A brand new zero-day vulnerability in NTLM found by researchers at 0patch permits attackers to steal NTLM credentials by having a person view a specifically crafted malicious file in Home windows Explorer — no want for the person to open the file. These password hashes can be utilized for authentication relay assaults or for dictionary assaults on the password, each for identification takeover.
NTLM refers to a collection of outdated authentication protocols from Microsoft that present authentication, integrity, and confidentiality to customers.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 person accounts commonly authenticate with NTLM — proof that NTLM continues to be extensively used regardless of its recognized weaknesses.
The flaw is exploitable even in environments utilizing NTLM v2, making it a big danger to enterprises that haven’t but moved to Kerberos and are nonetheless counting on NTLM. Contemplating Microsoft might not patch this subject 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, a number of hardening steps, and multifactor authentication (MFA) may help restrict makes an attempt to use this vulnerability. Upgrading the protocol, the place doable, might eradicate the difficulty utterly.
What Is the NTLM Vulnerability?
When a person 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 person 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 methods. Attackers may doubtlessly use the uncovered passwords to entry the group’s software-as-a-service (SaaS) setting 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 newest 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The elemental downside with NTLM lies in its outdated protocol design.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 help trendy safety features, similar to MFA, leaving methods open to quite a lot of credential theft methods, similar to pass-the-hash and hash relaying.
What Defenders Have to Do
To mitigate this vulnerability, Microsoft has up to date earlier steerage on learn how to allow Prolonged Safety for Authentication (EPA) on LDAP, Energetic Listing Certificates Providers (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 offered by Microsoft to activate EPA manually on Change Server 2016. The place doable, replace to the newest Home windows Server 2025 because it ships with EPA and channel binding enabled by default for each AD CS and LDAP.
Some organizations should be depending on NTLM because of legacy methods. These groups ought to take into account extra authentication layers, similar to dynamic risk-based insurance policies, for safeguarding current NTLM legacy methods in opposition to exploitations.
Harden LDAP configurations. Configure LDAP to implement channel binding and monitor for legacy purchasers that will not help these settings.
Test impression on SaaS. In case you are uncertain whether or not there are functions or purchasers in your setting that depend on NTLMv2, you should use Group Coverage to allow the Community Safety: Limit 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 establish which shopper functions, servers, or providers nonetheless depend on NTLMv2, so you can also make focused changes or updates.
Utilizing Group Coverage to restrict or disable NTLM authentication by way of the Community Safety: Limit NTLM setting will cut back the danger of fallback situations the place NTLM is unintentionally used.
Monitor SMB visitors. Enabling SMB signing and encryption may help stop attackers from impersonating professional servers and triggering NTLM authentication. Blocking outbound SMB visitors to untrusted networks may also cut back the danger of NTLM credential leakage to rogue servers. Implement community monitoring and alerting for uncommon SMB visitors patterns, significantly outbound requests to unknown or untrusted IP addresses.
Go away NTLM behind. NTLM has been deprecated. Directors ought to audit NTLM utilization to establish which methods nonetheless depend on NTLM. Organizations ought to prioritize transitioning these methods away from NTLM to extra trendy authentication protocols, similar to Kerberos. As soon as a extra trendy protocol is in place, implement MFA so as to add a further layer of safety.
Taking these steps will assist organizations tackle the basic flaws in NTLM and enhance their safety posture.
