Secrets

A Secret is an object that contains a small amount of sensitive data such as a password, a token, or a key. Such information might otherwise be put in a Pod specification or in a container image. Using a Secret means that you don't need to include confidential data in your application code.

Because Secrets can be created independently of the Pods that use them, there is less risk of the Secret (and its data) being exposed during the workflow of creating, viewing, and editing Pods. Kubernetes, and applications that run in your cluster, can also take additional precautions with Secrets, such as avoiding writing secret data to nonvolatile storage.

Secrets are similar to ConfigMaps but are specifically intended to hold confidential data.

See Information security for Secrets for more details.

Uses for Secrets

There are three main ways for a Pod to use a Secret:

The Kubernetes control plane also uses Secrets; for example, bootstrap token Secrets are a mechanism to help automate node registration.

Alternatives to Secrets

Rather than using a Secret to protect confidential data, you can pick from alternatives.

Here are some of your options:

  • if your cloud-native component needs to authenticate to another application that you know is running within the same Kubernetes cluster, you can use a ServiceAccount and its tokens to identify your client.
  • there are third-party tools that you can run, either within or outside your cluster, that provide secrets management. For example, a service that Pods access over HTTPS, that reveals a secret if the client correctly authenticates (for example, with a ServiceAccount token).
  • for authentication, you can implement a custom signer for X.509 certificates, and use CertificateSigningRequests to let that custom signer issue certificates to Pods that need them.
  • you can use a device plugin to expose node-local encryption hardware to a specific Pod. For example, you can schedule trusted Pods onto nodes that provide a Trusted Platform Module, configured out-of-band.

You can also combine two or more of those options, including the option to use Secret objects themselves.

For example: implement (or deploy) an operator that fetches short-lived session tokens from an external service, and then creates Secrets based on those short-lived session tokens. Pods running in your cluster can make use of the session tokens, and operator ensures they are valid. This separation means that you can run Pods that are unaware of the exact mechanisms for issuing and refreshing those session tokens.

Working with Secrets

Creating a Secret

There are several options to create a Secret:

Constraints on Secret names and data

The name of a Secret object must be a valid DNS subdomain name.

You can specify the data and/or the stringData field when creating a configuration file for a Secret. The data and the stringData fields are optional. The values for all keys in the data field have to be base64-encoded strings. If the conversion to base64 string is not desirable, you can choose to specify the stringData field instead, which accepts arbitrary strings as values.

The keys of data and stringData must consist of alphanumeric characters, -, _ or .. All key-value pairs in the stringData field are internally merged into the data field. If a key appears in both the data and the stringData field, the value specified in the stringData field takes precedence.

Size limit

Individual secrets are limited to 1MiB in size. This is to discourage creation of very large secrets that could exhaust the API server and kubelet memory. However, creation of many smaller secrets could also exhaust memory. You can use a resource quota to limit the number of Secrets (or other resources) in a namespace.

Editing a Secret

You can edit an existing Secret using kubectl:

kubectl edit secrets mysecret

This opens your default editor and allows you to update the base64 encoded Secret values in the data field; for example:

# Please edit the object below. Lines beginning with a '#' will be ignored,
# and an empty file will abort the edit. If an error occurs while saving this file, it will be
# reopened with the relevant failures.
#
apiVersion: v1
data:
  username: YWRtaW4=
  password: MWYyZDFlMmU2N2Rm
kind: Secret
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: { ... }
  creationTimestamp: 2020-01-22T18:41:56Z
  name: mysecret
  namespace: default
  resourceVersion: "164619"
  uid: cfee02d6-c137-11e5-8d73-42010af00002
type: Opaque

That example manifest defines a Secret with two keys in the data field: username and password. The values are Base64 strings in the manifest; however, when you use the Secret with a Pod then the kubelet provides the decoded data to the Pod and its containers.

You can package many keys and values into one Secret, or use many Secrets, whichever is convenient.

Using a Secret

Secrets can be mounted as data volumes or exposed as environment variables to be used by a container in a Pod. Secrets can also be used by other parts of the system, without being directly exposed to the Pod. For example, Secrets can hold credentials that other parts of the system should use to interact with external systems on your behalf.

