GKE Anonymous Request Authorized by Unusual User Agent

[metadata]
creation_date = "2026/07/16"
integration = ["gcp"]
maturity = "production"
updated_date = "2026/07/16"

[rule]
author = ["Elastic"]
description = """
Detects successful GKE API requests from unauthenticated anonymous identities using an unusual user agent.
Attackers may rely on anonymous access for initial cluster access or to avoid attribution. Matches
"system:anonymous" / "system:unauthenticated" and GKE audit rows where the principal is missing (common for
unauthenticated clients). Common kube-probe health checks (readyz/livez/healthz/version) are excluded.
"""
false_positives = [
    """
    Anonymous API access is a dangerous default. Health probes are excluded; investigate all other authorized
    anonymous requests and disable anonymous authentication where possible.
    """,
]
from = "now-6m"
index = ["logs-gcp.audit-*"]
language = "kuery"
license = "Elastic License v2"
name = "GKE Anonymous Request Authorized by Unusual User Agent"
note = """## Triage and analysis

### Investigating GKE Anonymous Request Authorized by Unusual User Agent

Anonymous success outside health endpoints can indicate a publicly reachable API server or overly permissive RBAC for
`system:anonymous` / `system:unauthenticated`. On GKE via GCP audit, some unauthenticated clients omit
`client.user.email`; those successes are included when the user agent is unusual.

### Investigation steps

- Review `client.user.email`, `event.action`, `gcp.audit.resource_name`, `source.ip`, and `user_agent.original`.
- Confirm whether the API server is Internet-exposed and whether anonymous auth is intentionally enabled.
- Hunt for follow-on anonymous pod mutations, secret reads, or RBAC changes from the same source.

### False positives

- Custom health or readiness endpoints not covered by the built-in exclusions; add environment-specific exceptions after review.

"""
references = [
    "https://media.defense.gov/2022/Aug/29/2003066362/-1/-1/0/CTR_KUBERNETES_HARDENING_GUIDANCE_1.2_20220829.PDF",
    "https://cloud.google.com/kubernetes-engine/docs/how-to/audit-logging",
]
risk_score = 47
rule_id = "189f0f31-8f31-48eb-bb3e-c0db8c8b8c4c"
severity = "medium"
tags = [
    "Domain: Cloud",
    "Domain: Kubernetes",
    "Data Source: GCP",
    "Data Source: Google Cloud Platform",
    "Use Case: Threat Detection",
    "Tactic: Initial Access",
    "Tactic: Defense Evasion",
    "Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "new_terms"
setup = "The GCP Fleet integration with GKE audit logs enabled is required to be compatible with this rule."

query = '''
data_stream.dataset:gcp.audit and service.name:k8s.io and event.outcome:success and client.user.email:("system:anonymous" or "system:unauthenticated" or not *) and user_agent.original:(* and not (*kubernetes/$Format or kube-probe*)) and not gcp.audit.resource_name:(healthz or livez or readyz or version or .well-known*)
'''

[rule.new_terms]
field = "new_terms_fields"
value = ["user_agent.original"]

[[rule.new_terms.history_window_start]]
field = "history_window_start"
value = "now-7d"

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

[[rule.threat.technique]]
id = "T1078"
name = "Valid Accounts"
reference = "https://attack.mitre.org/techniques/T1078/"

[[rule.threat.technique.subtechnique]]
id = "T1078.001"
name = "Default Accounts"
reference = "https://attack.mitre.org/techniques/T1078/001/"

[rule.threat.tactic]
id = "TA0001"
name = "Initial Access"
reference = "https://attack.mitre.org/tactics/TA0001/"

[rule.investigation_fields]
field_names = [
    "@timestamp",
    "client.user.email",
    "source.ip",
    "user_agent.original",
    "event.action",
    "event.outcome",
    "gcp.audit.resource_name",
    "orchestrator.resource.name",
    "orchestrator.namespace",
]