Zerologon Explained: How a Math Mistake Could Hand Over a Windows Domain

What Happened

Netlogon is the protocol Windows machines use to authenticate to a domain controller. Part of its cryptographic handshake used a mode of AES encryption (AES-CFB8) with a fixed, all-zero initialization vector. Because of how that specific mode behaves, roughly 1 in 256 attempts at sending a crafted authentication request would succeed by pure chance — and an attacker could just keep trying. Once successful, they could reset the domain controller's own computer account password to a known value.

What This Means

This is a cryptographic implementation flaw, not a logic bug in application code: the math was being used incorrectly, which broke the security guarantee the protocol was supposed to provide. Resetting the domain controller's password effectively handed an attacker the keys to the entire Windows domain — every user, every computer, every policy.

Why You Should Care

Active Directory domain controllers are the trust anchor for most enterprise Windows networks. Zerologon mattered because it required no valid username or password at all — just network access to a domain controller — and within weeks of public proof-of-concept code being released, ransomware groups were actively using it to take over entire corporate networks in a single step.

What You Can Do

• Apply Microsoft's Zerologon patches and, critically, complete the enforcement-mode rollout — the initial patch alone left systems in a compatibility mode that could still be bypassed.
• Restrict and monitor network access to domain controllers; they shouldn't be reachable from general user network segments.
• Watch for repeated failed Netlogon authentication attempts, which is the signature of an active Zerologon attempt.
• Don't assume cryptography is "correct by default" just because it uses a well-known algorithm like AES — the mode and parameters matter just as much as the algorithm.