CSC News Table of Contents
NTLM hash leak risks are now harder to exploit on Windows. Microsoft has disabled certain downloaded file previews that silently triggered credential exposure. The change blocks background network requests from risky preview handlers and cuts a common path to stolen hashes.
The update targets low friction attacks where a preview in File Explorer could send hashed credentials over SMB or WebDAV. It reduces opportunistic credential theft in mixed Windows domains.
Microsoft positions the change as part of a broader plan to deprecate NTLM and harden default authentication paths across Windows.
NTLM hash leak: What You Need to Know
- Microsoft blocked downloaded file previews from making background lookups that could cause an NTLM hash leak.
What changed and why Microsoft moved fast
Microsoft is preventing an NTLM hash leak by stopping preview handlers from fetching external resources when files carry a Mark of the Web flag.
When Windows previews a document, image, archive, or shortcut that references remote content, legacy authentication can attempt SMB or WebDAV negotiation. That silent exchange may send hashed credentials to attacker infrastructure.
As noted in a SecurityWeek report, the fix prevents Explorer and related components from performing background network lookups that could trigger an NTLM hash leak. A downloaded file’s presence in a preview pane is now far less likely to expose credentials.
Bitdefender provides endpoint protection, network attack defense, and exploit prevention.
1Password offers an enterprise password manager to reduce NTLM reuse and credential sprawl.
Passpack delivers a team password vault with access controls to limit lateral movement.
IDrive enables encrypted backups that protect data after credential theft.
Auvik monitors networks to spot suspicious SMB and WebDAV activity tied to hash leakage attempts.
Tenable identifies legacy NTLM exposures and weak configurations.
Tresorit supports end to end encrypted file sharing that reduces risky file handling paths.
Windows file preview vulnerability, how the attack chain worked
In this Windows file preview vulnerability, a malicious file or a document with a remote reference is saved locally and then viewed in Explorer.
The preview triggers a background request to an external resource, often over SMB or WebDAV, which leads to an NTLM hash leak to the attacker.
Captured hashes can be cracked offline or used in relay attacks where NTLM is still enabled. That made the NTLM hash leak valuable for initial access brokers and red teams because it required only folder browsing.
Inside the Microsoft security update NTLM hardening
The Microsoft security update NTLM change modifies default behaviors tied to the MOTW flag on downloaded content.
Where previews once pulled remote metadata, thumbnails, or embedded links, the system now blocks those calls if they could trigger an NTLM hash leak. Organizations should see fewer unsolicited outbound requests to untrusted hosts during preview events.
Microsoft has signaled a steady shift away from NTLM across Windows. See the plan to deprecate NTLM and policy guidance for restricting legacy authentication in domains via Network Security, Restrict NTLM.
What security teams should do now
Defense in depth remains essential to contain any NTLM hash leak and related credential theft.
- Patch Windows endpoints and servers promptly. Review recent fixes for critical issues, including multiple Microsoft zero day patches and February Windows zero day fixes.
- Disable NTLM where feasible and enforce SMB signing to disrupt relays. See CISA guidance.
- Harden outbound egress for SMB on TCP 445 and 139 and WebDAV. Monitor for anomalous name resolution or authentication traffic.
- Apply UNC hardening, prefer Kerberos, and consult MITRE ATT&CK on credential relays at T1557.
- Educate users to treat downloaded files carefully. Be wary of archives, shortcuts, and documents with embedded content.
- Track related paths such as Windows LDAP attack techniques that intersect with legacy authentication.
Operational impacts to expect
Most organizations will see minimal disruption. Some third party preview handlers or workflows that rely on remote metadata in previews may change behavior.
In those cases, administrators should avoid policies that re enable risky previews that could cause an NTLM hash leak. Favor safer patterns such as signed metadata or offline thumbnails.
What this change means for security teams
Advantages
By removing the easiest route to an NTLM hash leak, Microsoft lowers the success rate of broadly targeted credential theft. The change aligns with the deprecation of NTLM and encourages modern authentication.
Telemetry should show fewer unsolicited outbound connections from Explorer previews, which simplifies detection and incident response.
Disadvantages
Legacy applications that depend on remote lookups during previews may lose convenience or need updates.
Blue teams must still watch for alternate paths to an NTLM hash leak, such as malicious LNK files opened directly or embedded resources in unsandboxed viewers. Environments that rely on NTLM need a measured migration plan to avoid breaking workflows.
Tenable Nessus identifies systems at risk from legacy NTLM and SMB exposures.
EasyDMARC reduces phishing that can precede credential capture and relay attacks.
Optery removes exposed personal data that can aid targeted credential theft.
Auvik provides visibility into SMB and WebDAV traffic patterns for early detection.
IDrive offers immutable backups that withstand extortion after compromise.
Bitdefender blocks payloads that exploit stolen hashes to move laterally.
Conclusion
Microsoft closed a quiet but potent path to an NTLM hash leak through File Explorer previews. The update curbs background network lookups that exposed credentials.
Security teams should pair the change with NTLM reduction, SMB signing, and Kerberos enforcement. These steps limit relay attacks and blunt lateral movement.
Continue monitoring for new preview or handler techniques that could revive an NTLM hash leak. Layered controls and strong egress policies remain the safest approach.
Questions Worth Answering
What is an NTLM hash leak?
It occurs when Windows sends a hashed version of credentials over the network during SMB or WebDAV authentication, which attackers can capture or relay.
Does this disable all file previews?
No. It targets previews of downloaded files that could initiate risky network calls. Local and trusted previews continue to work.
Is this a fix for a Windows file preview vulnerability?
Yes. It reduces the Windows file preview vulnerability where background lookups during previews could cause credential exposure.
Can attackers still relay NTLM after this change?
Yes through other vectors. Disable NTLM where possible, enforce SMB signing, and prefer Kerberos to block relays.
How can I tell if my environment still uses NTLM?
Audit Windows event logs, run vulnerability scans, and review domain policy. Microsoft provides domain guidance for restricting NTLM.
Why is Mark of the Web relevant?
The MOTW flag marks downloaded content. Windows now blocks preview lookups for MOTW tagged files to prevent an NTLM hash leak.
About Microsoft
Microsoft develops Windows, Microsoft 365, Azure, and security technologies used worldwide. Its platforms underpin identity, productivity, and cloud services.
The company advances modern authentication and reduces legacy protocols while improving enterprise security baselines across the ecosystem.
Through the MSRC and product teams, Microsoft ships patches, advisories, and tools that help organizations mitigate evolving threats at scale.
