Through the exploitation of how service accounts leverage Kerberos authentication with Service Principal Names (SPNs), the adversary obtains and subsequently cracks the hashed credentials of a service account target to exploit its privileges. The Kerberos authentication protocol centers around a ticketing system which is used to request/grant access to services and to then access the requested services. As an authenticated user, the adversary may request Active Directory and obtain a service ticket with portions encrypted via RC4 with the private key of the authenticated account. By extracting the local ticket and saving it disk, the adversary can brute force the hashed value to reveal the target account credentials.

Execution Flow

1) Explore

Scan for user accounts with set SPN values

Technique

• These can be found via Powershell or LDAP queries, as well as enumerating startup name accounts and other means.

2) Explore

Request service tickets

Technique

• Using user account's SPN value, request other service tickets from Active Directory

3) Experiment

Extract ticket and save to disk

Technique

• Certain tools like Mimikatz can extract local tickets and save them to memory/disk.

4) Exploit

Crack the encrypted ticket to harvest plain text credentials

Technique

• Leverage a brute force application/script on the hashed value offline until cracked. The shorter the password, the easier it is to crack.

Prerequisites

The adversary requires access as an authenticated user on the system. This attack pattern relates to elevating privileges.
The adversary requires use of a third-party credential harvesting tool (e.g., Mimikatz).
The adversary requires a brute force tool.

Skills Required

(Level : Medium)

Mitigations

Monitor system and domain logs for abnormal access.
Employ a robust password policy for service accounts. Passwords should be of adequate length and complexity, and they should expire after a period of time.
Employ the principle of least privilege: limit service accounts privileges to what is required for functionality and no more.
Enable AES Kerberos encryption (or another stronger encryption algorithm), rather than RC4, where possible.

Related Weaknesses

References

REF-559

Extracting Service Account Passwords with Kerberoasting
Jeff Warren.
https://blog.stealthbits.com/extracting-service-account-passwords-with-kerberoasting/

REF-585

Kerberoasting Without Mimikatz
https://www.harmj0y.net/blog/powershell/kerberoasting-without-mimikatz/

REF-586

Invoke-Kerberoast
https://powersploit.readthedocs.io/en/latest/Recon/Invoke-Kerberoast/

Submission

Modifications