Secret volume sources are validated to ensure that the specified object reference actually points to an object of type Secret. Therefore, a Secret needs to be created before any Pods that depend on it.

If the Secret cannot be fetched (perhaps because it does not exist, or due to a temporary lack of connection to the API server) the kubelet periodically retries running that Pod. The kubelet also reports an Event for that Pod, including details of the problem fetching the Secret.

Optional Secrets

When you define a container environment variable based on a Secret, you can mark it as optional. The default is for the Secret to be required.

None of a Pod's containers will start until all non-optional Secrets are available.

If a Pod references a specific key in a Secret and that Secret does exist, but is missing the named key, the Pod fails during startup.

Using Secrets as files from a Pod

If you want to access data from a Secret in a Pod, one way to do that is to have Kubernetes make the value of that Secret be available as a file inside the filesystem of one or more of the Pod's containers.

To configure that, you:

  1. Create a secret or use an existing one. Multiple Pods can reference the same secret.
  2. Modify your Pod definition to add a volume under .spec.volumes[]. Name the volume anything, and have a .spec.volumes[].secret.secretName field equal to the name of the Secret object.
  3. Add a .spec.containers[].volumeMounts[] to each container that needs the secret. Specify .spec.containers[].volumeMounts[].readOnly = true and .spec.containers[].volumeMounts[].mountPath to an unused directory name where you would like the secrets to appear.
  4. Modify your image or command line so that the program looks for files in that directory. Each key in the secret data map becomes the filename under mountPath.

This is an example of a Pod that mounts a Secret named mysecret in a volume:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: foo
      mountPath: "/etc/foo"
      readOnly: true
  volumes:
  - name: foo
    secret:
      secretName: mysecret
      optional: false # default setting; "mysecret" must exist

Each Secret you want to use needs to be referred to in .spec.volumes.

If there are multiple containers in the Pod, then each container needs its own volumeMounts block, but only one .spec.volumes is needed per Secret.

Projection of Secret keys to specific paths

You can also control the paths within the volume where Secret keys are projected. You can use the .spec.volumes[].secret.items field to change the target path of each key:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: foo
      mountPath: "/etc/foo"
      readOnly: true
  volumes:
  - name: foo
    secret:
      secretName: mysecret
      items:
      - key: username
        path: my-group/my-username

What will happen:

  • the username key from mysecret is available to the container at the path /etc/foo/my-group/my-username instead of at /etc/foo/username.
  • the password key from that Secret object is not projected.

If .spec.volumes[].secret.items is used, only keys specified in items are projected. To consume all keys from the Secret, all of them must be listed in the items field.

If you list keys explicitly, then all listed keys must exist in the corresponding Secret. Otherwise, the volume is not created.

Secret files permissions

You can set the POSIX file access permission bits for a single Secret key. If you don't specify any permissions, 0644 is used by default. You can also set a default mode for the entire Secret volume and override per key if needed.

For example, you can specify a default mode like this:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - name: mypod
    image: redis
    volumeMounts:
    - name: foo
      mountPath: "/etc/foo"
  volumes:
  - name: foo
    secret:
      secretName: mysecret
      defaultMode: 0400

The secret is mounted on /etc/foo; all the files created by the secret volume mount have permission 0400.

Consuming Secret values from volumes

Inside the container that mounts a secret volume, the secret keys appear as files. The secret values are base64 decoded and stored inside these files.

This is the result of commands executed inside the container from the example above:

ls /etc/foo/

The output is similar to:

username
password
cat /etc/foo/username

The output is similar to:

admin
cat /etc/foo/password

The output is similar to:

1f2d1e2e67df

The program in a container is responsible for reading the secret data from these files, as needed.

Mounted Secrets are updated automatically

When a volume contains data from a Secret, and that Secret is updated, Kubernetes tracks this and updates the data in the volume, using an eventually-consistent approach.

