KRBTGT Delegation Backdoor

[metadata]
creation_date = "2022/01/27"
integration = ["system", "windows"]
maturity = "production"
updated_date = "2026/05/03"

[rule]
author = ["Elastic"]
description = """
Identifies the modification of the msDS-AllowedToDelegateTo attribute to KRBTGT. Attackers can use this technique to
maintain persistence to the domain by having the ability to request tickets for the KRBTGT service.
"""
from = "now-9m"
index = ["logs-system.security*", "logs-windows.forwarded*", "winlogbeat-*"]
language = "eql"
license = "Elastic License v2"
name = "KRBTGT Delegation Backdoor"
references = [
    "https://skyblue.team/posts/delegate-krbtgt",
    "https://github.com/atc-project/atomic-threat-coverage/blob/master/Atomic_Threat_Coverage/Logging_Policies/LP_0026_windows_audit_user_account_management.md",
]
risk_score = 73
rule_id = "e052c845-48d0-4f46-8a13-7d0aba05df82"
severity = "high"
tags = [
    "Domain: Endpoint",
    "OS: Windows",
    "Use Case: Threat Detection",
    "Tactic: Persistence",
    "Use Case: Active Directory Monitoring",
    "Data Source: Active Directory",
    "Data Source: Windows Security Event Logs",
    "Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "eql"

query = '''
iam where host.os.type == "windows" and event.code == "4738" and winlog.event_data.AllowedToDelegateTo : "*krbtgt*"
'''

note = """## Triage and analysis

### Investigating KRBTGT Delegation Backdoor

#### Possible investigation steps

- What account was changed, and what krbtgt delegation value did the alert preserve?
  - Focus: alert 4738 evidence in `winlog.event_data.TargetSid`, `winlog.event_data.TargetUserName`, `winlog.event_data.TargetDomainName`, `winlog.event_data.AllowedToDelegateTo`, and `winlog.computer_name`.
  - Implication: Escalate when the target now lists a krbtgt service target such as "krbtgt/DOMAIN"; lower suspicion only when the target, value, and controller match a time-boxed security validation or emergency delegation repair explicitly naming krbtgt, which is not routine delegation.

- Who initiated the account change?
  - Focus: `winlog.event_data.SubjectUserSid`, `winlog.event_data.SubjectUserName`, `winlog.event_data.SubjectDomainName`, and `winlog.event_data.SubjectLogonId`.
  - Implication: Escalate when a human admin, newly introduced service identity, or unexpected controller-local context made the change; lower suspicion only when the same stable tier-0 identity owns this exact delegation-maintenance path.

- What source and authentication method created the modifying session?
  - Focus: authentication events on the same `host.id` where `winlog.event_data.TargetLogonId` equals alert `winlog.event_data.SubjectLogonId`; review `source.ip`, `winlog.logon.type`, and `winlog.event_data.AuthenticationPackageName`. $investigate_0
  - Hint: missing authentication telemetry, or absent `source.ip` on local/service sessions, is unresolved, not benign; review same-session 4648 records when credential-source context matters. $investigate_1
  - Implication: Escalate for an unusual source, remote-interactive access, unexpected NTLM, or explicit credentials; lower suspicion when the session maps to the expected tier-0 admin path for this controller.

- Did surrounding directory changes prepare, repeat, or roll back the krbtgt delegation?
  - Focus: surrounding 4738 records for the same `winlog.event_data.TargetSid`, plus 5136 records on the same `winlog.computer_name` and modifying subject/session; use `winlog.event_data.ObjectDN`, `winlog.event_data.AttributeLDAPDisplayName`, and `winlog.event_data.AttributeValue` to identify the affected object and value.
  - Hint: reconstruct the burst from same-target 4738 and same-controller directory-service changes; if 5136 grouping is thin, use `winlog.record_id` order on the same controller plus subject/session and object DN. Absent 5136 evidence leaves prerequisite and rollback context unresolved, not benign.
    - $investigate_2
    - $investigate_3
  - Implication: Escalate when `winlog.event_data.AttributeLDAPDisplayName` and `winlog.event_data.AttributeValue` show delegation-enabling changes, repeated msDS-AllowedToDelegateTo writes, or no rollback; prompt removal narrows the persistence window but does not clear actor or session intent.

