The End of NTLMv1: Mandiant Releases Rainbow Tables to Kill Legacy Logins

Mandiant has disseminated an exhaustive repository of rainbow tables engineered to decrypt the antiquated Net-NTLMv1 protocol. This initiative is designed to accelerate the obsolescence of a technology deemed insecure since the late 1990s. Notwithstanding the fact that its vulnerabilities have been meticulously documented for decades, it persists within corporate infrastructures, where it is still sporadically employed for authentication.

Experts posit that the enduring presence of Net-NTLMv1 is the result of technical inertia coupled with a perceived lack of imminent threat. However, demonstrating its inherent peril has now become considerably more straightforward. Previously, such an incursion necessitated either the utilization of third-party online services—thereby jeopardizing confidential data—or the acquisition of exorbitant hardware. With the publication of these tables, key recovery is achievable within hours, even on consumer-grade hardware valued at less than $600.

The tables are accessible via the Google Cloud research data portal, with authenticity verified through SHA512 hashes. Members of the password-cracking community have already derived specialized versions and hosted them on independent platforms. The execution of the attack is predicated on extracting a Net-NTLMv1 hash without Extended Session Security (ESS). By leveraging a predictable sequence, such as 1122334455667788, it becomes possible to execute a known-plaintext attack. This methodology facilitates the restoration of the NT hash for an Active Directory object—be it a user or a computer—thereby paving the way for privilege escalation.

A particularly grave scenario involves coerced authentication on behalf of a Domain Controller. Upon securing its account hash, an adversary may utilize DCSync to replicate credentials for other objects within the domain. To harvest the requisite hash, practitioners frequently employ the Responder utility with ESS disabled and configured to a fixed authentication value. Once captured, the hash is bifurcated into segments corresponding to DES components, which are then processed via specialized rainbow table search software.

The final stage involves reconstituting the complete NT hash and verifying it through a specialized mode in Hashcat. Subsequently, a DCSync attack can be orchestrated utilizing secretsdump.py from the Impacket toolkit.

Mandiant emphasizes that organizations must expeditiously disable Net-NTLMv1 via Windows Group Policy. However, merely altering configurations is insufficient; a persistent threat remains if an intruder gains local access and manually reverts the system to a vulnerable state. For timely detection, it is recommended to monitor Windows Event Logs for Event ID 4624, scrutinizing the authentication information for “LM” or “NTLMv1” within the package name field. This publication was curated with the support of the research community, drawing upon open-source intelligence, repository code, and expert discourse.