Kubernetes Admission Controller Webhook Created

Severity: medium
Group by: username
Log types: Amazon.EKS.Audit, Azure.MonitorActivity, GCP.AuditLog
Tags: Kubernetes, Persistence, Credential Access, Collection, Unified Detection
Reference: https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/
Source: github.com/panther-labs/panther-analysis

This detection monitors for creation of MutatingWebhookConfiguration or ValidatingWebhookConfiguration resources. Admission controller webhooks can intercept all API requests to the Kubernetes API server, allowing attackers to inspect, modify, or block any resource creation or modification. This provides powerful capabilities for persistence (modifying deployments to inject backdoors), credential theft (intercepting secrets), and reconnaissance (enumerating all cluster activity).

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1546` Event Triggered Execution
Credential Access	`T1552` Unsecured Credentials
Collection	`T1530` Data from Cloud Storage

Rule body yaml

AnalysisType: rule
RuleID: "Kubernetes.AdmissionController.Created"
DisplayName: "Kubernetes Admission Controller Webhook Created"
Enabled: true
Filename: k8s_admission_controller_created.py
LogTypes:
  - Amazon.EKS.Audit
  - Azure.MonitorActivity
  - GCP.AuditLog
Tags:
  - Kubernetes
  - Persistence
  - Credential Access
  - Collection
  - Unified Detection
Severity: Medium
Description: >
  This detection monitors for creation of MutatingWebhookConfiguration or ValidatingWebhookConfiguration
  resources. Admission controller webhooks can intercept all API requests to the Kubernetes API server,
  allowing attackers to inspect, modify, or block any resource creation or modification. This provides
  powerful capabilities for persistence (modifying deployments to inject backdoors), credential theft
  (intercepting secrets), and reconnaissance (enumerating all cluster activity).
Runbook: |
  1. Review the webhook configuration details including the target webhook service URL and failure policy
  2. Identify all API operations performed by the username in the 48 hours before webhook creation to establish intent
  3. Search for other webhook configurations or suspicious API activity from this user across all clusters in the past 7 days
Reports:
  MITRE ATT&CK:
    - TA0003:T1546 # Persistence: Event Triggered Execution
    - TA0006:T1552 # Credential Access: Unsecured Credentials
    - TA0009:T1530 # Collection: Data from Cloud Storage Object
Reference: https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/
DedupPeriodMinutes: 60
SummaryAttributes:
  - username
  - p_source_label
Tests:
  - Name: EKS MutatingWebhookConfiguration Created
    ExpectedResult: true
    Log:
      {
        "kind": "Event",
        "apiVersion": "audit.k8s.io/v1",
        "verb": "create",
        "user": {"username": "admin@example.com"},
        "sourceIPs": ["1.2.3.4"],
        "userAgent": "kubectl/v1.28.0",
        "objectRef": {
          "resource": "mutatingwebhookconfigurations",
          "name": "custom-mutator",
          "apiGroup": "admissionregistration.k8s.io",
          "apiVersion": "v1"
        },
        "responseStatus": {"code": 201},
        "requestObject": {
          "kind": "MutatingWebhookConfiguration",
          "metadata": {"name": "custom-mutator"},
          "webhooks": [{
            "name": "mutate.example.com",
            "clientConfig": {
              "url": "https://webhook.example.com/mutate"
            },
            "rules": [{
              "operations": ["CREATE", "UPDATE"],
              "apiGroups": ["*"],
              "apiVersions": ["*"],
              "resources": ["*"]
            }]
          }]
        },
        "p_log_type": "Amazon.EKS.Audit",
        "p_source_label": "eks-cluster"
      }
  - Name: AKS ValidatingWebhookConfiguration Created
    ExpectedResult: true
    Log:
      {
        "p_log_type": "Azure.MonitorActivity",
        "category": "kube-audit",
        "operationName": "Microsoft.ContainerService/managedClusters/diagnosticLogs/Read",
        "properties": {
