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Kubernetes Admission Webhook

Overview​

Casbin K8s-Gatekeeper is a Kubernetes admission webhook that uses Casbin for authorization. You define models and policies declaratively to allow or deny operations on any Kubernetes resource—no custom code in the webhook. Maintained by the Casbin community: github.com/casbin/k8s-gatekeeper.

Basic example​

Example: deny deployments that use images with a specific tag, using only config:

Model:

[request_definition]
r =  obj

[policy_definition]
p =  obj,eft

[policy_effect]
e = !some(where (p.eft == deny))

[matchers]
m = r.obj.Request.Namespace == "default" && r.obj.Request.Resource.Resource =="deployments" && \
contain(split(accessWithWildcard(${OBJECT}.Spec.Template.Spec.Containers , "*", "Image"),":",1) , p.obj)

Policy:

p, "1.14.1",deny

This uses standard Casbin ACL language, which should be straightforward if you've read the introductory chapters.

Casbin K8s-Gatekeeper offers several advantages:

  • Simple to use—write ACL configurations instead of extensive code
  • Supports live configuration updates without plugin restarts
  • Flexible—apply arbitrary rules to any Kubernetes resource using kubectl gatekeeper
  • Simplifies Kubernetes admission webhook implementation—no need to understand webhook internals or write webhook code. Just define constraints and write Casbin ACL.
  • Community-maintained—contact us with questions or issues

1.1 How Casbin K8s-Gatekeeper Works​

K8s-Gatekeeper is an admission webhook for Kubernetes that uses Casbin to enforce custom access control rules, preventing unwanted operations on Kubernetes resources.

Casbin is an efficient open-source access control library supporting various authorization models. For details, see the Overview.

Admission webhooks in Kubernetes are HTTP callbacks that receive and process admission requests. K8s-Gatekeeper is a ValidatingAdmissionWebhook that accepts or rejects admission requests. Admission requests are HTTP requests describing operations on Kubernetes resources (e.g., creating or deleting a deployment). For more information, see the Kubernetes documentation.

1.2 Example Workflow​

When someone creates a deployment with an nginx pod (via kubectl or Kubernetes clients), Kubernetes generates an admission request like this (in YAML format):

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 1
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.1
        ports:
        - containerPort: 80

This request passes through middleware layers, including K8s-Gatekeeper. K8s-Gatekeeper detects all Casbin enforcers stored in Kubernetes etcd (created and maintained by users via kubectl or the provided Go client). Each enforcer contains a Casbin model and policy. The admission request is evaluated by each enforcer sequentially, and must pass all enforcers to be accepted.

(If you're unfamiliar with Casbin enforcers, models, or policies, see Get Started).

For instance, if an administrator wants to block the 'nginx:1.14.1' image while allowing 'nginx:1.3.1', they can create an enforcer with this model and policy (creation and configuration details follow in subsequent sections):

Model:

[request_definition]
r =  obj

[policy_definition]
p =  obj,eft

[policy_effect]
e = !some(where (p.eft == deny))

[matchers]
m = r.obj.Request.Namespace == "default" && r.obj.Request.Resource.Resource =="deployments" && \
access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Image") == p.obj

Policy:

p, "nginx:1.13.1",allow
p, "nginx:1.14.1",deny

Creating an enforcer with this model and policy will reject the admission request, preventing Kubernetes from creating the deployment.

2. Installing K8s-gatekeeper​

Three installation methods are available: External webhook, Internal webhook, and Helm.

заметка

These installation methods are for evaluation purposes only. For production deployments, review Chapter 5. Advanced settings and apply necessary security modifications before installation.

2.1 Internal Webhook​

2.1.1 Step 1: Build the Image​

For internal webhook deployment, K8s-gatekeeper runs as a Kubernetes service. Build the image:

docker build --target webhook -t k8s-gatekeeper .

This creates a local image named 'k8s-gatekeeper:latest'.

заметка

For minikube users, run eval $(minikube -p minikube docker-env) before 'docker build'.

2.1.2 Step 2: Deploy Services and Resources​

Run these commands:

kubectl apply -f config/rbac.yaml
kubectl apply -f config/webhook_deployment.yaml 
kubectl apply -f config/webhook_internal.yaml

Verify deployment with kubectl get pods.

2.1.3 Step 3: Install CRD Resources​

Install custom resource definitions:

kubectl apply -f config/auth.casbin.org_casbinmodels.yaml 
kubectl apply -f config/auth.casbin.org_casbinpolicies.yaml

2.2 External Webhook​

For external webhook deployment, K8s-gatekeeper runs outside Kubernetes. Kubernetes requires HTTPS for admission webhooks. We provide test certificates and a private key (not production-secure). For custom certificates, see Chapter 5. Advanced settings.

The provided certificate is issued for 'webhook.domain.local'. Update your hosts file (e.g., /etc/hosts) to point 'webhook.domain.local' to the IP address where K8s-gatekeeper runs.

