have completely wasted the attackers’ pre-computation effort.  Ideally, every account uses a different salt, so that even if your user database is taken, every password must be brute forced separately. 

Salted Hash Example

Password    Salt  SHA-1 Hash

V3ryStrong  1     88 93 EB 7A 28 DA A4 95 89 B1 B5 A1 E0 C6 A0 83 9D 38 A3 39

V3ryStrong  2     59 86 41 B2 62 B7 C3 C7 54 27 78 94 FB D6 59 24 5F 77 74 40


The example is purposely weak to make for easy understanding.  Usually the salt would be 8 – 16 bytes. It shows the amplification provided by SHA-1.  With a single bit change the data is completely different and we see that there is no way to pre-compute all passwords, unless we pre-compute all passwords with all possible salt values, which in not feasible.   

While this is fun background data, it really has little to do with passing the hash, as PTH lets an attacker who has the hash assume the user’s identity without ever knowing the password.  This is because windows stores the user’s NT hash in Active Directory DIT or the SAM.  The first step of NTLM and NTLMv2 handshakes is that the system converts the user’s password to the NT hash, and the handshake does some fun handshake math that proves the user knew the hash, without the hash ever crossing the network. 

This system makes the hash “password equivalent”.  If an attacker has the user’s hash, they just skip the “convert password to NT hash” and go straight to using it in the handshake.   There are many tools for doing this, depending on where NTLM is being used. NTLM authentication occurs in-band with the protocol that is being used to access data or access a system.  NTLM auth occurs inside remote file access via CIFS/SMB, TDS for accessing SQL server, RPC for all sorts or remote administration and access, HTTP for web auth, and I’m sure a few more things. 

In order for an attacker to pass the hash, they need four things:

1.       The valid NT hash (and username)

2.       A channel to pass the hash through

3.       A server hosting a service that utilizes/allows NTLM auth

4.       A client (or tool) that allows the attacker to input the hash rather than the password

In a world where all four elements are available to an attacker, there is no stopping PTH.  Keeping in mind the four requirements, defenders can limit, and more importantly, and understand their level of exposure. 

Securing the Hash

The most important thing a defender can do is keep the hash from falling into the wrong hands.  The hash resides on a computer in two main areas; the SAM (Security Account Manager) db and in RAM, for users that are logged in.  There is a common misconception that the NT hash is also stored in the registry for cached logon of domain user accounts on domain joined computers.  This is just that, a misconception. Depending on system settings, data is stored that allows a domain user to logon to a machine when no domain controller is available; however this data is an MD4 of the NT hash + username.  It might be worth an attackers’ time to pre-compute the hashes for common user names, such as ‘Administrator’. 

Looking at the threat model, one can conclude that the horse is out of the barn if the hash is stolen.  In order to steal the hash, the attacker had to gain full control of the system.  The attacker must either execute hash stealing code as a member of local administrators, or gain physical access to the poorly protected computer.  While the horse is indeed out of the barn on that host, the attacker should be able to go no further.  At least that is the defenders hope. This is, of course, not the case.  If the user has the other three required elements, she can act as the user at will. 

Stealing the Hash from RAM

In order for an attacker to steal a user’s NT hash from RAM, the attacker must run code with administrator rights.  As with all the advice that will be offered, all the controls are fairly standard and map to basic security concepts.

·         Reduce surface area

o   Don’t leave sessions logged in unnecessarily, as another administrator’s failure can compromise your account.

o   Avoid logging in with administrator rights.  If you screw up and run attackers code, they can’t steal the hash if you aren’t an admin.

·         Use up to date AV software

Stealing the Hash via Physical Access

We all know that if an attacker has physical access to a host then they own the host.  The last thing we want to do is allow the attacker to use that host as a foothold to even more systems and data.  In the case of PTH attacks, we probably have good news here.  The NT hashes are stored in the local SAM. If the computers are not domain joined, the username and password usually don’t cross hosts, unless you took the time to sync up your usernames and passwords across them all.  As I mentioned earlier, the cached domain credentials DO NOT contain the NT hash. This means the SAM from one computer is of no value on anther.  

The one place where there IS a great deal of risk is in an enterprise, where standardization probably means that the local admin password is the same across every domain member.   This means that the compromise of any one server is equivalent to the compromise of every server.  This means that that dev server that you left up running PHP 1.2 can be a great foothold to the rest of your servers and data.

To reduce this risk:

·         Use full disk encryption that requires a token or Password Based Key Derivation Function

·         Enable syskey protection > 1.

o   This requires a password or floppy to boot and decrypt the goodies in the SAM.

o   This scales poorly for an enterprise.

·         Make sure all local admin passwords are different.

Stealing the Hash from the Network

The NTLM and NTLMv2 handshakes assure that the NT hash itself can be verified, without it ever appearing on the network.  There is nothing to see here.

If an attacker were to figure out a way to get the hash from the network, it would be an impressive feat.

Securing the Channels

If the attacker steals the hash, but has no path to pass it over, then the hash is of little value.  One nice thing about NTLM auth is that it occurs in band with the protocol utilizing it, so as a defender, you can better understand and limit your exposure.  If you block ports 80, 138,139, and 445, but open 1433, you know that the stolen hash can only be used to access SQL data but not remote file access or websites.