          "log": "{\"kind\":\"Event\",\"apiVersion\":\"audit.k8s.io/v1\",\"verb\":\"create\",\"user\":{\"username\":\"policy-admin@example.com\"},\"sourceIPs\":[\"10.0.0.1\"],\"objectRef\":{\"resource\":\"validatingwebhookconfigurations\",\"name\":\"policy-validator\",\"apiGroup\":\"admissionregistration.k8s.io\"},\"responseStatus\":{\"code\":201},\"requestObject\":{\"kind\":\"ValidatingWebhookConfiguration\",\"webhooks\":[{\"name\":\"validate.policy.com\"}]}}"
        },
        "p_source_label": "aks-cluster"
      }
  - Name: GKE MutatingWebhookConfiguration Created
    ExpectedResult: true
    Log:
      {
        "protoPayload": {
          "authenticationInfo": {"principalEmail": "user@company.com"},
          "authorizationInfo": [{
            "granted": true,
            "permission": "io.k8s.admissionregistration.v1.mutatingwebhookconfigurations.create",
            "resource": "admissionregistration.k8s.io/v1/mutatingwebhookconfigurations/istio-sidecar"
          }],
          "methodName": "io.k8s.admissionregistration.v1.mutatingwebhookconfigurations.create",
          "requestMetadata": {"callerIP": "1.2.3.4"},
          "resourceName": "admissionregistration.k8s.io/v1/mutatingwebhookconfigurations/istio-sidecar",
          "serviceName": "k8s.io",
          "request": {
            "kind": "MutatingWebhookConfiguration",
            "metadata": {"name": "istio-sidecar"},
            "webhooks": [{
              "name": "sidecar-injector.istio.io",
              "clientConfig": {
                "service": {
                  "name": "istio-sidecar-injector",
                  "namespace": "istio-system"
                }
              }
            }]
          }
        },
        "resource": {
          "type": "k8s_cluster",
          "labels": {"project_id": "test-project"}
        },
        "p_log_type": "GCP.AuditLog",
        "p_source_label": "gke-cluster"
      }
  - Name: Webhook Update (Not Create)
    ExpectedResult: false
    Log:
      {
        "kind": "Event",
        "verb": "update",
        "objectRef": {
          "resource": "mutatingwebhookconfigurations",
          "name": "existing-webhook"
        },
        "responseStatus": {"code": 200},
        "p_log_type": "Amazon.EKS.Audit"
      }
  - Name: Different Resource Type
    ExpectedResult: false
    Log:
      {
        "kind": "Event",
        "verb": "create",
        "objectRef": {
          "resource": "configmaps",
          "name": "test-config"
        },
        "responseStatus": {"code": 201},
        "p_log_type": "Amazon.EKS.Audit"
      }
  - Name: Webhook Creation Failed
    ExpectedResult: false
    Log:
      {
        "kind": "Event",
        "verb": "create",
        "objectRef": {
          "resource": "validatingwebhookconfigurations",
          "name": "test-webhook"
        },
        "responseStatus": {"code": 403},
        "p_log_type": "Amazon.EKS.Audit"
      }

Detection logic

Condition

verb eq "create"
resource in ["mutatingwebhookconfigurations", "validatingwebhookconfigurations"]
not (responseStatus is_not_null and (responseStatus.code ge "400" or (responseStatus.code ge "1" and responseStatus.code le "16")))
not (username is_not_null and (username in ["masterclient", "aksService"] or (username starts_with "system:" and username not contains "serviceaccount")))

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
`username`	contains	`serviceaccount`
`username`	starts_with	`system:`
`username`	in	`aksService`, `masterclient`
`username`	is_not_null	(no value, null check)
`responseStatus.code`	ge	`1`
`responseStatus.code`	le	`16`
`responseStatus.code`	ge	`400`
`responseStatus`	is_not_null	(no value, null check)

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
`resource`	in	`mutatingwebhookconfigurations` `validatingwebhookconfigurations`
`username`	contains	`serviceaccount`
`verb`	eq	`create`

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	Source
`username`
`sourceIPs`
`userAgent`
`namespace`
`verb`
`resource`
`requestURI`
`responseStatus`
`cluster`	`p_source_label`
`name`

`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)