One Weird Trick To Secure You PCs

Avecto released a report which analyzed recent Microsoft vulnerabilities and found that 92% of all critical vulnerabilities reported by Microsoft were mitigated if when the exploit attempt happened on an account without local administrator permissions. Subsequently, there has been a lot of renewed discussion about removing admin rights as a mitigation from these kinds of vulnerabilities.

Generally, I think it’s a good idea to remove admin rights if possible, but there are a number of items to think about which I discuss below.

First, when a user does not have local administrator rights, a help desk person will generally need to remotely perform software installs or other administrative activities on the user’s behalf. This typically involves a support person logging on to the PC using some manner of privileged domain account which was configured to have local administrator rights of the PCs. Once this happens, a cached copy of the login credentials used by the support staff are saved to the PC, albeit in a hashed manner. Should an attacker be able to obtain access to a PC using some form of malware, she may be able to either brute force recover the password from the hash or use a pass-the-hash attack, which would grant the attacker far broader permissions on the victim organization’s network than a standard user ID would. Additionally, an attacker who already has a presence on a PC may use a tool such as mimikatz to directly obtain the plain text password of the administrative account.

You might be thinking “but, if I remove administrator rights, attackers would be very unlikely to gain access to the PC in manner to steal hashes or run mimikatz, both of which require at least administrator level access. What gives?”

That is a good question which dovetails into my second point. The Avecto report covers vulnerabilities which Microsoft deems the severity to be critical. However, most local privilege escalation vulnerabilities I could find are only rated Important by Microsoft. This means that if even if you don’t have administrator rights, I can trick you into running a piece of code of my choosing, such as one delivered through an email attachment or even using a vulnerability in another piece of code like Java, Flash Player or PDF reader, and my code initially would be running with your restricted permissions, however my code could then leverage a privilege escalation flaw to obtain administrator or system privileges. From there, I can then steal hashes or run mimikatz. Chaining exploits in attacks is not all that uncommon any longer, and we shouldn’t consider this scenario to be so unlikely that it isn’t worth our attention.

I’ll also point out that many organizations don’t quickly patch local privilege escalation flaws, because they tend to carry a lower severity rating and they intuitively seem less important to focus on, as compared to other vulnerabilities which are rated critical.

Lastly, many of the recent high profile, pervasive breaches in recent history heavily leveraged Active Directory by means of credential theft and subsequent lateral movement using those stolen credentials. This means that the know-how for navigating Active Directory environments through credential stealing is out there.

Removing administrator rights is generally a prudent thing to do from a security standpoint. A spirited debate has been raging for years on whether removing administrator rights costs money, in the form of additional help desk staff who now have to perform some activities which users used to do themselves and related productivity loss by the users who now have to call the help desk, or is a net savings because there are less malware infections, less misconfigurations by users, less incident response costs, and associate higher user productivity, or if those two factors simply cancel each other out. I can’t add a lot to that debate, as the economics are going to be very specific to each organization considering removing administrator rights.

My recommendations for security precautions to take when implementing a program to remove admin rights are:
1. Prevent Domain Administrator or other accounts with high privileges from logging into PCs. Help desk technicians should be using a purpose-created account which only has local admin rights on PCs, and systems administrators should not be logging in to their own PCs with domain admin rights.
2. Do not disable UAC.
3. Patch local privilege escalation bugs promptly.
4. Use tools like EMET to prevent exploitation of some Oday privilege escalation vulnerabilities.
5. Disable cached passwords if possible, noting that this isn’t practical in many environments.
6. Use application whitelisting to block tools like mimikatz from running.
7. Follow a security configuration standard like the USGCB.

Please leave a comment below if you disagree or have any other thoughts on what can be done.

H/T @lerg for sending me the story and giving me the idea for this post.

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