- Does the same actor or target touch other delegation-relevant accounts in the case window?
  - Focus: run this only if earlier answers are suspicious or unresolved; search 4738 and 5136 records for the same modifying subject/session or `winlog.event_data.TargetSid`, then compare `winlog.event_data.TargetUserName`, `winlog.event_data.ObjectDN`, `winlog.event_data.AttributeLDAPDisplayName`, and `winlog.event_data.AttributeValue`.
  - Implication: Broaden scope when the same actor or target appears in additional delegation writes across objects or controllers; keep it narrow when changes stay confined to one exact target and resolved maintenance path. $investigate_4

- Escalate when a non-routine actor/session adds krbtgt delegation, supporting directory changes show setup, the value remains, or same-actor/target scope expands; close only when the modified account, krbtgt value, actor/session, source/authentication context, surrounding changes, rollback, and scope all align with one tightly controlled authorized workflow; preserve raw 4738, authentication, and directory-change evidence and escalate when findings stay mixed or incomplete.

### False positive analysis

- Authorized security validation or emergency delegation repair can trigger this rule only when krbtgt delegation is the explicit planned action. Confirm the target (`winlog.event_data.TargetSid` plus `winlog.event_data.AllowedToDelegateTo`), actor/session (`winlog.event_data.SubjectUserSid`, `winlog.event_data.SubjectLogonId`, and recovered authentication context), controller, and surrounding attribute evidence all align. If change records are unavailable, telemetry-only closure must still bind one exact workflow with no contradictions; prior benign recurrence strengthens confidence but is not required.
- Do not close generic service onboarding, migration, or constrained-delegation cutover as benign unless outside confirmation explicitly names krbtgt delegation and telemetry matches that exact target, actor, session, and controller path. A normal service-delegation explanation that does not account for the krbtgt value is incomplete.
- Before creating an exception, validate that the same `winlog.event_data.SubjectUserSid`, `winlog.event_data.TargetSid`, exact `winlog.event_data.AllowedToDelegateTo`, `winlog.computer_name`, and recovered session pattern identify the authorized workflow across prior benign cases or a tightly controlled test plan. Build the exception from that minimum confirmed pattern, and avoid exceptions on "krbtgt", event 4738, or "msDS-AllowedToDelegateTo" alone.

### Response and remediation

- If confirmed benign, document the actor, target account, krbtgt value, controller, recovered session context, and surrounding delegation-change pattern before reversing temporary containment. Create an exception only if that same pattern is stable across prior benign cases.
- If suspicious but unconfirmed, first export the alert, raw 4738 record, matching authentication events, and surrounding 4738 or 5136 records. Preserve the modified account, krbtgt value, actor/session, source/auth context, and rollback evidence before reversible containment such as heightened monitoring or temporary delegation-administration restrictions.
- If confirmed malicious, preserve the same identity, session, and directory-change evidence first, then restrict or disable the modifying account. Restrict the modified target only when it was intentionally backdoored or used for follow-on Kerberos abuse. Contain the recovered source host when session evidence identifies one, or hand off the preserved evidence set to Active Directory or incident response.
- After containment, review recent 4738 and 5136 records for the same actor, target, and controller before cleanup. Remove the unauthorized krbtgt value from the `msDS-AllowedToDelegateTo` attribute, roll back related delegation-prerequisite changes identified in the same change set, and verify clean replication across domain controllers.
- If ticket abuse or broader Active Directory compromise is confirmed, activate the domain-compromise plan, including the required double reset of krbtgt after scoping and coordination with directory owners.
- Post-incident hardening: restrict delegation administration and SeEnableDelegationPrivilege to dedicated tier-0 identities, keep Audit User Account Management plus supporting 5136, 4624, and 4648 visibility on domain controllers, and document any recurring benign validation pattern for future triage.
"""

setup = """## Setup

Audit User Account Management must be enabled to generate the events used by this rule.
Setup instructions: https://ela.st/audit-user-account-management
"""

