This article was cross-posted on Corelight’s blog.

CVE-2020-1472 aka Zerologon, disclosed by Tom Tervoort of Secura, is an illustrative case study of how a small implementation mistake in cryptographic routines cascades into a privilege escalation vulnerability that allows an attacker to change the password of any unpatched Active Directory domain controllers to which they have network access. Upon successful exploitation, the attacker is free to alter additional credentials, escalate to the level of a domain admin, and move laterally to other machines in the domain. At a high level, the encryption scheme as implemented has a 1256 chance of encrypting a plaintext message of all zeroes to a ciphertext message of all zeroes, which eventually leads to setting a zero length password. If this sounds as interesting to you as it was to me, I’d recommend reading the more in-depth technical report also from Secura.

To assist, we’ve open sourced a Zeek package that detects both attempted and successful exploits. Using Secura’s excellent, defanged proof-of-concept Python tool, we generated sample PCAPs for unsuccessful and successful attacks on both Windows Server 2016 and 2019 domain controllers. These pcaps are included in the repository. I would be remiss if I didn’t also mention these techniques for detection: Sigma rule by SOCPrime, and Splunk by Shannon Davis. These served as inspiration for the Zeek package.

There are fully functional exploit tools for this CVSS 10.0 rated vulnerability already floating around publicly, so I recommend reading Microsoft’s support guide entry, patching your domain controllers, and looking for signs of historic exploitation attempts in your logs, and looking for future attempts with this Zeek package.

Below are snippets from the Zeek package that define the Notices that are generated and the global “knobs” that when redefed can be used to tune for different network traffic profiles. The package will run in both clustered and non-clustered environments.

redef enum Notice::Type += {
    Zerologon_Attempt,
    Zerologon_Password_Change
};

export {
    # Time window of attack. A higher value will catch more careful
    # attackers, but at the potential cost of more false positives.
    global expire = 2min &redef;
    # Minimum required number of NetrServerReqChallenge and
    # NetrServerAuthenticate3 pairs before considering it to be an
    # attempted attack.
    global cutoff = 20 &redef;
    # Change this to T if you only want a notice to be generated for a
    # successful exploit.
    global notice_on_exploit_only = F &redef;
}

Happy hunting!