Windows Server 2025 must prevent NTLM from falling back to a Null session.
Severity | Group ID | Group Title | Version | Rule ID | Date | STIG Version |
|---|---|---|---|---|---|---|
| medium | V-278221 | SRG-OS-000480-GPOS-00227 | WN25-SO-000270 | SV-278221r1181369_rule | 2026-02-20 | 1 |
Description
NTLM sessions that are allowed to fall back to Null (unauthenticated) sessions may gain unauthorized access.
ℹ️ Check
If the following registry value does not exist or is not configured as specified, this is a finding:
Registry Hive: HKEY_LOCAL_MACHINE
Registry Path: \SYSTEM\CurrentControlSet\Control\LSA\MSV1_0\
Value Name: allownullsessionfallback
Type: REG_DWORD
Value: 0x00000000 (0)
✔️ Fix
Configure the policy value for Computer Configuration >> Windows Settings >> Security Settings >> Local Policies >> Security Options >> Network security: Allow LocalSystem NULL session fallback to "Disabled".