Execute:

go mod tidy
go mod vendor
go run cmd/webhook/main.go
kubectl apply -f config/auth.casbin.org_casbinmodels.yaml 
kubectl apply -f config/auth.casbin.org_casbinpolicies.yaml
kubectl apply -f config/webhook_external.yaml

2.3 Installing via Helm​

2.3.1 Step 1: Build the Image​

See Chapter 2.1.1.

2.3.2 Helm Installation​

Run: helm install k8sgatekeeper ./k8sgatekeeper

3. Using K8s-gatekeeper​

3.1 Creating Casbin Models and Policies​

Create models and policies using either kubectl or the provided Go client.

3.1.1 Create/Update via kubectl​

In K8s-gatekeeper, Casbin models are stored as 'CasbinModel' CRD resources. The definition is in config/auth.casbin.org_casbinmodels.yaml.

See examples in example/allowed_repo/model.yaml. Important fields:

  • metadata.name: Model name. Must match the associated CasbinPolicy object name for K8s-gatekeeper to pair them correctly.
  • spec.enable: Set to "false" to disable this model and its associated policy.
  • spec.modelText: String containing the Casbin model definition.

Casbin policies are stored as 'CasbinPolicy' CRD resources, defined in config/auth.casbin.org_casbinpolicies.yaml.

See examples in example/allowed_repo/policy.yaml. Important fields:

  • metadata.name: Policy name. Must match the associated CasbinModel object name.
  • spec.policyItem: String containing the Casbin policy definition.

Apply your CasbinModel and CasbinPolicy files:

kubectl apply -f <filename>

K8s-gatekeeper detects new CasbinModel/CasbinPolicy pairs within 5 seconds.

3.1.2 Create/Update via Go Client​

For scenarios where direct shell access to cluster nodes is impractical (e.g., when building automated cloud platforms), we provide a Go client for managing CasbinModel and CasbinPolicy resources.

The Go client library is in pkg/client.

In client.go, use this function to create a client:

func NewK8sGateKeeperClient(externalClient bool) (*K8sGateKeeperClient, error)

The externalClient parameter indicates whether K8s-gatekeeper runs inside or outside the Kubernetes cluster.

In model.go, functions are provided for creating, deleting, and modifying CasbinModel. See model_test.go for usage examples.

In policy.go, functions are provided for creating, deleting, and modifying CasbinPolicy. See policy_test.go for usage examples.

3.1.2 Testing K8s-gatekeeper​

After creating the model and policy from example/allowed_repo, test with:

kubectl apply -f example/allowed_repo/testcase/reject_1.yaml

Kubernetes should reject this request, citing the webhook as the rejection reason. However, applying example/allowed_repo/testcase/approve_2.yaml should succeed.

4. Writing Models and Policies for K8s-gatekeeper​

Ensure you understand Casbin model and policy syntax before proceeding. If not, read the Get Started section first. This chapter assumes familiarity with Casbin models and policies.

4.1 Request Definition​

When K8s-gatekeeper evaluates a request, the input is always an AdmissionReview Go object. The enforcer is used as follows:

ok, err := enforcer.Enforce(admission)

where admission is an AdmissionReview object from Kubernetes' official Go API "k8s.io/api/admission/v1". The struct definition is available at: https://github.com/kubernetes/api/blob/master/admission/v1/types.go. Additional documentation: https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#webhook-request-and-response.

For K8s-gatekeeper models, the request_definition should always follow this format:

    [request_definition]
    r =  obj

The name 'obj' can be changed as long as it's used consistently in the [matchers] section.

4.2 Model Matchers​

Use Casbin's ABAC feature to write rules. However, Casbin's expression evaluator doesn't natively support map/array indexing or array expansion. K8s-gatekeeper provides extension functions to address this. If you need additional functionality, please open an issue or submit a pull request.

For background on Casbin functions, see Function.

Extension functions:

4.2.1 Extension Functions​

4.2.1.1 доступ​

The access function enables map and array indexing. See example/allowed_repo/model.yaml:

[matchers]
m = r.obj.Request.Namespace == "default" && r.obj.Request.Resource.Resource =="deployments" && \
access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Image") == p.obj

Here, access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Image") is equivalent to r.obj.Request.Object.Object.Spec.Template.Spec.Containers[0].Image, where r.obj.Request.Object.Object.Spec.Template.Spec.Containers is a slice.

access can also call parameterless functions that return a single value. See example/container_resource_limit/model.yaml:

[matchers]
  m = r.obj.Request.Namespace == "default" && r.obj.Request.Resource.Resource =="deployments" && \
  parseFloat(access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Resources","Limits","cpu","Value")) >= parseFloat(p.cpu) && \
  parseFloat(access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Resources","Limits","memory","Value")) >= parseFloat(p.memory)

Here, access(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , 0, "Resources","Limits","cpu","Value") equals r.obj.Request.Object.Object.Spec.Template.Spec.Containers[0].Resources.Limits["cpu"].Value(), where r.obj.Request.Object.Object.Spec.Template.Spec.Containers[0].Resources.Limits is a map and Value() is a parameterless function returning a single value.

