Kerberos Authentication

For a protocol-level walkthrough of the exchanges summarized below, see Tarlogic’s Kerberos article.^[3]

TL;DR for attackers

Kerberos is the default AD auth protocol; most lateral-movement chains will touch it.
Think in three operator phases:^[3]
- AS-REQ / AS-REP → password/hash/certificate to obtain a TGT. This is where AS-REP roasting, over-pass-the-hash / pass-the-key, and PKINIT live.
- TGS-REQ / TGS-REP → use a TGT to obtain service tickets. This is where Kerberoasting, S4U abuse, delegation abuse, and most ticket-forging tradecraft become relevant.
- AP-REQ / AP-REP → present the ticket to the service. This is where pass-the-ticket and service-specific lateral movement happen.
For hands-on cheatsheets (AS-REP/Kerberoasting, ticket forgery, delegation abuse, etc.) see: Readme
Use this page as the overview / “what changed recently” index, then jump to the dedicated pages for Kerberoast, Resource-Based Constrained Delegation, AD Certificates / PKINIT abuse, or BadSuccessor / dMSA abuse.

Fresh attack notes (2024-2026)

RC4 hardening changed the defaults, not Kerberos itself – modern DC hardening focuses on the default assumed encryption types for accounts that do not explicitly set msDS-SupportedEncryptionTypes. After the 2026 rollout, those accounts increasingly default to AES-only on patched DCs, so blind /rc4 Kerberoast assumptions fail more often. However, explicitly RC4-enabled service accounts remain excellent offline-crack targets.^[1]
PAC validation enforcement matters for forged tickets – 2024 PAC-signature hardening means that golden/diamond/sapphire/extraSID-style abuses need more realistic PAC data and the correct signing context. Unpatched domains or domains left in compatibility/audit-style deployments stay softer targets.^[2]
Certificate-based Kerberos changed twice:
- Strong certificate binding (KB5014754 timeline) makes sloppy certificate-to-account mappings less reliable in fully enforced environments.
- CVE-2025-26647 added another hardening layer around altSecurityIdentities mappings that use a certificate’s Subject Key Identifier. Patch level, enforcement or audit state, and explicit mapping configuration therefore matter when evaluating pass-the-certificate and related certificate-based paths.^[5]^[6] For PKINIT, the KDC also validates the certificate path and checks that the issuer is trusted through the NTAuth store.^[8]
Cross-domain / cross-forest delegation abuse is still very alive – Windows supports modern cross-realm S4U2Self/S4U2Proxy flows, so writable delegation attributes in another domain are still valuable. The blocker is usually tooling fidelity and trust/policy details, not protocol support.
Recursive multi-domain RBCD matters operationally – in 3+ domain forests, S4U2Self/S4U2Proxy can recurse through trust referrals, and SPN-less abuse may require a final S4U2Self+U2U hop plus RC4-dependent ticket handling. See Resource-Based Constrained Delegation.^[4]
Windows Server 2025 introduced delegated Managed Service Accounts (dMSAs) and their migration logic. If you see delegated rights over OUs or service-account objects in a 2025 domain, check the dedicated BadSuccessor page instead of treating it like “just another gMSA”.^[7]

Fast operator checks in modern domains

Before choosing a Kerberos attack path, quickly answer four questions:

Which accounts are still RC4-friendly?
Which users do not require pre-auth?
Which objects expose delegation abuse?
Which parts of the domain are new enough to enforce recent hardening?

# 1) Service accounts explicitly pinned to RC4 / legacy etypes
Get-ADObject -LDAPFilter '(|(msDS-SupportedEncryptionTypes=4)(msDS-SupportedEncryptionTypes=12))' \
  -Properties samAccountName,servicePrincipalName,msDS-SupportedEncryptionTypes

# 2) Service accounts with no explicit etype config
#    (these increasingly inherit AES-only defaults on patched 2026 DCs)
Get-ADObject -LDAPFilter '(&(servicePrincipalName=*)(!(msDS-SupportedEncryptionTypes=*)))' \
  -Properties samAccountName,servicePrincipalName

# 3) AS-REP roastable users
Get-ADUser -LDAPFilter '(&(samAccountType=805306368)(userAccountControl:1.2.840.113556.1.4.803:=4194304))' \
  -Properties userAccountControl

# 4) Delegation hot spots
Get-ADComputer -LDAPFilter '(msDS-AllowedToActOnBehalfOfOtherIdentity=*)' \
  -Properties msDS-AllowedToActOnBehalfOfOtherIdentity
Get-ADObject -LDAPFilter '(|(userAccountControl:1.2.840.113556.1.4.803:=524288)(userAccountControl:1.2.840.113556.1.4.803:=16777216))' \
  -Properties samAccountName,servicePrincipalName,userAccountControl

# 5) DC-side RC4 hardening / compatibility clues
Get-WinEvent -LogName System | Where-Object {
  $_.ProviderName -eq 'Microsoft-Windows-Kerberos-Key-Distribution-Center' -and $_.Id -in 201..209
}

Practical interpretation:

If interesting SPN accounts are explicitly RC4-capable, Kerberoasting stays cheap and fast.
If most service accounts have no explicit etype configuration, expect AES-only behavior on updated 2026 DCs and plan for slower offline cracking or a different path.
If RBCD / KCD / unconstrained delegation is present, S4U often beats brute-force.
If certificate auth is in play, remember that a failed PKINIT path does not always mean the cert is useless; in many environments the same cert still works for Schannel/LDAPS abuse (see AD Certificates / PKINIT abuse).

Common Kerberos errors that change the attack plan

KDC_ERR_ETYPE_NOTSUPP → The target account / DC will not use the encryption type you asked for. Stop retrying with RC4 only; supply AES keys or request AES roast material instead.
KRB_AP_ERR_MODIFIED → You likely have the wrong service key, the wrong SPN, or a forged ticket that does not match the service account actually decrypting it.
KRB_AP_ERR_SKEW → Your time is off. Sync to the DC before debugging anything else.
KDC_ERR_BADOPTION during S4U / delegation flows → frequently means sensitive/not-delegable users, the wrong delegation model, or that you are trying to do classic KCD where only RBCD would accept a non-forwardable S4U2Self ticket.

Kerberos Authentication

Kerberos Authentication

TL;DR for attackers

Fresh attack notes (2024-2026)

Fast operator checks in modern domains

Common Kerberos errors that change the attack plan

References

☧ CPTS · ordered posts