Data-driven policies for reuse and intelligent decision making

Variables make policies smarter and reusable by enabling references to data in the policy definition, the admission review request, and external data sources like ConfigMaps, the Kubernetes API Server, and even OCI image registries.

Variables are stored as JSON and Kyverno supports using JMESPath (pronounced “James path”) to select and transform JSON data. With JMESPath, values from data sources are referenced in the format of {{key1.key2.key3}}. For example, to reference the name of an new/incoming resource during a kubectl apply action such as a Namespace, you would write this as a variable reference: {{}}. The policy engine will substitute any values with the format {{ <JMESPath> }} with the variable value before processing the rule. For a page dedicated to exploring JMESPath’s use in Kyverno see here.

Pre-defined Variables

Kyverno automatically creates a few useful variables and makes them available within rules:

  1. serviceAccountName: the “userName” which is the last part of a service account (i.e. without the prefix system:serviceaccount:<namespace>:). For example, when processing a request from system:serviceaccount:nirmata:user1 Kyverno will store the value user1 in the variable serviceAccountName.

  2. serviceAccountNamespace: the “namespace” part of the serviceAccount. For example, when processing a request from system:serviceaccount:nirmata:user1 Kyverno will store nirmata in the variable serviceAccountNamespace.

  3. request.roles: a list of roles stored in an array the given account may have. For example, ["foo:dave"].

  4. request.clusterRoles: a list of cluster roles stored in an array. For example, ["dave-admin","system:basic-user","system:discovery","system:public-info-viewer"]

  5. images: a map of container image information, if available. See Variables from container images for more information.

Variables from policy definitions

Kyverno policy definitions can refer to other fields in the policy definition as a form of “shortcut”. This can be a useful way to analyze and compare values without having to explicitly define them.

In order for Kyverno to refer to these existing values in a manifest, it uses the notation $(./../key_1/key_2). This may look familiar as it is essentially the same way Linux/Unix systems refer to relative paths. For example, consider the policy manifest snippet below.

 1validationFailureAction: enforce
 3- name: check-tcpSocket
 4  match:
 5    any:
 6    - resources:
 7        kinds:
 8        - Pod
 9  validate:
10    message: "Port number for the livenessProbe must be less than that of the readinessProbe."
11    pattern:
12      spec:
13        ^(containers):
14        - livenessProbe:
15            tcpSocket:
16              port: "$(./../../../readinessProbe/tcpSocket/port)"
17          readinessProbe:
18            tcpSocket:
19              port: "3000"

In this above example, for any containers found in a Pod spec, the field readinessProbe.tcpSocket.port must be 3000 and the field livenessProbe.tcpSocket.port must be the same value. The lookup expression can be thought of as a cd back three levels and down into the readinessProbe object.

Operators also work on manifest lookup variables as well so the previous snippet could be modified as such.

1- livenessProbe:
2    tcpSocket:
3      port: "$(<./../../../readinessProbe/tcpSocket/port)"
4  readinessProbe:
5    tcpSocket:
6      port: "3000"

In this case, the field livenessProbe.tcpSocket.port must now be less than the value specified in readinessProbe.tcpSocket.port.

For more information on operators see the Operators section.

Escaping Variables

In some cases, you wish to write a rule containing a variable for action on by another program or process flow and not for Kyverno’s use. For example, with the variables in $() notation, as of Kyverno 1.5.0 these can be escaped with a leading backslash (\) and Kyverno will not attempt to substitute values. Variables written in JMESPath notation can also be escaped using the same syntax, for example \{{ }}.

In the below policy, the value of OTEL_RESOURCE_ATTRIBUTES contains references to other environment variables which will be quoted literally as, for example, $(POD_NAMESPACE).