The kubelet keeps a cache of the current keys and values for the Secrets that are used in volumes for pods on that node. You can configure the way that the kubelet detects changes from the cached values. The configMapAndSecretChangeDetectionStrategy field in the kubelet configuration controls which strategy the kubelet uses. The default strategy is Watch.

Updates to Secrets can be either propagated by an API watch mechanism (the default), based on a cache with a defined time-to-live, or polled from the cluster API server on each kubelet synchronisation loop.

As a result, the total delay from the moment when the Secret is updated to the moment when new keys are projected to the Pod can be as long as the kubelet sync period + cache propagation delay, where the cache propagation delay depends on the chosen cache type (following the same order listed in the previous paragraph, these are: watch propagation delay, the configured cache TTL, or zero for direct polling).

Using Secrets as environment variables

To use a Secret in an environment variable in a Pod:

  1. Create a Secret (or use an existing one). Multiple Pods can reference the same Secret.
  2. Modify your Pod definition in each container that you wish to consume the value of a secret key to add an environment variable for each secret key you wish to consume. The environment variable that consumes the secret key should populate the secret's name and key in env[].valueFrom.secretKeyRef.
  3. Modify your image and/or command line so that the program looks for values in the specified environment variables.

This is an example of a Pod that uses a Secret via environment variables:

apiVersion: v1
kind: Pod
metadata:
  name: secret-env-pod
spec:
  containers:
  - name: mycontainer
    image: redis
    env:
      - name: SECRET_USERNAME
        valueFrom:
          secretKeyRef:
            name: mysecret
            key: username
            optional: false # same as default; "mysecret" must exist
                            # and include a key named "username"
      - name: SECRET_PASSWORD
        valueFrom:
          secretKeyRef:
            name: mysecret
            key: password
            optional: false # same as default; "mysecret" must exist
                            # and include a key named "password"
  restartPolicy: Never

Invalid environment variables

Secrets used to populate environment variables by the envFrom field that have keys that are considered invalid environment variable names will have those keys skipped. The Pod is allowed to start.

If you define a Pod with an invalid variable name, the failed Pod startup includes an event with the reason set to InvalidVariableNames and a message that lists the skipped invalid keys. The following example shows a Pod that refers to a Secret named mysecret, where mysecret contains 2 invalid keys: 1badkey and 2alsobad.

kubectl get events

The output is similar to:

LASTSEEN   FIRSTSEEN   COUNT     NAME            KIND      SUBOBJECT                         TYPE      REASON
0s         0s          1         dapi-test-pod   Pod                                         Warning   InvalidEnvironmentVariableNames   kubelet, 127.0.0.1      Keys [1badkey, 2alsobad] from the EnvFrom secret default/mysecret were skipped since they are considered invalid environment variable names.

Consuming Secret values from environment variables

Inside a container that consumes a Secret using environment variables, the secret keys appear as normal environment variables. The values of those variables are the base64 decoded values of the secret data.

This is the result of commands executed inside the container from the example above:

echo "$SECRET_USERNAME"

The output is similar to:

admin
echo "$SECRET_PASSWORD"

The output is similar to:

1f2d1e2e67df

Container image pull secrets

If you want to fetch container images from a private repository, you need a way for the kubelet on each node to authenticate to that repository. You can configure image pull secrets to make this possible. These secrets are configured at the Pod level.

The imagePullSecrets field for a Pod is a list of references to Secrets in the same namespace as the Pod. You can use an imagePullSecrets to pass image registry access credentials to the kubelet. The kubelet uses this information to pull a private image on behalf of your Pod. See PodSpec in the Pod API reference for more information about the imagePullSecrets field.

Using imagePullSecrets

The imagePullSecrets field is a list of references to secrets in the same namespace. You can use an imagePullSecrets to pass a secret that contains a Docker (or other) image registry password to the kubelet. The kubelet uses this information to pull a private image on behalf of your Pod. See the PodSpec API for more information about the imagePullSecrets field.

Manually specifying an imagePullSecret

You can learn how to specify imagePullSecrets from the container images documentation.

Arranging for imagePullSecrets to be automatically attached

You can manually create imagePullSecrets, and reference these from a ServiceAccount. Any Pods created with that ServiceAccount or created with that ServiceAccount by default, will get their imagePullSecrets field set to that of the service account. See Add ImagePullSecrets to a service account for a detailed explanation of that process.

