Reversible Encryption

What the technique is

An adversary may abuse Active Directory authentication encryption properties to gain access to credentials on Windows systems. The AllowReversiblePasswordEncryption property specifies whether reversible password encryption for an account is enabled or disabled. By default this property is disabled (instead storing user credentials as the output of one-way hashing functions) and should not be enabled unless legacy or other software require it. If the property is enabled and/or a user changes their password after it is enabled, an adversary may be able to obtain the plaintext of passwords created/changed after the property was enabled. To decrypt the passwords, an adversary needs four components: 1. Encrypted password (G$RADIUSCHAP) from the Active Directory user-structure userParameters 2. 16 byte randomly-generated value (G$RADIUSCHAPKEY) also from userParameters 3. Global LSA secret (G$MSRADIUSCHAPKEY) 4. Static key hardcoded in the Remote Access Subauthentication DLL (RASSFM.DLL) With this information, an adversary may be able to reproduce the encryption key and subsequently decrypt the encrypted password value. An adversary may set this property at various scopes through Local Group Policy Editor, user properties, Fine-Grained Password Policy (FGPP), or via the ActiveDirectory PowerShell module. […]

What MITRE says you would watch

• AN1621

  Detects enabling of reversible password encryption in Active Directory or Group Policy, suspicious PowerShell commands modifying AD user properties, and unusual account configuration changes correlated with policy modifications. Multi-event correlation links Group Policy edits, PowerShell command execution, and user account property changes to identify tampering with authentication encryption settings.