 2kind: Policy
 4  name: add-otel-resource-env
 6  background: false
 7  rules:
 8  - name: imbue-pod-spec
 9    match:
10      any:
11      - resources:
12          kinds:
13          - v1/Pod
14    mutate:
15      patchStrategicMerge:
16        spec:
17          containers:
18          - (name): "?*"
19            env:
20            - name: NODE_NAME
21              value: "mutated_name"
22            - name: POD_IP_ADDRESS
23              valueFrom:
24                fieldRef:
25                  fieldPath: status.podIP
26            - name: POD_NAME
27              valueFrom:
28                fieldRef:
29                  fieldPath:
30            - name: POD_NAMESPACE
31              valueFrom:
32                fieldRef:
33                  fieldPath: metadata.namespace
34            - name: POD_SERVICE_ACCOUNT
35              valueFrom:
36                fieldRef:
37                  fieldPath: spec.serviceAccountName
38            - name: OTEL_RESOURCE_ATTRIBUTES
39              value: >-
40      \$(POD_NAMESPACE),
41      \$(NODE_NAME),
42      \$(POD_NAME),
43                k8s.pod.primary_ip_address=\$(POD_IP_ADDRESS),
45                rule_applied=$(./../../../../../../../../name)                

Using a Pod definition as below, this can be tested.

 1apiVersion: v1
 2kind: Pod
 4  name: test-env-vars
 6  containers:
 7  - name: test-container
 8    image: busybox
 9    command: ["sh", "-c"]
10    args:
11    - while true; do
12      echo -en '\n';
14      sleep 10;
15      done;
16    env:
17    - name: NODE_NAME
18      value: "node_name"
19    - name: POD_NAME
20      valueFrom:
21        fieldRef:
22          fieldPath:
23    - name: POD_NAMESPACE
24      valueFrom:
25        fieldRef:
26          fieldPath: metadata.namespace
27    - name: POD_IP_ADDRESS
28      valueFrom:
29        fieldRef:
30          fieldPath: status.podIP
31  restartPolicy: Never

The result of the mutation of this Pod with respect to the OTEL_RESOURCE_ATTRIBUTES environment variable will be as follows.

2      value:$(POD_NAMESPACE),$(NODE_NAME),$(POD_NAME),
3        k8s.pod.primary_ip_address=$(POD_IP_ADDRESS),$(POD_SERVICE_ACCOUNT),
4        rule_applied=imbue-pod-spec

Variables in Helm

Both Kyverno and Helm use Golang-style variable substitution syntax and, as a result, Kyverno policies containing variables deployed through Helm may need to be “wrapped” to avoid Helm interpreting them as Helm variables.

Because Helm executes its templating routine prior to Kyverno, a Kyverno policy with a variable {{ request.userInfo.username }} must be padded with Helm’s templating so that the resulting value, after the chart is deployed, remains {{ request.userInfo.username }}. Wrap the Kyverno variables in following way shown below:

{{`{{ request.userInfo.username }}`}}

Variables from admission review requests

Kyverno operates as a webhook inside Kubernetes. Whenever a new request is made to the Kubernetes API server, for example to create a Pod, the API server sends this information to the webhooks registered to listen to the creation of Pod resources. This incoming data to a webhook is passed as a AdmissionReview object. There are four commonly used data properties available in any AdmissionReview request:

  • {{request.operation}}: the type of API action being performed (CREATE, UPDATE, DELETE, or CONNECT).
  • {{request.object}}: the object being created or modified. It is null for DELETE requests.
  • {{request.oldObject}}: the object being modified. It is null for CREATE and CONNECT requests.
  • {{request.userInfo}}: contains information on who/what submitted the request which includes the groups and username keys.
  • {{request.namespace}}: the Namespace of the object subject to the operation.

Here are some examples of looking up this data:

  1. Reference a resource name (type string)


  1. Reference the metadata (type object)


  1. Reference the name of a new Namespace resource being created


  1. Reference the name of a user who submitted a request


Variables from the AdmissionReview can also be combined with user-defined strings to create values for messages and other fields.

  1. Build a name from multiple variables (type string)


Let’s look at an example of how this AdmissionReview data can be used in Kyverno policies.