Using Secrets with static Pods

You cannot use ConfigMaps or Secrets with static Pods.

Use cases

Use case: As container environment variables

Create a secret

apiVersion: v1
kind: Secret
metadata:
  name: mysecret
type: Opaque
data:
  USER_NAME: YWRtaW4=
  PASSWORD: MWYyZDFlMmU2N2Rm

Create the Secret:

kubectl apply -f mysecret.yaml

Use envFrom to define all of the Secret's data as container environment variables. The key from the Secret becomes the environment variable name in the Pod.

apiVersion: v1
kind: Pod
metadata:
  name: secret-test-pod
spec:
  containers:
    - name: test-container
      image: k8s.gcr.io/busybox
      command: [ "/bin/sh", "-c", "env" ]
      envFrom:
      - secretRef:
          name: mysecret
  restartPolicy: Never

Use case: Pod with SSH keys

Create a Secret containing some SSH keys:

kubectl create secret generic ssh-key-secret --from-file=ssh-privatekey=/path/to/.ssh/id_rsa --from-file=ssh-publickey=/path/to/.ssh/id_rsa.pub

The output is similar to:

secret "ssh-key-secret" created

You can also create a kustomization.yaml with a secretGenerator field containing ssh keys.

Now you can create a Pod which references the secret with the SSH key and consumes it in a volume:

apiVersion: v1
kind: Pod
metadata:
  name: secret-test-pod
  labels:
    name: secret-test
spec:
  volumes:
  - name: secret-volume
    secret:
      secretName: ssh-key-secret
  containers:
  - name: ssh-test-container
    image: mySshImage
    volumeMounts:
    - name: secret-volume
      readOnly: true
      mountPath: "/etc/secret-volume"

When the container's command runs, the pieces of the key will be available in:

/etc/secret-volume/ssh-publickey
/etc/secret-volume/ssh-privatekey

The container is then free to use the secret data to establish an SSH connection.

Use case: Pods with prod / test credentials

This example illustrates a Pod which consumes a secret containing production credentials and another Pod which consumes a secret with test environment credentials.

You can create a kustomization.yaml with a secretGenerator field or run kubectl create secret.

kubectl create secret generic prod-db-secret --from-literal=username=produser --from-literal=password=Y4nys7f11

The output is similar to:

secret "prod-db-secret" created

You can also create a secret for test environment credentials.

kubectl create secret generic test-db-secret --from-literal=username=testuser --from-literal=password=iluvtests

The output is similar to:

secret "test-db-secret" created

Now make the Pods:

cat <<EOF > pod.yaml
apiVersion: v1
kind: List
items:
- kind: Pod
  apiVersion: v1
  metadata:
    name: prod-db-client-pod
    labels:
      name: prod-db-client
  spec:
    volumes:
    - name: secret-volume
      secret:
        secretName: prod-db-secret
    containers:
    - name: db-client-container
      image: myClientImage
      volumeMounts:
      - name: secret-volume
        readOnly: true
        mountPath: "/etc/secret-volume"
- kind: Pod
  apiVersion: v1
  metadata:
    name: test-db-client-pod
    labels:
      name: test-db-client
  spec:
    volumes:
    - name: secret-volume
      secret:
        secretName: test-db-secret
    containers:
    - name: db-client-container
      image: myClientImage
      volumeMounts:
      - name: secret-volume
        readOnly: true
        mountPath: "/etc/secret-volume"
EOF

Add the pods to the same kustomization.yaml:

cat <<EOF >> kustomization.yaml
resources:
- pod.yaml
EOF

Apply all those objects on the API server by running:

kubectl apply -k .

Both containers will have the following files present on their filesystems with the values for each container's environment:

/etc/secret-volume/username
/etc/secret-volume/password

Note how the specs for the two Pods differ only in one field; this facilitates creating Pods with different capabilities from a common Pod template.

You could further simplify the base Pod specification by using two service accounts:

  1. prod-user with the prod-db-secret
  2. test-user with the test-db-secret

The Pod specification is shortened to:

apiVersion: v1
kind: Pod
metadata:
  name: prod-db-client-pod
  labels:
    name: prod-db-client
spec:
  serviceAccount: prod-db-client
  containers:
  - name: db-client-container
    image: myClientImage

Use case: dotfiles in a secret volume

You can make your data "hidden" by defining a key that begins with a dot. This key represents a dotfile or "hidden" file. For example, when the following secret is mounted into a volume, secret-volume:

apiVersion: v1
kind: Secret
metadata:
  name: dotfile-secret
data:
  .secret-file: dmFsdWUtMg0KDQo=
---
apiVersion: v1
kind: Pod
metadata:
  name: secret-dotfiles-pod
spec:
  volumes:
  - name: secret-volume
    secret:
      secretName: dotfile-secret
  containers:
  - name: dotfile-test-container
    image: k8s.gcr.io/busybox
    command:
    - ls
    - "-l"
    - "/etc/secret-volume"
    volumeMounts:
    - name: secret-volume
      readOnly: true
      mountPath: "/etc/secret-volume"

The volume will contain a single file, called .secret-file, and the dotfile-test-container will have this file present at the path /etc/secret-volume/.secret-file.

Use case: Secret visible to one container in a Pod

Consider a program that needs to handle HTTP requests, do some complex business logic, and then sign some messages with an HMAC. Because it has complex application logic, there might be an unnoticed remote file reading exploit in the server, which could expose the private key to an attacker.

This could be divided into two processes in two containers: a frontend container which handles user interaction and business logic, but which cannot see the private key; and a signer container that can see the private key, and responds to simple signing requests from the frontend (for example, over localhost networking).

With this partitioned approach, an attacker now has to trick the application server into doing something rather arbitrary, which may be harder than getting it to read a file.

Types of Secret

When creating a Secret, you can specify its type using the type field of the Secret resource, or certain equivalent kubectl command line flags (if available). The Secret type is used to facilitate programmatic handling of the Secret data.

Kubernetes provides several built-in types for some common usage scenarios. These types vary in terms of the validations performed and the constraints Kubernetes imposes on them.

Built-in Type Usage
Opaque arbitrary user-defined data
kubernetes.io/service-account-token ServiceAccount token
kubernetes.io/dockercfg serialized ~/.dockercfg file
kubernetes.io/dockerconfigjson serialized ~/.docker/config.json file
kubernetes.io/basic-auth credentials for basic authentication
kubernetes.io/ssh-auth credentials for SSH authentication
kubernetes.io/tls data for a TLS client or server
bootstrap.kubernetes.io/token bootstrap token data

You can define and use your own Secret type by assigning a non-empty string as the type value for a Secret object (an empty string is treated as an Opaque type).

Kubernetes doesn't impose any constraints on the type name. However, if you are using one of the built-in types, you must meet all the requirements defined for that type.

If you are defining a type of secret that's for public use, follow the convention and structure the secret type to have your domain name before the name, separated by a /. For example: cloud-hosting.example.net/cloud-api-credentials.

Opaque secrets

Opaque is the default Secret type if omitted from a Secret configuration file. When you create a Secret using kubectl, you will use the generic subcommand to indicate an Opaque Secret type. For example, the following command creates an empty Secret of type Opaque.

kubectl create secret generic empty-secret
kubectl get secret empty-secret

The output looks like:

NAME           TYPE     DATA   AGE
empty-secret   Opaque   0      2m6s

The DATA column shows the number of data items stored in the Secret. In this case, 0 means you have created an empty Secret.

Service account token Secrets

A kubernetes.io/service-account-token type of Secret is used to store a token that identifies a service account. When using this Secret type, you need to ensure that the kubernetes.io/service-account.name annotation is set to an existing service account name. A Kubernetes controller fills in some other fields such as the kubernetes.io/service-account.uid annotation, and the token key in the data field, which is set to contain an authentication token.

