GKE Anonymous Endpoint Permission Enumeration

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GKE Anonymous Endpoint Permission Enumeration

Detects bursts of GKE API requests from an anonymous identity that probe many distinct actions and resources with mostly failed outcomes. This pattern is consistent with unauthenticated permission enumeration against an exposed API server. On GKE GCP audit logs, unauthenticated probes often omit "client.user.email" (null principal) with Unauthorized failures; those events are included alongside "system:anonymous" / "system:unauthenticated".

Rule type: esql

Rule indices: None

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-6m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

Domain: Cloud
Domain: Kubernetes
Data Source: GCP
Data Source: Google Cloud Platform
Use Case: Threat Detection
Tactic: Discovery
Tactic: Reconnaissance
Resources: Investigation Guide

Version: 1

Rule authors:

Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Investigating GKE Anonymous Endpoint Permission Enumeration

Anonymous multi-endpoint failure bursts map which APIs are reachable before credential theft or exploitation. Treat missing client.user.email with Unauthorized/failure bursts as anonymous on GKE.

Investigation steps

Review Esql.event_action_values and Esql.resource_name_values for targeted APIs (secrets, RBAC, CRDs).
Confirm whether source.ip is Internet-routable and whether the API endpoint is publicly exposed.
Hunt for later successful anonymous or authenticated activity from the same source or user agent.

False positives

Misconfigured auth proxies that strip credentials can make legitimate clients appear anonymous during outages.

Setup

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The GCP Fleet integration with GKE audit logs enabled is required to be compatible with this rule.

Rule query

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from logs-gcp.audit-* metadata _id, _index, _version
| where data_stream.dataset == "gcp.audit"
    and service.name == "k8s.io"
    and (
      client.user.email in ("system:anonymous", "system:unauthenticated")
      or client.user.email is null
    )
    and not gcp.audit.resource_name in ("readyz", "livez", "healthz", "version")
| stats
    Esql.document_count = count(),
    Esql.failure_count = sum(case(event.outcome == "failure", 1, 0)),
    Esql.event_action_count_distinct = count_distinct(event.action),
    Esql.resource_name_count_distinct = count_distinct(gcp.audit.resource_name),
    Esql.event_action_values = values(event.action),
    Esql.resource_name_values = values(gcp.audit.resource_name),
    Esql.event_outcome_values = values(event.outcome),
    Esql.client_user_email_values = values(client.user.email),
    Esql.timestamp = VALUES(@timestamp),
    Esql.data_stream_namespace = VALUES(data_stream.namespace),
    Esql.user_agent_original_values = VALUES(user_agent.original)
  by source.ip
| where Esql.event_action_count_distinct > 5
    and Esql.resource_name_count_distinct > 3
    and Esql.document_count < 50
    and Esql.failure_count >= 1
| keep Esql.*, source.ip

Framework: MITRE ATT&CK^TM

Tactic:
- Name: Discovery
- ID: TA0007
- Reference URL: https://attack.mitre.org/tactics/TA0007/
Technique:
- Name: Container and Resource Discovery
- ID: T1613
- Reference URL: https://attack.mitre.org/techniques/T1613/
Tactic:
- Name: Reconnaissance
- ID: TA0043
- Reference URL: https://attack.mitre.org/tactics/TA0043/
Technique:
- Name: Active Scanning
- ID: T1595
- Reference URL: https://attack.mitre.org/techniques/T1595/
Sub-technique:
- Name: Wordlist Scanning
- ID: T1595.003
- Reference URL: https://attack.mitre.org/techniques/T1595/003/

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