In the below ClusterPolicy, we wish to know which account created a given Pod resource. We can use information from the AdmissionReview contents, specifically the username key, to write this information out in the form of a label. Apply the following sample.

 2kind: ClusterPolicy
 4  name: who-created-this
 6  background: false
 7  rules:
 8  - name: who-created-this
 9    match:
10      any:
11      - resources:
12          kinds:
13          - Pod
14    mutate:
15      patchStrategicMerge:
16        metadata:
17          labels:
18            created-by: "{{request.userInfo.username}}"

This sample will mutate all incoming Pod creation requests with a label named created-by and the value of the authenticated user based on their kubeconfig.

Create a simple Pod resource.

1kubectl run busybox --image busybox:1.28

Now get the newly-created busybox Pod.

1kubectl get po busybox --show-labels
4busybox   0/1     Pending   0          25m   created-by=kubernetes-admin,run=busybox

In the output, we can clearly see the value of our created-by label is kubernetes-admin which, in this case, is the user who created the Pod.

Variables from container images

Kyverno extracts image data from the AdmissionReview request and makes this available as a variable named images of type map in the rule context. Here is an example:

 2  "containers": {
 3    "tomcat": {
 4      "registry": "",
 5      "path": "tomcat",
 6      "name": "tomcat",
 7      "tag": "9"
 8    }
 9  },
10  "initContainers": {
11    "vault": {
12      "registry": "",
13      "path": "vault",
14      "name": "vault",
15      "tag": "v3"
16    }
17  }

Whenever an AdmissionReview request has containers or initContainers defined, the images variable can be referenced as shown in the examples below:

Reference the image properties of container tomcat:

  1. Reference the registry URL


  1. Reference the path to the image


  1. Reference the image name


  1. Reference the image tag


  1. Reference the digest


Reference the image properties of initContainer vault:

  1. Reference the registry URL


  1. Reference the path to the image


  1. Reference the image name


  1. Reference the image tag


  1. Reference the digest


This same pattern and image variable arrangement also works for ephemeral containers.

Kyverno by default sets an empty registry to and an empty tag to latest.

You can also fetch image properties of all containers for further processing. For example, {{ images.containers.*.name }} creates a string list of all image names.

Inline Variables

Variables may be defined in a context for consumption by Kyverno rules. This can be as simple as a static value, another variable, or a nested object. Variables may also be redefined by using the same variable name. The last value that is set is used. The below sets a context variable with a value of foo.

1    context:
2    # A unique name for the for the variable 
3    # if the user redeclares a variable with the same name it should be re-assigned
4    - name: foodata
5      variable:
6        # value defines the value that the variable must have, it may contain jmespath variables or any yaml object that can be represented as a json object.
7        # value, default, and jmespath are optional but either value or jmespath must be defined.
8        value: "foo"

This snippet sets a context variable to the value of If the value field is not defined, the contents of jmesPath will act on the entire context.

2- name: objName
3  variable:
4    jmesPath:

And below allows for an inline variable with a nested object as well as a default value for that object if it cannot be resolved. Even if the value is not defined, the default can still be set to global values such as other request.object.* variables from AdmissionReview requests.

 1    context:
 2    - name: nested-metadata
 3      variable:
 4        value:
 5          metadata:
 6            labels: 
 7              name: {{ }}
 8        # the default value a variable may have if after jmespath processing the value ends up being nil
 9        # the default value may also be another variable, for example something from the AdmissionReview
10        default: '{}'
11        # jmespath expression that can be used to modify the `value` before it is assigned to the variable
12        jmespath: 'to_string(@)'

Variables can reference other variables as well as shown below. Note that context variables are ordered; a variable consumed by another variable must be defined higher in the list of context variables.

