EKS Anonymous API Access Detected

Severity: low
Group by: userAgent
Entities: aws_account_ids, aws_arns, ip_addresses, usernames
Log types: Amazon.EKS.Audit
Tags: EKS, Security Control, API, Initial Access:Exploit Public-Facing Application
Reference: https://raesene.github.io/blog/2023/03/18/lets-talk-about-anonymous-access-to-Kubernetes/
Source: github.com/panther-labs/panther-analysis

This rule detects anonymous API requests made to the Kubernetes API server. In production environments, anonymous access should be disabled to prevent unauthorized access to the API server.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application

Rule body yaml

AnalysisType: rule
Filename: anonymous_api_access.py
RuleID: "Amazon.EKS.AnonymousAPIAccess"
DisplayName: "EKS Anonymous API Access Detected"
Enabled: true
LogTypes:
  - Amazon.EKS.Audit
Severity: Low
Reports:
  MITRE ATT&CK:
    - "TA0001:T1190" # Initial Access: Exploit Public-Facing Application
Description: >
  This rule detects anonymous API requests made to the Kubernetes API server.
  In production environments, anonymous access should be disabled to prevent
  unauthorized access to the API server.
DedupPeriodMinutes: 60
Reference: 
  https://raesene.github.io/blog/2023/03/18/lets-talk-about-anonymous-access-to-Kubernetes/
Runbook: |
  Check the EKS cluster configuration and ensure that anonymous access
  to the Kubernetes API server is disabled. This can be done by verifying the  API
  server arguments and authentication webhook configuration.
SummaryAttributes:
  - user:username
  - p_any_ip_addresses
  - p_source_label
Tags:
  - EKS
  - Security Control
  - API
  - Initial Access:Exploit Public-Facing Application
Tests:
  - Name: Anonymous API Access
    ExpectedResult: true
    Log:
      {
        "annotations": {
          "authorization.k8s.io/decision": "allow",
          "authorization.k8s.io/reason": "RBAC: allowed by ClusterRoleBinding system:public-info-viewer"
        },
        "apiVersion": "audit.k8s.io/v1",
        "auditID": "abcde12345",
        "kind": "Event",
        "level": "Request",
        "objectRef": {
          "apiVersion": "v1",
          "name": "test-pod",
          "namespace": "default",
          "resource": "pods"
        },
        "p_any_aws_account_ids": [
          "123412341234"
        ],
        "p_any_aws_arns": [
          "arn:aws:iam::123412341234:role/DevAdministrator"
        ],
        "p_any_ip_addresses": [
          "8.8.8.8"
        ],
        "p_any_usernames": [
          "system:anonymous"
        ],
        "p_event_time": "2022-11-29 00:09:04.38",
        "p_log_type": "Amazon.EKS.Audit",
        "p_parse_time": "2022-11-29 00:10:25.067",
        "p_row_id": "2e4ab474b0f0f7a4a8fff4f014a9b32a",
        "p_source_id": "4c859cd4-9406-469b-9e0e-c2dc1bee24fa",
        "p_source_label": "example-cluster-eks-logs",
        "requestReceivedTimestamp": "2022-11-29 00:09:04.38",
        "requestURI": "/api/v1/namespaces/default/pods/test-pod",
        "responseStatus": {
          "code": 200
        },
        "sourceIPs": [
          "8.8.8.8"
        ],
        "stage": "ResponseComplete",
        "user": {
          "username": "system:anonymous"
        },
        "userAgent": "kubectl/v1.25.4"
      }
  - Name: Non-Anonymous API Access
    ExpectedResult: false
    Log:
      {
        "annotations": {
          "authorization.k8s.io/decision": "allow",
          "authorization.k8s.io/reason": "RBAC: allowed by ClusterRoleBinding system:public-info-viewer"
        },
        "apiVersion": "audit.k8s.io/v1",
        "auditID": "abcde12345",
        "kind": "Event",
        "level": "Request",
        "objectRef": {
          "apiVersion": "v1",
          "name": "test-pod",
          "namespace": "default",
