Monday, March 3, 2014

Guest Post: Let's talk about Pass-the-Hash by Scriptjunkie

March is Pass-the-Hash awareness month. (#PtHAwareness) Pass-the-Hash affects virtually all of us with Windows networks, but authentication issues like Pass-the-Hash are often misunderstood. This month, take time to ensure you are PtH aware and help us spread awareness.  

What are the problems?

Pass-the-Hash refers to using a password hash to authenticate to a remote Windows system. Password hashes for all local accounts are stored locally on all Windows computers, hashes for all domain accounts are stored on all domain controllers for a Windows domain, and hashes for currently-logged-in users whether local or domain are usually stored in memory in the computer the user is logged into. (some exceptions apply, see below for details) But there are actually many attacks that pass hashes, and stopping one does not stop the rest. It's debatable whether any one of these is more important or more significant than the others; all of them have been frequently used by pentesters and real hackers. Which one matters most changes depending who you ask, which is why one person can think X solves the problem, while another person doesn't think X touches the main problem, so why you need to understand all of the problems. To better explain, I'll give you them as a series of stories:  

  1. Evil hacker Eve compromises a workstation on the DOH windows domain with local admin privileges. (Maybe Eve compromised user DOH\Alice with a drive-by-download exploit or an Excel macro and escalated privileges using an HP driver vulnerability. Or maybe Eve physically snuck in and added a backdoor to the hard drive; it doesn't really matter; these things happen.) The first thing Eve does (after sitting back and evilly cackling Buwahahah!) is to dump the local hash database, which local administrators are allowed to do at least three different ways.* Eve now has the hashes for all local accounts. Eve now uses those user accounts and hashes to try to remotely compromise other systems on the same LAN. If the password to a local administrator accounts is the same, and the account is not prohibited from remotely logging in, Eve will succeed in remotely compromising those systems, owning more of the network and moving inevitably closer to world domination.  
  2. Evil hacker Eve compromises Alice's workstation on the DOH windows domain with local admin privileges, just like before. But this time, Eve searches memory for tokens of a DOH admin. Since Alice's computer needed a Java update yesterday, DOH\adminBob, a help desk admin remotely logged in via Remote Desktop to install it. Bob's hash is left in memory (along with his password if he used a password, and his Kerberos ticket). Eve steals Bob's hash and passes it to other computers, compromising all the workstations in the DOH domain, and stealing everybody's data. World dominated, since the adminBob account had privileges to remotely control all the workstations.
  3. Evil hacker Eve got access to the DOH hash database. Maybe Eve found a backup of a domain controller, maybe Eve snuck into the server racks, maybe Eve hacked one of the DOH domain admins or stole a ticket at one point in time. Eve now has the hashes to every account on the domain. Eve then uses that as a way to maintain elevated privileges. Eve can also use them to remotely pull all of the email from all users’ accounts if OWA accepts NTLM authentication. Eve can use these creds to log in as any domain user and quickly obtain access to any sharepoint page, database, etc. Eve can also use these hashes to compromise other domains if any user account or group in DOH has been granted privileges on another domain that has a trust relationship with DOH. If there is a VPN server using an MS-CHAP variant that accepts users’ passwords, Eve can now VPN in as any of those users. Eve can create a "golden ticket" (forged Kerberos ticket) using the hash of the KRBTGT account, which will allow Eve to authenticate as an admin even after the admin changes his or her password.
*inject LSASS & dump, export registry backup and extract, or raw disk volume access. One of Microsoft's new mitigations is restrictions preventing injecting into LSASS without a signed kernel driver, but since there are many ways to dump hashes and get kernel code running, it doesn't make a huge difference.

It didn't have to be this way

Some people simply accept all of these flaws as inherent in single-sign-on and centralized authentication. But they don't have to be. For example, it's easy to dismiss 3. saying "Well, Eve already has full access to the domain" but there's a few reasons why this doesn't have to be so bad. Let's suppose Alice & Bob find out their DOH domain was hacked. They change all the passwords to all the users. They look for and get rid of all malware. But Eve is able to quickly regain full control of the domain. Or maybe Alice & Bob added something like DNS whitelisting, which broke Eve's access until Eve sent another phishing email. But Eve doesn't have to do any work to re-escalate privileges since Eve has permanent stored credentials. "Re-entry attacks" are a real threat to real networks, and dramatically increase the cost of recovery. Let's think about some other ways Microsoft could have done it:

  • Instead of saving a shared secret based on each user's password, Microsoft could have generated a password-derived public/private key pair, and only stored the public part. Story 1 and story 3 wouldn't exist, since stored hashes could not be re-used for authentication; there would be no pass-the-hash.
  • Instead of saving creds in memory, Microsoft could have simply required users to enter their password every time they access a new network resource and used salted hashes or something like that. This would help address 2, but wouldn't be practical since everyone hates re-entering passwords.
  • Microsoft could have stored a public key and required smart card auth for every network resource. This might slow down things, but would address 1, 2, and 3.

