A brand new zero-day vulnerability in NTLM found by researchers at 0patch permits attackers to steal NTLM credentials by having a consumer view a specifically crafted malicious file in Home windows Explorer — no want for the consumer to open the file. These password hashes can be utilized for authentication relay assaults or for dictionary assaults on the password, each for id takeover.
NTLM refers to a set of outdated authentication protocols from Microsoft that present authentication, integrity, and confidentiality to customers. Whereas NTLM was formally deprecated as of June, our analysis exhibits that 64% of Energetic Listing consumer accounts often authenticate with NTLM — proof that NTLM remains to be extensively used regardless of its identified weaknesses.
The flaw is exploitable even in environments utilizing NTLM v2, making it a major danger to enterprises that haven’t but moved to Kerberos and are nonetheless counting on NTLM. Contemplating Microsoft could not patch this situation for some time, enterprise defenders ought to take steps to mitigate the vulnerability of their environments. This Tech Tip outlines how dynamic entry insurance policies, just a few hardening steps, and multifactor authentication (MFA) can assist restrict makes an attempt to take advantage of this vulnerability. Upgrading the protocol, the place doable, might get rid of the problem utterly.
What Is the NTLM Vulnerability?
When a consumer views a malicious file in Home windows Explorer — whether or not by navigating to a shared folder, inserting a USB drive containing the malicious file, or simply viewing a file within the Downloads folder that was robotically downloaded from a malicious Net web page — an outbound NTLM connection is triggered. This causes Home windows to robotically ship NTLM hashes of the at present logged-in consumer to a distant attacker-controlled share.
These NTLM hashes can then be intercepted and used for authentication relay assaults and even dictionary assaults, granting attackers unauthorized entry to delicate programs. Attackers can even doubtlessly use the uncovered passwords to entry the group’s software-as-a-service (SaaS) surroundings because of the excessive charges of synced customers.
The problem impacts all Home windows variations from Home windows 7 and Server 2008 R2 as much as the most recent Home windows 11 24H2 and Server 2022.
The elemental downside with NTLM lies in its outdated protocol design. NTLM transmits password hashes as an alternative of verifying plaintext passwords, making it susceptible to interception and exploitation. Even with NTLM v2, which makes use of stronger encryption, the hashes can nonetheless be captured and relayed by attackers. NTLM’s reliance on weak cryptographic practices and lack of safety in opposition to relay assaults are key weaknesses that make it extremely exploitable. Furthermore, NTLM authentication doesn’t assist fashionable safety features, reminiscent of MFA, leaving programs open to quite a lot of credential theft methods, reminiscent of pass-the-hash and hash relaying.
What Defenders Have to Do
To mitigate this vulnerability, Microsoft has up to date earlier steerage on how you can allow Prolonged Safety for Authentication (EPA) on LDAP, Energetic Listing Certificates Companies (AD CS), and Change Server. On Home windows Server 2022 and 2019, directors can manually allow EPA for AD CS and channel binding for LDAP. There are scripts supplied by Microsoft to activate EPA manually on Change Server 2016. The place doable, replace to the most recent Home windows Server 2025 because it ships with EPA and channel binding enabled by default for each AD CS and LDAP.
Some organizations should still be depending on NTLM on account of legacy programs. These groups ought to take into account further authentication layers, reminiscent of dynamic risk-based insurance policies, for shielding current NTLM legacy programs in opposition to exploitations.
Harden LDAP configurations. Configure LDAP to implement channel binding and monitor for legacy shoppers that won’t assist these settings.
Verify influence on SaaS. If you’re not sure whether or not there are functions or shoppers in your surroundings that depend on NTLMv2, you should utilize Group Coverage to allow the Community Safety: Prohibit NTLM: Audit incoming NTLM visitors coverage setting. This is not going to block NTLMv2 visitors however will log all makes an attempt to authenticate utilizing NTLMv2 within the Operations Log. By analyzing these logs, you may determine which shopper functions, servers, or companies nonetheless depend on NTLMv2, so you can also make focused changes or updates.
Utilizing Group Coverage to restrict or disable NTLM authentication through the Community Safety: Prohibit NTLM setting will scale back the danger of fallback situations the place NTLM is unintentionally used.
Monitor SMB visitors. Enabling SMB signing and encryption can assist forestall attackers from impersonating respectable servers and triggering NTLM authentication. Blocking outbound SMB visitors to untrusted networks will even scale back the danger of NTLM credential leakage to rogue servers. Implement community monitoring and alerting for uncommon SMB visitors patterns, notably outbound requests to unknown or untrusted IP addresses.
Go away NTLM behind. NTLM has been deprecated. Directors ought to audit NTLM utilization to determine which programs nonetheless depend on NTLM. Organizations ought to prioritize transitioning these programs away from NTLM to extra fashionable authentication protocols, reminiscent of Kerberos. As soon as a extra fashionable protocol is in place, implement MFA so as to add an extra layer of safety.
Taking these steps will assist organizations deal with the elemental flaws in NTLM and enhance their safety posture.
