Microsoft released NTLMv1 in the 1980s with the release of OS/2. In 1999, cryptanalyst Bruce Schneier and Mudge published research that exposed key weaknesses in the NTLMv1 underpinnings. At the 2012 Defcon 20 conference, researchers released a tool set that allowed attackers to move from untrusted network guest to admin in 60 seconds, by attacking the underlying weakness. With the 1998 release of Windows NT SP4 in 1998, Microsoft introduced NTLMv2, which fixed the weakness.
Organizations that rely on Windows networking aren’t the only laggards. Microsoft only announced plans to deprecate NTLMv1 last August.
Despite the public awareness that NTLMv1 is weak, “Mandiant consultants continue to identify its use in active environments,” the company said. “This legacy protocol leaves organizations vulnerable to trivial credential theft, yet it remains prevalent due to inertia and a lack of demonstrated immediate risk.”
The table first assists attackers in providing the proper answer to a challenge that Windows sends during the authentication process by using a known plaintext attack with the challenge 1122334455667788. Once the challenge has been solved, the attacker obtains the Net-NTLMv1 hash and uses the table to rapidly crack it. Typically tools including Responder, PetitPotam, and DFSCoerce are involved.
In a thread on Mastodon, researchers and admins applauded the move, because they said it would give them added ammunition when trying to convince decision makers to make the investments to move off the insecure function.
“I’ve had more than one instance in my (admittedly short) infosec career where I’ve had to prove the weakness of a system and it usually involves me dropping a sheet of paper on their desk with their password on it the next morning,” one person said. “These rainbow tables aren’t going to mean much for attackers as they’ve likely already got them or have far better methods, but where it will help is in making the argument that NTLMv1 is unsafe.”
The Mandiant post provides basic steps required to move off of NTLMv1. It links to more detailed instructions.
“Organizations should immediately disable the use of Net-NTLMv1,” Mandiant said. Organizations that get hacked because they failed to heed will have only themselves to blame.
