Net-NTLMv1 Is No Longer Safe: Rainbow Tables Explained

Category: Threat Intelligence / Infrastructure Security

In the world of cybersecurity, some protocols are like old buildings—they’ve been standing so long that we forget they are structural hazards. On January 15, 2026, Mandiant (a Google Cloud company) performed a "controlled demolition" of one such protocol. By releasing a massive dataset of Net-NTLMv1 Rainbow Tables to the public, they have made this 30-year-old Windows standard a massive liability for most organizations.

At ZyberWalls, we’ve broken down exactly what this means, why it’s dangerous, and how to close the door before a hacker walks through it.

Illustration of a hacker cracking an old Windows Net-NTLMv1 password using rainbow tables, showing a steel door with weak padlocks representing password chunks

1. The "Cheat Code" Concept: What are Rainbow Tables?

Imagine you are trying to guess a 4-digit PIN. You could sit there and try 0000, 0001, 0002... this is Brute Force. It works, but it takes time.

Now, imagine someone hands you a book that has every possible 4-digit PIN on the left and its encrypted "digital fingerprint" (hash) on the right. If you find a fingerprint, you just look it up in the book and instantly see the PIN.

That "book" is a Rainbow Table. It’s a massive database of pre-solved math problems that is built long before the attack, not created live.

  • The Mandiant Release: They released a "library" that covers almost every possible Net-NTLMv1 password hash.

  • The Hardware: Previously, you needed a supercomputer. Now, a hacker can crack a high-value account password in under 12 hours (depending on password length) using a normal gaming PC.

2. The Weak Link: Why Net-NTLMv1 is the Target

Windows uses a system called NTLM (New Technology LAN Manager) to let you log into network resources. The oldest version, Net-NTLMv1, has a fatal design flaw: it uses very old encryption from the 1970s.

The "Three Padlocks" Example

Imagine your password is a giant, heavy steel door. To make it "compatible" with old systems, Net-NTLMv1 breaks your password into three separate chunks and puts a cheap, tiny padlock on each one.

  • Chunk 1: About 7 characters

  • Chunk 2: About 7 characters

  • Chunk 3: The remaining pieces (usually weak because it’s padded with empty data)

A hacker doesn't have to break your whole password at once. They pick the three tiny locks one by one. Because the locks are so small (56-bit by today’s standards), modern computers can "pick" them in seconds.

3. The Attack: From a "Network Request" to Full Control

How does a hacker actually use this? They use a technique called Coerced Authentication. This does not require malware on the target.

  1. The Trigger: A hacker sends a fake network request to a high-level server pretending to be a printer or file share.

  2. The Response: The server says, "Okay, let me log in to you." It sends its "digital fingerprint" (the Net-NTLMv1 hash) to the hacker.

  3. The Look-up: The hacker takes that fingerprint, runs it through the Mandiant tables, and can recover the password.

  4. The Takeover: If the cracked account has high privileges, they can create a "Silver Ticket"—a permanent, invisible pass to access any file, email, or camera on that system or service.

4. Why is this happening now?

Mandiant released these tables to force companies to stop delaying upgrades. Many businesses keep Net-NTLMv1 turned on because they have old printers or ancient accounting software. The risk has moved from unlikely to easily abused.

5. The ZyberWalls "Kill Switch" (What to do)

Check your LMCompatibilityLevel setting in the Windows Registry:

  • Levels 0–2: Danger Zone. Your network is wide open.

  • Level 3: Better, but still allows risky behavior if a server requests it.

  • Level 5: Safest. This rejects Net-NTLMv1 entirely.

    • Note: This may break very old devices—that is expected. If something breaks, that device is too old to be trusted.

The "XYZ" Test: Try to connect to your server using a fake username like XYZ-User. If logs show the server tried a Net-NTLMv1 handshake, your door is unlocked. (Important: This test is for admins with logging enabled. Do not test on production during business hours.)

6. Indicators of Compromise (IOCs)

These indicators matter only before Net-NTLMv1 is fully disabled. Since the "cracking" happens offline, watch for the Capture and Abuse phases:

  • Static Challenge: Watch for the NTLM "Server Challenge" value 1122334455667788 in network traffic. This is the "key" for the Mandiant tables.

  • Event ID 4776: Look for "Success" audits where the Authentication Package is MICROSOFT_AUTHENTICATION_PACKAGE_V1_0.

  • Event ID 4624: Watch for Logon Type 3 (Network) where the "Package Name" is NTLMv1.

  • Unexpected RPC Calls: High-value servers (like Domain Controllers) making unrequested outgoing connections to random internal workstations.

ZyberWalls Verdict

The Mandiant release is a wake-up call. In 2026, compatibility is no longer an excuse. If you're still using Net-NTLMv1, you aren't "old school"—you’re exposed by design.

Stay Alert. Stay Human. Stay Safe. — ZyberWalls Research Team

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