In each of those alternatives, the attacker in 3 couldn't conduct many of those attacks without changing the password to a user, which is a big no-no for most serious attackers since users would notice and might blow the whole operation. PtH allows attackers to easily and silently obtain access to all users' data without being detected.

But here we are; with two main ways to authenticate to a system. Either you log in and give your creds to the system or you don't. If you do, you probably logged in via RDP, or locally at the console, or via a saved password (e.g. service or batch logon). If you didn't, you used network authentication, either with Kerberos or NTLM. The hash is used directly in NTLM, but not in Kerberos. (If you're using passwords, the password hash will be used to get a Kerberos ticket, but not if you're using smart cards. In all cases, the hash of the krbtgt account will be used as the Kerberos controller's secret to authenticate and encrypt Kerberos tickets.) So as long as NTLM is enabled, the hash can be passed. And the krbtgt hash can be used to get yourself a Kerberos ticket at any time. Those can be stolen anywhere there is interactive authentication and used anywhere there is network authentication.

Stop Leaving Your Creds Lying All Around The Network

For 2, Microsoft introduced new security features in Windows 8.1; by default domain accounts no longer store plaintext passwords in memory on a system, should clean up the hash and ticket better on logout, and if the account is a Protected User, the hash will not be present either (although a Kerberos TGT will be). If the account logged in remotely via RestrictedAdmin RDP, no stealable credentials will be put on the remote system at all. But that means in order to authenticate, RestrictedAdmin RDP now uses only network authentication, which means either NTLM or Kerberos will work by default. So you can now pass-the-hash (or ticket) to RDP. Of course, if you already had an admin hash, you could pass-the-hash with psexec and take over the remote system that way if windows SMB/RPC (ports 445,135,139...) were exposed, but because of the increased risk of these ports, there are millions of systems that are only exposed over RDP, and not SMB or RPC. They may soon become vulnerable to pass-the-hash. The advantage is that one of our biggest problems, the one I call "leaving your creds lying all around the network" is dramatically reduced, assuming your admins stick to RestrictedAdmin RDP. This is a big win, assuming you don't allow RDP anywhere you don't allow SMB.

Protected Users highlight the importance of 3; if a domain admin was a Protected User and logged into a compromised server, the attacker who controls that server would have temporary access to that domain admin's account via the TGT. By dumping the domain's hash database with that TGT, the attacker now has permanent access with full control over the domain. However, I still consider Protected Users an improvement since the exposure duration of kerberos tickets (a few hours) is vastly less than the exposure duration of hashes (good forever until password change). Microsoft also introduced authentication silos, which restrict where an account can log in from. If you can write that detailed of policies for your admins, e.g. only allow login from admin workstations, that will also help a lot. Anyone elsewhere in the network with their hash will be unable to pass-the-hash with their accounts. Of course that doesn't address all the user PtH.

No Permanent Stored Credentials

For 3, if an attacker obtains access to your domain hash database, your only good option is to adopt a policy I call "No permanent stored credentials." In order to do this, you must disable NTLM in your domain, and mandate that any account that can remotely authenticate uses a smart card. Then you must also shorten the computer account password change timeline and frequently (every few days) change the password to the krbtgt account to a random value as well. This will prevent any users' hashes from being used in remote authentication, and no permanent credentials will ever be stored on disk or in-memory. You may not be able to implement this policy if you have a lot of legacy equipment and software, so some of my PtH cohorts don't like it, but I'm the build-a-secure-network-guy and I highly recommend it if you are building a new network. No reason to keep doing stupid just because you've always done stupid. This has big benefits; in addition to significantly reducing the exposure of credentials to theft, attackers will be forced to maintain access with malware on critical systems which you have a chance of finding, rather than simply holding creds to pull data at will, which you will have very little chance of finding. Attackers will find re-entry attacks much harder since any stolen or forged credentials will quickly become invalid.

Note that there are a lot of other problems with Windows authentication, especially if you're using passwords. People leave them lying around in shared drives and sticky notes, they can be guessed, everyone reuses them, they're hard to remember, people type them into the pages that look like login pages but aren't, they're easy to lock out or brute force online, and their hashes whether salted, unsalted, or captured in a challenge-response can be cracked because hardware is cheap but human memory is terrible. And NTLM is just all-around bad. But this month, we're just talking about Pass-the-Hash.

Bottom Line

is that hashes come from many places and can be used many ways. Microsoft's new mitigations can help you if you're careful, Keep Local Local; if you watch out for the other pitfalls, Stop Leaving Your Creds Lying All Around The Network; but I still recommend you have No Permanent Stored Credentials as well. And for crying out loud, get rid of passwords or this guy will hunt you down and crack them all.

1 comment: