Secrets

Kubernetes Secrets let you store and manage sensitive information, such as passwords, OAuth tokens, and ssh keys. Storing confidential information in a Secret is safer and more flexible than putting it verbatim in a PodA Pod represents a set of running containers in your cluster. definition or in a container imageStored instance of a container that holds a set of software needed to run an application. . See Secrets design document for more information.

Overview of 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 an image. Users can create secrets and the system also creates some secrets.

To use a secret, a Pod needs to reference the secret. A secret can be used with a Pod in three ways:

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

The keys of data and stringData must consist of alphanumeric characters, -, _ or ..

Built-in Secrets

Service accounts automatically create and attach Secrets with API credentials

Kubernetes automatically creates secrets which contain credentials for accessing the API and automatically modifies your Pods to use this type of secret.

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.

Creating a Secret

There are several options to create a Secret:

Editing a Secret

An existing Secret may be edited with the following command:

kubectl edit secrets mysecret

This will open the default configured editor and allow for updating the base64 encoded Secret values in the data field:

# 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 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: 2016-01-22T18:41:56Z
  name: mysecret
  namespace: default
  resourceVersion: "164619"
  uid: cfee02d6-c137-11e5-8d73-42010af00002
type: Opaque

Using Secrets

Secrets can be mounted as data volumes or exposed as environment variablesContainer environment variables are name=value pairs that provide useful information into containers running in a Pod. 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.

Using Secrets as files from a Pod

To consume a Secret in a volume in a Pod:

  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 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

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.

You can package many files into one secret, or use many secrets, whichever is convenient.

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:

  • username secret is stored under /etc/foo/my-group/my-username file instead of /etc/foo/username.
  • password secret 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. All listed keys must exist in the corresponding secret. Otherwise, the volume is not created.

Secret files permissions

You can set the 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

Then, the secret will be mounted on /etc/foo and all the files created by the secret volume mount will have permission 0400.

Note that the JSON spec doesn't support octal notation, so use the value 256 for 0400 permissions. If you use YAML instead of JSON for the Pod, you can use octal notation to specify permissions in a more natural way.

Note if you kubectl exec into the Pod, you need to follow the symlink to find the expected file mode. For example,

Check the secrets file mode on the pod.

kubectl exec mypod -it sh

cd /etc/foo
ls -l

The output is similar to this:

total 0
lrwxrwxrwx 1 root root 15 May 18 00:18 password -> ..data/password
lrwxrwxrwx 1 root root 15 May 18 00:18 username -> ..data/username

Follow the symlink to find the correct file mode.

cd /etc/foo/..data
ls -l

The output is similar to this:

total 8
-r-------- 1 root root 12 May 18 00:18 password
-r-------- 1 root root  5 May 18 00:18 username

You can also use mapping, as in the previous example, and specify different permissions for different files 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
      items:
      - key: username
        path: my-group/my-username
        mode: 0777

In this case, the file resulting in /etc/foo/my-group/my-username will have permission value of 0777. If you use JSON, owing to JSON limitations, you must specify the mode in decimal notation, 511.

Note that this permission value might be displayed in decimal notation if you read it later.

Consuming Secret values from volumes

Inside the container that mounts a secret volume, the secret keys appear as files and 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 secrets from the files.

Mounted Secrets are updated automatically

When a secret currently consumed in a volume is updated, projected keys are eventually updated as well. The kubelet checks whether the mounted secret is fresh on every periodic sync. However, the kubelet uses its local cache for getting the current value of the Secret. The type of the cache is configurable using the ConfigMapAndSecretChangeDetectionStrategy field in the KubeletConfiguration struct. A Secret can be either propagated by watch (default), ttl-based, or simply redirecting all requests directly to the API server. 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 (it equals to watch propagation delay, ttl of cache, or zero correspondingly).

Note: A container using a Secret as a subPath volume mount will not receive Secret updates.

Using Secrets as environment variables

To use a secret in an environment variableContainer environment variables are name=value pairs that provide useful information into containers running in a Pod. 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 secrets from 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
      - name: SECRET_PASSWORD
        valueFrom:
          secretKeyRef:
            name: mysecret
            key: password
  restartPolicy: Never

Consuming Secret Values from environment variables

Inside a container that consumes a secret in an environment variables, the secret keys appear as normal environment variables containing 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

Immutable Secrets

FEATURE STATE: Kubernetes v1.19 [beta]

The Kubernetes beta feature Immutable Secrets and ConfigMaps provides an option to set individual Secrets and ConfigMaps as immutable. For clusters that extensively use Secrets (at least tens of thousands of unique Secret to Pod mounts), preventing changes to their data has the following advantages:

  • protects you from accidental (or unwanted) updates that could cause applications outages
  • improves performance of your cluster by significantly reducing load on kube-apiserver, by closing watches for secrets marked as immutable.

This feature is controlled by the ImmutableEphemeralVolumes feature gate, which is enabled by default since v1.19. You can create an immutable Secret by setting the immutable field to true. For example,

apiVersion: v1
kind: Secret
metadata:
  ...
data:
  ...
immutable: true
Note: Once a Secret or ConfigMap is marked as immutable, it is not possible to revert this change nor to mutate the contents of the data field. You can only delete and recreate the Secret. Existing Pods maintain a mount point to the deleted Secret - it is recommended to recreate these pods.

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 it 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.

Automatic mounting of manually created Secrets

Manually created secrets (for example, one containing a token for accessing a GitHub account) can be automatically attached to pods based on their service account. See Injecting Information into Pods Using a PodPreset for a detailed explanation of that process.