The following example configuration declares a service account token Secret:

apiVersion: v1
kind: Secret
metadata:
  name: secret-sa-sample
  annotations:
    kubernetes.io/service-account.name: "sa-name"
type: kubernetes.io/service-account-token
data:
  # You can include additional key value pairs as you do with Opaque Secrets
  extra: YmFyCg==

When creating a Pod, Kubernetes automatically finds or creates a service account Secret and then automatically modifies your Pod to use this Secret. The service account token Secret contains credentials for accessing the Kubernetes API.

The automatic creation and use of API credentials can be disabled or overridden if desired. However, if all you need to do is securely access the API server, this is the recommended workflow.

See the ServiceAccount documentation for more information on how service accounts work.
You can also check the automountServiceAccountToken field and the serviceAccountName field of the Pod for information on referencing service account from Pods.

Docker config Secrets

You can use one of the following type values to create a Secret to store the credentials for accessing a container image registry:

  • kubernetes.io/dockercfg
  • kubernetes.io/dockerconfigjson

The kubernetes.io/dockercfg type is reserved to store a serialized ~/.dockercfg which is the legacy format for configuring Docker command line. When using this Secret type, you have to ensure the Secret data field contains a .dockercfg key whose value is content of a ~/.dockercfg file encoded in the base64 format.

The kubernetes.io/dockerconfigjson type is designed for storing a serialized JSON that follows the same format rules as the ~/.docker/config.json file which is a new format for ~/.dockercfg. When using this Secret type, the data field of the Secret object must contain a .dockerconfigjson key, in which the content for the ~/.docker/config.json file is provided as a base64 encoded string.

Below is an example for a kubernetes.io/dockercfg type of Secret:

apiVersion: v1
kind: Secret
metadata:
  name: secret-dockercfg
type: kubernetes.io/dockercfg
data:
  .dockercfg: |
        "<base64 encoded ~/.dockercfg file>"

When you create these types of Secrets using a manifest, the API server checks whether the expected key exists in the data field, and it verifies if the value provided can be parsed as a valid JSON. The API server doesn't validate if the JSON actually is a Docker config file.

When you do not have a Docker config file, or you want to use kubectl to create a Secret for accessing a container registry, you can do:

kubectl create secret docker-registry secret-tiger-docker \
  --docker-email=[email protected] \
  --docker-username=tiger \
  --docker-password=pass1234 \
  --docker-server=my-registry.example:5000

That command creates a Secret of type kubernetes.io/dockerconfigjson. If you dump the .data.dockerconfigjson field from that new Secret and then decode it from base64:

kubectl get secret secret-tiger-docker -o jsonpath='{.data.*}' | base64 -d

then the output is equivalent to this JSON document (which is also a valid Docker configuration file):

{
  "auths": {
    "my-registry.example:5000": {
      "username": "tiger",
      "password": "pass1234",
      "email": "[email protected]",
      "auth": "dGlnZXI6cGFzczEyMzQ="
    }
  }
}

Basic authentication Secret

The kubernetes.io/basic-auth type is provided for storing credentials needed for basic authentication. When using this Secret type, the data field of the Secret must contain one of the following two keys:

  • username: the user name for authentication
  • password: the password or token for authentication

Both values for the above two keys are base64 encoded strings. You can, of course, provide the clear text content using the stringData for Secret creation.

The following manifest is an example of a basic authentication Secret:

apiVersion: v1
kind: Secret
metadata:
  name: secret-basic-auth
type: kubernetes.io/basic-auth
stringData:
  username: admin      # required field for kubernetes.io/basic-auth
  password: t0p-Secret # required field for kubernetes.io/basic-auth

The basic authentication Secret type is provided only for convenience. You can create an Opaque type for credentials used for basic authentication. However, using the defined and public Secret type (kubernetes.io/basic-auth) helps other people to understand the purpose of your Secret, and sets a convention for what key names to expect. The Kubernetes API verifies that the required keys are set for a Secret of this type.

SSH authentication secrets

The builtin type kubernetes.io/ssh-auth is provided for storing data used in SSH authentication. When using this Secret type, you will have to specify a ssh-privatekey key-value pair in the data (or stringData) field as the SSH credential to use.