[rule.investigation_fields]
field_names = [
    "@timestamp",
    "user.name",
    "user.id",
    "winlog.event_data.SubjectUserName",
    "winlog.event_data.SubjectUserSid",
    "winlog.event_data.SubjectDomainName",
    "winlog.event_data.SubjectLogonId",
    "winlog.logon.id",
    "winlog.event_data.TargetUserName",
    "winlog.event_data.TargetSid",
    "winlog.event_data.TargetDomainName",
    "winlog.event_data.AllowedToDelegateTo",
    "host.name",
    "host.id",
    "winlog.computer_name",
]

[transform]

[[transform.investigate]]
label = "Authentication events for the modifying session"
description = ""
providers = [
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "event.code", queryType = "phrase", value = "4624", valueType = "string" },
    { excluded = false, field = "winlog.event_data.TargetLogonId", queryType = "phrase", value = "{{winlog.event_data.SubjectLogonId}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[transform.investigate]]
label = "Explicit-credential events from the modifying session"
description = ""
providers = [
  [
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "event.code", queryType = "phrase", value = "4648", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectLogonId", queryType = "phrase", value = "{{winlog.event_data.SubjectLogonId}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[transform.investigate]]
label = "4738 changes for this delegation target"
description = ""
providers = [
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "4738", valueType = "string" },
    { excluded = false, field = "winlog.event_data.TargetSid", queryType = "phrase", value = "{{winlog.event_data.TargetSid}}", valueType = "string" }
  ],
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "4738", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectLogonId", queryType = "phrase", value = "{{winlog.event_data.SubjectLogonId}}", valueType = "string" }
  ]
]
relativeFrom = "now-24h/h"
relativeTo = "now"

[[transform.investigate]]
label = "5136 directory changes by this modifying session"
description = ""
providers = [
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "5136", valueType = "string" },
    { excluded = false, field = "winlog.computer_name", queryType = "phrase", value = "{{winlog.computer_name}}", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectLogonId", queryType = "phrase", value = "{{winlog.event_data.SubjectLogonId}}", valueType = "string" }
  ],
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "5136", valueType = "string" },
    { excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectUserSid", queryType = "phrase", value = "{{winlog.event_data.SubjectUserSid}}", valueType = "string" }
  ]
]
relativeFrom = "now-24h/h"
relativeTo = "now"

[[transform.investigate]]
label = "Delegation-sensitive changes by this actor"
description = ""
providers = [
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "4738", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectUserSid", queryType = "phrase", value = "{{winlog.event_data.SubjectUserSid}}", valueType = "string" }
  ],
  [
    { excluded = false, field = "event.code", queryType = "phrase", value = "5136", valueType = "string" },
    { excluded = false, field = "winlog.event_data.SubjectUserSid", queryType = "phrase", value = "{{winlog.event_data.SubjectUserSid}}", valueType = "string" }
  ]
]
relativeFrom = "now-48h/h"
relativeTo = "now"

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1098"
name = "Account Manipulation"
reference = "https://attack.mitre.org/techniques/T1098/"

[rule.threat.tactic]
id = "TA0003"
name = "Persistence"
reference = "https://attack.mitre.org/tactics/TA0003/"

[[rule.threat]]
framework = "MITRE ATT&CK"

[[rule.threat.technique]]
id = "T1558"
name = "Steal or Forge Kerberos Tickets"
reference = "https://attack.mitre.org/techniques/T1558/"

[rule.threat.tactic]
id = "TA0006"
name = "Credential Access"
reference = "https://attack.mitre.org/tactics/TA0006/"