Details

Restrictions

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.

Secret resources reside in a namespaceAn abstraction used by Kubernetes to support multiple virtual clusters on the same physical cluster. . Secrets can only be referenced by Pods in that same namespace.

Individual secrets are limited to 1MiB in size. This is to discourage creation of very large secrets which would exhaust the API server and kubelet memory. However, creation of many smaller secrets could also exhaust memory. More comprehensive limits on memory usage due to secrets is a planned feature.

The kubelet only supports the use of secrets for Pods where the secrets are obtained from the API server. This includes any Pods created using kubectl, or indirectly via a replication controller. It does not include Pods created as a result of the kubelet --manifest-url flag, its --config flag, or its REST API (these are not common ways to create Pods.)

Secrets must be created before they are consumed in Pods as environment variables unless they are marked as optional. References to secrets that do not exist will prevent the Pod from starting.

References (secretKeyRef field) to keys that do not exist in a named Secret will prevent the Pod from starting.

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 will be allowed to start. There will be an event whose reason is InvalidVariableNames and the message will contain the list of invalid keys that were skipped. The example shows a pod which refers to the default/mysecret that 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.

Secret and Pod lifetime interaction

When a Pod is created by calling the Kubernetes API, there is no check if a referenced secret exists. Once a Pod is scheduled, the kubelet will try to fetch the secret value. If the secret cannot be fetched because it does not exist or because of a temporary lack of connection to the API server, the kubelet will periodically retry. It will report an event about the Pod explaining the reason it is not started yet. Once the secret is fetched, the kubelet will create and mount a volume containing it. None of the Pod's containers will start until all the Pod's volumes are mounted.

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.

Caution: Think carefully before sending your own ssh keys: other users of the cluster may have access to the secret. Use a service account which you want to be accessible to all the users with whom you share the Kubernetes cluster, and can revoke this account if the users are compromised.

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
Note:

Special characters such as $, \, *, =, and ! will be interpreted by your shell and require escaping. In most shells, the easiest way to escape the password is to surround it with single quotes ('). For example, if your actual password is S!B\*d$zDsb=, you should execute the command this way:

kubectl create secret generic dev-db-secret --from-literal=username=devuser --from-literal=password='S!B\*d$zDsb='

You do not need to escape special characters in passwords from files (--from-file).

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.

Note: Files beginning with dot characters are hidden from the output of ls -l; you must use ls -la to see them when listing directory contents.

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.

Best practices

Clients that use the Secret API

When deploying applications that interact with the Secret API, you should limit access using authorization policies such as RBAC.

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.

For these reasons watch and list requests for secrets within a namespace are extremely powerful capabilities and should be avoided, since listing secrets allows the clients to inspect the values of all secrets that are in that namespace. The ability to watch and list all secrets in a cluster should be reserved for only the most privileged, system-level components.

Applications that need to access the Secret API should perform get requests on the secrets they need. This lets administrators restrict access to all secrets while white-listing access to individual instances that the app needs.

For improved performance over a looping get, clients can design resources that reference a secret then watch the resource, re-requesting the secret when the reference changes. Additionally, a "bulk watch" API to let clients watch individual resources has also been proposed, and will likely be available in future releases of Kubernetes.

Security properties

Protections

Because secrets can be created independently of the Pods that use them, there is less risk of the secret being exposed during the workflow of creating, viewing, and editing Pods. The system can also take additional precautions with Secrets, such as avoiding writing them to disk where possible.

A secret is only sent to a node if a Pod on that node requires it. The kubelet stores the secret into a tmpfs so that the secret is not written to disk storage. Once the Pod that depends on the secret is deleted, the kubelet will delete its local copy of the secret data as well.

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.

There may be several containers in a Pod. However, each container in a Pod has to request the secret volume in its volumeMounts for it to be visible within the container. This can be used to construct useful security partitions at the Pod level.

On most Kubernetes distributions, communication between users and the API server, and from the API server to the kubelets, is protected by SSL/TLS. Secrets are protected when transmitted over these channels.

FEATURE STATE: Kubernetes v1.13 [beta]

You can enable encryption at rest for secret data, so that the secrets are not stored in the clear into etcdConsistent and highly-available key value store used as Kubernetes' backing store for all cluster data. .

Risks

  • In the API server, secret data is stored in etcdConsistent and highly-available key value store used as Kubernetes' backing store for all cluster data. ; therefore:
    • Administrators should enable encryption at rest for cluster data (requires v1.13 or later).
    • Administrators should limit access to etcd to admin users.
    • Administrators may want to wipe/shred disks used by etcd when no longer in use.
    • If running etcd in a cluster, administrators should make sure to use SSL/TLS for etcd peer-to-peer communication.
  • If you configure the secret through a manifest (JSON or YAML) file which has the secret data encoded as base64, sharing this file or checking it in to a source repository means the secret is compromised. Base64 encoding is not an encryption method and is considered the same as plain text.
  • Applications still need to protect the value of secret after reading it from the volume, such as not accidentally logging it or transmitting it to an untrusted party.
  • A user who can create a Pod that uses a secret can also see the value of that secret. Even if the API server policy does not allow that user to read the Secret, the user could run a Pod which exposes the secret.
  • Currently, anyone with root permission on any node can read any secret from the API server, by impersonating the kubelet. It is a planned feature to only send secrets to nodes that actually require them, to restrict the impact of a root exploit on a single node.

What's next

Last modified September 10, 2020 at 2:27 PM PST: Separating tasks from Secret concept (995b067fd)