The following manifest is an example of a Secret used for SSH public/private key authentication:

apiVersion: v1
kind: Secret
metadata:
  name: secret-ssh-auth
type: kubernetes.io/ssh-auth
data:
  # the data is abbreviated in this example
  ssh-privatekey: |
          MIIEpQIBAAKCAQEAulqb/Y ...

The SSH authentication Secret type is provided only for user's convenience. You could instead create an Opaque type Secret for credentials used for SSH authentication. However, using the defined and public Secret type (kubernetes.io/ssh-auth) helps other people to understand the purpose of your Secret, and sets a convention for what key names to expect. and the API server does verify if the required keys are provided in a Secret configuration.

TLS secrets

Kubernetes provides a builtin Secret type kubernetes.io/tls for storing a certificate and its associated key that are typically used for TLS.

One common use for TLS secrets is to configure encryption in transit for an Ingress, but you can also use it with other resources or directly in your workload. When using this type of Secret, the tls.key and the tls.crt key must be provided in the data (or stringData) field of the Secret configuration, although the API server doesn't actually validate the values for each key.

The following YAML contains an example config for a TLS Secret:

apiVersion: v1
kind: Secret
metadata:
  name: secret-tls
type: kubernetes.io/tls
data:
  # the data is abbreviated in this example
  tls.crt: |
        MIIC2DCCAcCgAwIBAgIBATANBgkqh ...
  tls.key: |
        MIIEpgIBAAKCAQEA7yn3bRHQ5FHMQ ...

The TLS Secret type is provided for user's convenience. You can create an Opaque for credentials used for TLS server and/or client. However, using the builtin Secret type helps ensure the consistency of Secret format in your project; the API server does verify if the required keys are provided in a Secret configuration.

When creating a TLS Secret using kubectl, you can use the tls subcommand as shown in the following example:

kubectl create secret tls my-tls-secret \
  --cert=path/to/cert/file \
  --key=path/to/key/file

The public/private key pair must exist before hand. The public key certificate for --cert must be DER format as per Section 5.1 of RFC 7468, and must match the given private key for --key (PKCS #8 in DER format; Section 11 of RFC 7468).

Bootstrap token Secrets

A bootstrap token Secret can be created by explicitly specifying the Secret type to bootstrap.kubernetes.io/token. This type of Secret is designed for tokens used during the node bootstrap process. It stores tokens used to sign well-known ConfigMaps.

A bootstrap token Secret is usually created in the kube-system namespace and named in the form bootstrap-token-<token-id> where <token-id> is a 6 character string of the token ID.

As a Kubernetes manifest, a bootstrap token Secret might look like the following:

apiVersion: v1
kind: Secret
metadata:
  name: bootstrap-token-5emitj
  namespace: kube-system
type: bootstrap.kubernetes.io/token
data:
  auth-extra-groups: c3lzdGVtOmJvb3RzdHJhcHBlcnM6a3ViZWFkbTpkZWZhdWx0LW5vZGUtdG9rZW4=
  expiration: MjAyMC0wOS0xM1QwNDozOToxMFo=
  token-id: NWVtaXRq
  token-secret: a3E0Z2lodnN6emduMXAwcg==
  usage-bootstrap-authentication: dHJ1ZQ==
  usage-bootstrap-signing: dHJ1ZQ==

A bootstrap type Secret has the following keys specified under data:

  • token-id: A random 6 character string as the token identifier. Required.
  • token-secret: A random 16 character string as the actual token secret. Required.
  • description: A human-readable string that describes what the token is used for. Optional.
  • expiration: An absolute UTC time using RFC3339 specifying when the token should be expired. Optional.
  • usage-bootstrap-<usage>: A boolean flag indicating additional usage for the bootstrap token.
  • auth-extra-groups: A comma-separated list of group names that will be authenticated as in addition to the system:bootstrappers group.

