Steal or Forge Kerberos Tickets: AS-REP Roasting

Adversaries may reveal credentials of accounts that have disabled Kerberos preauthentication by Password Cracking Kerberos messages.[1]

Preauthentication offers protection against offline Password Cracking. When enabled, a user requesting access to a resource initiates communication with the Domain Controller (DC) by sending an Authentication Server Request (AS-REQ) message with a timestamp that is encrypted with the hash of their password. If and only if the DC is able to successfully decrypt the timestamp with the hash of the user’s password, it will then send an Authentication Server Response (AS-REP) message that contains the Ticket Granting Ticket (TGT) to the user. Part of the AS-REP message is signed with the user’s password.[2]

For each account found without preauthentication, an adversary may send an AS-REQ message without the encrypted timestamp and receive an AS-REP message with TGT data which may be encrypted with an insecure algorithm such as RC4. The recovered encrypted data may be vulnerable to offline Password Cracking attacks similarly to Kerberoasting and expose plaintext credentials. [1][3]

An account registered to a domain, with or without special privileges, can be abused to list all domain accounts that have preauthentication disabled by utilizing Windows tools like PowerShell with an LDAP filter. Alternatively, the adversary may send an AS-REQ message for each user. If the DC responds without errors, the account does not require preauthentication and the AS-REP message will already contain the encrypted data. [1][3]

Cracked hashes may enable Persistence, Privilege Escalation, and Lateral Movement via access to Valid Accounts.[4]

ID: T1558.004
Sub-technique of:  T1558
Tactic: Credential Access
Platforms: Windows
System Requirements: Valid domain account
Permissions Required: User
Data Sources: Authentication logs, Windows event logs
Contributors: Dan Nutting, @KerberToast; Jacques Pluviose, @Jacqueswildy_IT; James Dunn, @jamdunnDFW, EY; Swapnil Kumbhar
Version: 1.0
Created: 24 August 2020
Last Modified: 20 October 2020

Mitigations

Mitigation Description
Audit

Kerberos preauthentication is enabled by default. Older protocols might not support preauthentication therefore it is possible to have this setting disabled. Make sure that all accounts have preauthentication whenever possible and audit changes to setting. Windows tools such as PowerShell may be used to easily find which accounts have preauthentication disabled. [5][3]

Encrypt Sensitive Information

Enable AES Kerberos encryption (or another stronger encryption algorithm), rather than RC4, where possible.[6]

Password Policies

Ensure strong password length (ideally 25+ characters) and complexity for service accounts and that these passwords periodically expire. Also consider using Group Managed Service Accounts or another third party product such as password vaulting. [6]

Detection

Enable Audit Kerberos Service Ticket Operations to log Kerberos TGS service ticket requests. Particularly investigate irregular patterns of activity (ex: accounts making numerous requests, Event ID 4768 and 4769, within a small time frame, especially if they also request RC4 encryption [Type 0x17], pre-authentication not required [Type: 0x0]).[6][7][8]

References