By now, everybody has probably read many articles on the latest viral outbreak, ranging from detailed analysis of the virus itself to rants about why companies don’t apply updates in a timely fashion and why employees continue to open malicious attachments in emails.
In this post, I thought I’d take a step back and look at some of the virus’ features that make it interesting from my perspective as a penetration tester, as it mimics some of the attacks we use on a weekly basis during our penetration tests. It also highlights the importance of some of the recommendations we provide that are often overlooked because the threat is rarely fully understood.
Some of the details are still a little blurry, but one of the initial infection vectors seems to have been the update mechanism of a software used by many companies in Ukraine. This is far from the classic phishing attacks where everybody can blame the end user for the infection. And despite the fact that this new attack also uses the EternalBlue exploit to propagate internally, it is most likely not the entry point into any company, in part due to the recent WannaCry ransomware which urged companies to finally apply the appropriate updates. For once we can’t just blame missing updates and irresponsible employees!
The second interesting part of this particular malware is the fact that it leverages mimikatz-like functionality to steal passwords and re-use them to infect other machines using PsExec or WMIC. This is very similar to what is performed during targeted attacks or penetration tests. After compromising an initial host, the attacker moves laterally through the network with any compromised credentials in order to take control of as many other hosts until the desired sensitive data is compromised. Obviously, the more privileged the credentials that were initially compromised are, the easier it will be for the attacker to gain access to other hosts. In the case of the current outbreak, given the fact that there seems to be no recovery possibility, and on top of this your backups happen to be accessible on the network, you will lose everything.
If at any point during an attack a domain administrator account (or similarly privileged account) is compromised, it is pretty much game over for the company, as the attacker has essentially compromised the whole company, and recovering from this is an extremely painful process. Some people will say that their most sensitive applications and data are not located on Windows servers and therefore not impacted by this, but in all likelihood, the legitimate employees who have access to the data do it from a Windows workstation. An attacker could therefore profit from this legitimate access to compromise the “secure” data as well.
Given the Ukrainian connection to the infection vector, the impact in Switzerland will probably be relatively low, but it is not unreasonable to believe that a similar attack could be performed against software that is regularly used within this country too. I can certainly think of a number of banking software that are used by a majority of Swiss companies that might be an interesting target. It gets worse if you think about Adobe, Java or even Windows (anybody remember Flame?).
So what can we do to reduce the risk of this happening in the future?
The answer in my eyes lies within two of the fundamentals of information security that are unfortunately very rarely implemented correctly:
- Network segmentation & filtering
- Least privileges
Network segmentation is generally pretty well understood and most companies have dedicated VLANs for various types of devices, but too often there is no firewalling performed between the different networks, making it all too easy to move around laterally or exploit vulnerabilities in obscure protocols that run on exotic ports that could very easily be blocked.
On top of a network firewall, we often recommend the use of Windows’ integrated firewall to block off SMB and RDP ports to avoid lateral movement within the same subnet. Typically these ports should be accessed only by a restricted team to manage the workstations or for support purposes. These rules can for example be deployed by GPOs.
If these rules had been deployed in the case of the current outbreak, if an initial host was compromised, the attack would be contained to that machine, as it couldn’t use PsExec, WMIC or EternalBlue to replicate, therefore drastically reducing its impact.
Nowadays, every company has one or several firewalls, but are unfortunately rarely used to their full potential. They can be used for a lot more than just routing traffic from one subnet to another. It takes a little time to set them up correctly to allow only authorised, necessary traffic, but this remains one of the best preventive measures against virus outbreaks and attacks.
For the second aspect of “least privileges”, I have very rarely encountered a company that truly performs this well at all levels, and I will group several different weaknesses that we see in pretty much every penetration test we perform:
- Use of excessive privileges by services
- Use of the same local administrator password on multiple hosts
- Excessive use of domain administrator (or other privileged) accounts
Before delving into why these issues are so important, I have to mention that access to a Windows workstation or server can be granted to either local users that are defined on the host itself, or domain users (as long as the machine is part of a domain) that are defined in the Active Directory.
It is also important to understand that apart from in certain circumstances, a user with local administrator privileges can use tools such as mimikatz to recover not only the NTLM password of all locally defined accounts, but also the plain-text password (or some times only NTLM hash) of any domain account authenticated to the host. Given that the NTLM hash can essentially be used as a password thanks to pass-the-hash techniques, all local accounts and any connected domain accounts can be completely compromised by a local administrator.
This should illustrate quite well why the three weaknesses above are so important.
- Any compromised service that is running with local administrator (SYSTEM) privileges will give the attacker the possibility of compromising all the local and connected domain accounts. Some of the services that we exploit the most that are nearly always affected by this are Tomcat, JBoss and SQL Server.
- If a local administrator account is compromised and that the same password is used on other hosts, an attacker can very easily use pass-the-hash attacks to compromise all of these other servers. Essentially, by using the same password on multiple machines, you’re transforming a local administrator account into a “global” administrator account. Microsoft has published a tool called LAPS which makes it pretty easy to manage a separate password for all domain-connected Windows machines, though it is also possible to simply disable the account all together.
- “Domain admin” accounts should only ever be used to connect to a domain controller and as sparingly as possible. They should never be used in day-to-day administration tasks. As was explained above, any time you connect to a server or workstation that has previously been compromised by an attacker, you are gifting him your account and privileges. It is therefore pretty obvious that privileged accounts should be used as sparingly as possible to avoid this scenario. We’ll often recommend segmenting the Active Directory into various groups granting access to only a limited subset of the company’s servers or workstation to reduce the impact the compromise of such an account can have.
It is well understood nowadays that it is not a question of “if” an attacker or virus gets in, but “when”. This does not mean that securing your perimeter is pointless, but it means that you have to prepare for a breach and do your best to reduce its impact.
So I’ll wrap this up with the following recommendations that cover the points mentioned above and that we give in most of our penetration tests:
- Use your firewalls for what they were designed for and filter access between your various subnets
- Use Window’s built-in firewall to restrict access to management ports
- Use LAPS or a similar password management system to avoid having the same password on multiple systems
- Disable local administrator accounts if you don’t actually use them
- Avoid running services with local admin privileges when it is not required
- Make sure your day-to-day accounts do not have admin privileges, and have separate accounts for administrative tasks. (It seems obvious, but given recent pentest results, it’s worth mentioning that the passwords for these accounts should be different…)
- Put your privileged accounts in the “Protected Users” group to avoid credentials theft
- Only use your domain admin accounts on domain controllers, and use them as sparingly as possible
- Make sure your backups are stored securely and that they are recoverable even in the case of a complete domain compromise
Often in the past it has been difficult to promote security within a company because it has always been hard to establish the business impact of a breach, but by reading the current news, you’ll see many companies crippled by the fact they have lost control of all their Windows machines, and maybe in some cases their backups. I’ll let you imagine the consequences that could have in your company.