The above YAML may look confusing because the values are all in base64 encoded strings. In fact, you can create an identical Secret using the following YAML:

apiVersion: v1
kind: Secret
metadata:
  # Note how the Secret is named
  name: bootstrap-token-5emitj
  # A bootstrap token Secret usually resides in the kube-system namespace
  namespace: kube-system
type: bootstrap.kubernetes.io/token
stringData:
  auth-extra-groups: "system:bootstrappers:kubeadm:default-node-token"
  expiration: "2020-09-13T04:39:10Z"
  # This token ID is used in the name
  token-id: "5emitj"
  token-secret: "kq4gihvszzgn1p0r"
  # This token can be used for authentication
  usage-bootstrap-authentication: "true"
  # and it can be used for signing
  usage-bootstrap-signing: "true"

Immutable Secrets

FEATURE STATE: Kubernetes v1.21 [stable]

Kubernetes lets you mark specific Secrets (and ConfigMaps) as immutable. Preventing changes to the data of an existing Secret has the following benefits:

  • protects you from accidental (or unwanted) updates that could cause applications outages
  • (for clusters that extensively use Secrets - at least tens of thousands of unique Secret to Pod mounts), switching to immutable Secrets improves the performance of your cluster by significantly reducing load on kube-apiserver. The kubelet does not need to maintain a [watch] on any Secrets that are marked as immutable.

Marking a Secret as immutable

You can create an immutable Secret by setting the immutable field to true. For example,

apiVersion: v1
kind: Secret
metadata:
  ...
data:
  ...
immutable: true

You can also update any existing mutable Secret to make it immutable.

Information security for Secrets

Although ConfigMap and Secret work similarly, Kubernetes applies some additional protection for Secret objects.

Secrets often hold values that span a spectrum of importance, many of which can cause escalations within Kubernetes (e.g. service account tokens) and to external systems. Even if an individual app can reason about the power of the Secrets it expects to interact with, other apps within the same namespace can render those assumptions invalid.

A Secret is only sent to a node if a Pod on that node requires it. For mounting secrets into Pods, the kubelet stores a copy of the data into a tmpfs so that the confidential data is not written to durable storage. Once the Pod that depends on the Secret is deleted, the kubelet deletes its local copy of the confidential data from the Secret.

There may be several containers in a Pod. By default, containers you define only have access to the default ServiceAccount and its related Secret. You must explicitly define environment variables or map a volume into a container in order to provide access to any other Secret.

There may be Secrets for several Pods on the same node. However, only the Secrets that a Pod requests are potentially visible within its containers. Therefore, one Pod does not have access to the Secrets of another Pod.

Security recommendations for developers

  • Applications still need to protect the value of confidential information after reading it from an environment variable or volume. For example, your application must avoid logging the secret data in the clear or transmitting it to an untrusted party.
  • If you are defining multiple containers in a Pod, and only one of those containers needs access to a Secret, define the volume mount or environment variable configuration so that the other containers do not have access to that Secret.
  • If you configure a Secret through a manifest, with the secret data encoded as base64, sharing this file or checking it in to a source repository means the secret is available to everyone who can read the manifest. Base64 encoding is not an encryption method, it provides no additional confidentiality over plain text.
  • When deploying applications that interact with the Secret API, you should limit access using authorization policies such as RBAC.
  • In the Kubernetes API, watch and list requests for Secrets within a namespace are extremely powerful capabilities. Avoid granting this access where feasible, since listing Secrets allows the clients to inspect the values of every Secret in that namespace.

Security recommendations for cluster administrators

  • Reserve the ability to watch or list all secrets in a cluster (using the Kubernetes API), so that only the most privileged, system-level components can perform this action.
  • When deploying applications that interact with the Secret API, you should limit access using authorization policies such as RBAC.
  • In the API server, objects (including Secrets) are persisted into etcd; therefore:
    • only allow cluster admistrators to access etcd (this includes read-only access);
    • enable encryption at rest for Secret objects, so that the data of these Secrets are not stored in the clear into etcd;
    • consider wiping / shredding the durable storage used by etcd once it is no longer in use;
    • if there are multiple etcd instances, make sure that etcd is using SSL/TLS for communication between etcd peers.

What's next

Last modified May 21, 2022 at 2:17 AM PST: Fix Markdown link (cca0aca855)