Controls to consider:

·         Reduce Surface Area. Only open the ports you need.  Duh…

·         Authenticate end points with VPN or IPSec.

Stop using NTLM

This one is not so simple.  One can “kerberize” all sorts of servers.  That is, allow them to utilize Kerberos authentication rather than NTLM.  Microsoft has done a pretty good job kerberizing applications, but in almost all cases, if Kerberos fails, the app will allow fallback to NTLM.

I know of no options for a defender here.  This section is really a call to MS, and others, to allow for more granular options related to NTLM auth, Kerberos, and fallback options.

I suppose this is an opportunity for firewall vendors as well.  It would be fairly easy to block NTLM auth and allow Kerberos on a per protocol basis.  I suspect an F5 ninja could write a crazy iRule to do it too.

Stop Clients from Allowing the use of the NT Hash

Assuming you have dealt with the firewall rules, most attackers will have no channel to pass hashes over.  The last option would be to pass from hosts inside your network.  This seems plausible, as the attacker got enough access to steal the hash.  

There are two classes of tools that let you work from the hash rather than the password.  The first are tools like WCE, which run on windows and allow an administrator to inject arbitrary usernames and hashes into a user’s session.  This allows all subsequent uses of any tool in that session to authenticate as the injected user.  This means that any Windows tool that speaks NTLM will work.  The second class are custom tools that may run on any OS and speak the protocol in question, be it SMB, TDS/SQL, RPC, or whatever, AND they speak NTLM taking a hash rather than password.

For a good list of current tools and tons of great info on PTH, see Still Passing the Hash 15 Years Later.

A good list, granted probably dated, of AV test results for detection of the tools, can be found here.

Controls to consider:

·         Make sure EVERY host inside your firewall has up to date AV.

o   Enforce this with NAC or NAP.

o   Make sure hosts with AV match up to your CMDB, such that all hosts are accounted for.

·         Consider AppLocker and Software Restriction Polices.

o   Make sure to block known tools by hash so the attacker can’t just rename at tool

o   Make sure to block known tools by name, in case the attacker makes code changes but chooses to use the same executable name.

o   Consider white listing known good apps rather than trying to blacklist known bad ones.

·         Minimize the number of local administrators.  A local admin can stop AV or add exclusions prior to copying over and loading up tools.

·         Keep up on the latest version of the tools and test your AV yourself.

Understanding Why Pass the Hash Still Works

The best way to learn this stuff is to go to Eric Glass’s NTLM page and do the math along with him.  I am borrowing heavily here to summarize for lazy.  ;-)  Well, there is a lot more great stuff on his page that you don’t need to know to understand PTH.

There are three different types of NTLM responses:

1.       NTLM Response

2.       NTLM2 Response, not to be confused with NTLMv2 Response

3.       NTLMv2 Response

NTLM Response

1.       Convert password to Unicode and apply MD4 hash.  This is the NT hash.

2.       Pad the NT hash with 0s to make it 21 bytes.

3.       Break the 21 bytes in to 3 keys of 7 bytes each.  This is 56 bits.

4.       Apply odd parity to each key.  This makes each key a valid 64 bit DES key.

5.       Encrypt the NTLM challenge (from the server) with each of the three keys.

6.       Concatenate the three encrypted values, creating the response.

7.       Send it.

As you can see, the challenge is provided by the server, and could be substituted by an attacker with network access.  This allows an attacker to create pre-computed responses (rainbow tables) for that challenge.  Obviously this is scary.

NTLM2 Response

Once again, not to be confused with the NTLMv2 Response.  This one uses nearly the same math as the NTLM Response, but let’s the client add a “client challenge” thus negating the value of an attacker substituting their own challenge.   This is used when the 0x00080000- NTLM 2 session security flag is set.

1.       Convert password to Unicode and apply MD4 hash.  This is the NT hash.

2.       Pad the NT hash with 0s to make it 21 bytes.

3.       Break the 21 bytes in to 3 keys of 7 bytes each.  This is 56 bits.

4.       Apply odd parity to each key.  This makes each key a valid 64 bit DES key.

5.       The client creates a random 8 byte client challenge.

6.       The server challenge and client challenge are concatenated and MD5 is applied, creating the challenge data.

7.       Encrypt the challenge (MD5 output) with each of the three keys.

8.       Concatenate the three encrypted values, creating the response.

9.       Send it.

NTLMv2 Response

This response is next level.  It is totally different that the first two and includes a timestamp, the domain the user belongs to, as well as target information.

1.       Convert password to Unicode and apply MD4 hash.  This is the NT hash.

2.       Concatenate the upper case Unicode username with the Unicode authentication target.

3.       Perform HMAC-MD5 on the result of step 2, using the NT hash.

4.       Construct blob, containing the Blob signature, 4 bytes of all 0s, the time in NT time format, the client challenge, another 4 bytes of all 0s, and the target information from the server Type 2 (challenge) message, and another 4 bytes of all 0s.

5.       The server challenge form the Type 2 message is concatenated to the blob from step 4.

6.       The blob from step 4 is then HMAC-MD5’d using the result of the HMAC-MD5 in step 3.

7.       The HMAC response from step 6 and the blob from 4 are concatenated and constitute the response.

8.       Send it.


Pass the hash attacks are alive and well and are not going away anytime soon.  Protect your systems. 

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