          "resource": "pods"
        },
        "p_any_aws_account_ids": [
          "123412341234"
        ],
        "p_any_aws_arns": [
          "arn:aws:iam::123412341234:role/DevAdministrator"
        ],
        "p_any_ip_addresses": [
          "8.8.8.8"
        ],
        "p_any_usernames": [
          "kubernetes-admin"
        ],
        "p_event_time": "2022-11-29 00:09:04.38",
        "p_log_type": "Amazon.EKS.Audit",
        "p_parse_time": "2022-11-29 00:10:25.067",
        "p_row_id": "2e4ab474b0f0f7a4a8fff4f014a9b32a",
        "p_source_id": "4c859cd4-9406-469b-9e0e-c2dc1bee24fa",
        "p_source_label": "example-cluster-eks-logs",
        "requestReceivedTimestamp": "2022-11-29 00:09:04.38",
        "requestURI": "/api/v1/namespaces/default/pods/test-pod",
        "responseStatus": {
          "code": 200
        },
        "sourceIPs": [
          "8.8.8.8"
        ],
        "stage": "ResponseComplete",
        "user": {
          "username": "kubernetes-admin"
        },
        "userAgent": "kubectl/v1.25.4"
      }
  - Name: Anonymous API Access Web Scanner Allowed
    ExpectedResult: true
    Log:
      {
        "annotations": {
          "authorization.k8s.io/decision": "allow",
          "authorization.k8s.io/reason": "RBAC: allowed by ClusterRoleBinding \"system:public-info-viewer\" of ClusterRole \"system:public-info-viewer\" to Group \"system:unauthenticated\""
        },
        "apiVersion": "audit.k8s.io/v1",
        "auditID": "d976bfc6-a2bc-49d5-bdeb-074441e0b875",
        "kind": "Event",
        "level": "Metadata",
        "requestReceivedTimestamp": "2024-11-13 18:34:10.595141000",
        "requestURI": "/version",
        "responseStatus": {
          "code": 200
        },
        "sourceIPs": [
          "44.238.138.237"
        ],
        "stage": "ResponseComplete",
        "stageTimestamp": "2024-11-13 18:34:10.595494000",
        "user": {
          "groups": [
            "system:unauthenticated"
          ],
          "username": "system:anonymous"
        },
        "userAgent": "python-requests/2.31.0",
        "verb": "get"
      }
  - Name: Anonymous API Access Web Scanner Denied
    ExpectedResult: true
    Log:
      {
        "annotations": {
          "authorization.k8s.io/decision": "forbid",
          "authorization.k8s.io/reason": ""
        },
        "apiVersion": "audit.k8s.io/v1",
        "auditID": "edf35e8d-92c3-4507-9bc6-4dd9cf068bcf",
        "kind": "Event",
        "level": "Metadata",
        "requestReceivedTimestamp": "2024-11-13 23:50:32.672347000",
        "requestURI": "/vendor/phpunit/src/Util/PHP/eval-stdin.php",
        "responseStatus": {
          "code": 403,
          "message": "forbidden: User \"system:anonymous\" cannot get path \"/vendor/phpunit/src/Util/PHP/eval-stdin.php\"",
          "reason": "Forbidden",
          "status": "Failure"
        },
        "sourceIPs": [
          "8.216.81.10"
        ],
        "stage": "ResponseComplete",
        "stageTimestamp": "2024-11-13 23:50:32.673504000",
        "user": {
          "groups": [
            "system:unauthenticated"
          ],
          "username": "system:anonymous"
        },
        "userAgent": "Custom-AsyncHttpClient",
        "verb": "get"
      }

Detection logic

Condition

userAgent not eq "ELB-HealthChecker/2.0"
user.username eq "system:anonymous"

This rule also runs imperative logic the parser cannot express as a filter; the conditions above are the structured part it could extract.

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`userAgent`	eq	`ELB-HealthChecker/2.0`

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`user.username`	eq	`system:anonymous`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field
`requestURI`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)