Modify Authentication Process: Password Filter DLL

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.[1]

ID: T1556.002
Sub-technique of:  T1556
Tactics: Credential Access, Defense Evasion
Platforms: Windows
Permissions Required: Administrator, SYSTEM
Data Sources: DLL monitoring, File monitoring
Contributors: Vincent Le Toux
Version: 1.0
Created: 11 February 2020
Last Modified: 25 March 2020

Procedure Examples

Name Description
Remsec

Remsec harvests plain-text credentials as a password filter registered on domain controllers.[2]

Strider

Strider has registered its persistence module on domain controllers as a Windows LSA (Local System Authority) password filter to acquire credentials any time a domain, local user, or administrator logs in or changes a password.[2]

Mitigations

Mitigation Description
Operating System Configuration

Ensure only valid password filters are registered. Filter DLLs must be present in Windows installation directory (C:\Windows\System32\ by default) of a domain controller and/or local computer with a corresponding entry in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Notification Packages.

Detection

Monitor for new, unfamiliar DLL files written to a domain controller and/or local computer. Monitor for changes to Registry entries for password filters (ex: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Notification Packages) and correlate then investigate the DLL files these files reference.

Password filters will also show up as an autorun and loaded DLL in lsass.exe.[3]

References