4.2.1.2 accessWithWildcard​

To check conditions across all array elements (e.g., "all elements must start with 'aaa'") without for loops, use accessWithWildcard with map/slice expansion.

If a.b.c is the array [aaa,bbb,ccc,ddd,eee], then accessWithWildcard(a,"b","c","*") returns the slice [aaa,bbb,ccc,ddd,eee]. The wildcard * expands the slice.

Multiple wildcards are supported. For example, accessWithWildcard(a,"b","c","*","*") returns [a.b.c[0][0], a.b.c[0][1], ..., a.b.c[1][0], a.b.c[1][1], ...].

4.2.1.3 Variable-Length Argument Functions​

Casbin's expression evaluator automatically expands arrays as variable-length arguments. Leveraging this for array/slice/map expansion, several functions accept arrays/slices:

  • contain(): Accepts multiple parameters and returns whether any parameter (except the last) equals the last parameter.
  • split(a,b,c...,sep,index): Returns [splits(a,sep)[index], splits(b,sep)[index], splits(c,sep)[index], ...].
  • len(): Returns the variable-length argument count.
  • matchRegex(a,b,c...,regex): Returns whether all parameters (a, b, c, ...) match the regex.

Example from example/disallowed_tag/model.yaml:

    [matchers]
    m = r.obj.Request.Namespace == "default" && r.obj.Request.Resource.Resource =="deployments" && \
    contain(split(accessWithWildcard(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , "*", "Image"),":",1) , p.obj)

If accessWithWildcard(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , "*", "Image") returns ["a:b", "c:d", "e:f", "g:h"], the split operation processes each element with variable-length argument support, selecting index 1 from each. Thus, split(accessWithWildcard(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , "*", "Image"),":",1) returns ["b","d","f","h"]. Finally, contain(split(accessWithWildcard(r.obj.Request.Object.Object.Spec.Template.Spec.Containers , "*", "Image"),":",1) , p.obj) checks if p.obj exists in ["b","d","f","h"].

4.2.1.2 Функции преобразования типов​

  • ParseFloat(): Converts integers to floats (required because comparisons need float type).
  • ToString(): Converts objects to strings. The object must have an underlying string type (e.g., type XXX string).
  • IsNil(): Возвращает, является ли параметр nil.

5. Расширенные настройки​

5.1 Certificate Configuration​

Kubernetes requires webhooks to use HTTPS. Two options are available:

  • Self-signed certificates (used in this repository's examples)
  • Publicly trusted certificates

5.1.1 Self-Signed Certificates​

Self-signed certificates use a Certificate Authority (CA) that isn't publicly recognized. You must configure Kubernetes to trust this CA.

The repository examples use a custom CA with private key and certificate stored in config/certificate/ca.key and config/certificate/ca.crt. The webhook certificate config/certificate/server.crt is issued by this CA for domains "webhook.domain.local" (external webhook) and "casbin-webhook-svc.default.svc" (internal webhook).

CA information is passed to Kubernetes via webhook configuration files. Both config/webhook_external.yaml and config/webhook_internal.yaml contain a "CABundle" field with the base64-encoded CA certificate.

To change the certificate/domain (e.g., moving the webhook to another namespace or changing domains):

  1. Создать новый CA:

    • Generate the CA private key:

      openssl genrsa -des3 -out ca.key 2048

    • Remove password protection:

      openssl rsa -in ca.key -out ca.key

  2. Generate a webhook server private key:

    openssl genrsa -des3 -out server.key 2048
    openssl rsa -in server.key -out server.key

  3. Sign the webhook certificate with the CA:

    • Copy your system's OpenSSL config file (find it with openssl version -a, typically openssl.cnf).

    • Modify the config file:

      • In the [req] section, add: req_extensions = v3_req

      • In the [v3_req] section, add: subjectAltName = @alt_names

      • Append:

        [alt_names]
        DNS.2=<Your desired domain>

        Note: Replace 'casbin-webhook-svc.default.svc' with your actual service name if modified.

    • Generate a certificate request:

      openssl req -new -nodes -keyout server.key -out server.csr -config openssl.cnf

    • Sign the certificate with the CA:

      openssl x509 -req -days 3650 -in server.csr -out server.crt -CA ca.crt -CAkey ca.key -CAcreateserial -extensions v3_req -extensions SAN -extfile openssl.cnf

  4. Update the 'CABundle' field in both config/webhook_external.yaml and config/webhook_internal.yaml with the base64-encoded new CA certificate.

  5. For Helm deployments, apply equivalent changes to the Helm charts.

5.1.2 Publicly Trusted Certificates​

With publicly trusted certificates, skip the above procedures. Remove the "CABundle" field from config/webhook_external.yaml and config/webhook_internal.yaml, and set the domain to your registered domain.