2- name: jpExpression
3  variable:
4    value: name
5- name: objName
6  variable:
7    value:
8      name: "{{ }}"
9    jmesPath: "{{ jpExpression }}"

Variables from external data sources

Some policy decisions require access to cluster resources and data managed by other Kubernetes controllers or external applications. For these types of policies, Kyverno allows HTTP calls to the Kubernetes API server and the use of ConfigMaps.

Data fetched from external sources is stored in a per-rule processing context that is used to evaluate variables by the policy engine. Once the data from external sources is stored in the context, it can be referenced like any other variable data.

Learn more about ConfigMap lookups and API Server calls in the External Data Sources section.

Nested Lookups

It is also possible to nest JMESPath expressions inside one another when mixing data sourced from a ConfigMap and AdmissionReview, for example. By including one JMESPath expression inside the other, Kyverno will first substitute the inner expression before building the outer one as seen in the below example.

 2kind: ClusterPolicy
 4  name: resource-annotater
 6  background: false
 7  rules:
 8  - name: add-resource-annotations
 9    context:
10    - name: LabelsCM
11      configMap:
12        name: resource-annotater-reference
13        namespace: default
14    match:
15      any:
16      - resources:
17          kinds:
18          - Pod
19    mutate:
20      patchStrategicMerge:
21        metadata:
22          annotations:
23            foo: "{{{{ }}}}"

In this example, AdmissionReview data is first collected in the inner expression in the form of {{}} while the outer expression is built from a ConfigMap context named LabelsCM.

Evaluation Order

Kyverno policies can contain variables in:

  • Rule context
  • Rule preconditions
  • Rule definitions:
    • Validation patterns
    • Validation deny rules
    • Mutate strategic merge patches (patchesStrategicMerge)
    • Generate resource data definitions

Variables are not supported in the match and exclude elements, so that rules can be matched quickly without having to load and process data. Variables are also not supported in the patchesJson6902.path key.

Since variables can be nested, it is important to understand the order in which the variables are evaluated. During admission control, here is how the engine processes rules:

  1. The set of matching rules is determined by creating a hash from the request information to retrieve all matching rules based on the rule and resource types.
  2. Each matched rule is further processed to fully evaluate the match and retrieve conditions.
  3. The rule context is then evaluated and variables are loaded from data sources.
  4. The preconditions are then checked.
  5. The rule body is processed.

This ordering makes it possible to use request data when defining the context, and context variables in preconditions. Within the context itself, each variable is evaluated in the order of definition. Hence, if required, a variable can reference a prior variable but attempts to use a subsequent definition will result in errors.

JMESPath custom functions

In addition to the list of built-in functions JMESPath offers, Kyverno augments these by adding several others which makes it even easier to craft Kyverno policies.


 1base64_decode(string) string
 2base64_encode(string) string
 3compare(string, string) integer
 4equal_fold(string, string) bool
 5label_match(object, object) bool (object arguments must be enclosed in backticks; ex. `{{request.object.spec.template.metadata.labels}}`)
 6parse_json(string) any (decodes a valid JSON encoded string to the appropriate type. Opposite of `to_string` function)
 7parse_yaml(string) any
 8path_canonicalize(string) string
 9pattern_match(pattern string, string|number) bool ('*' matches zero or more alphanumeric characters, '?' matches a single alphanumeric character)
10regex_match(string, string|number) bool
11regex_replace_all(regex string, src string|number, replace string|number) string (converts all parameters to string)
12regex_replace_all_literal(regex string, src string|number, replace string|number) string (converts all parameters to string)
13replace(str string, old string, new string, n float64) string
14replace_all(str string, old string, new string) string
15semver_compare(string, string) bool (Use operators [>, <, etc] with string inputs for comparison logic)
16split(str string, sep string) []string
17time_since(<layout>, <time1>, <time2>) string (all inputs as string)
18to_upper(string) string
19to_lower(string) string
20trim(str string, cutset string) string
21truncate(str string, length float64) string (length argument must be enclosed in backticks; ex. "{{ | truncate(@, `9`)}}")


 1add(number, number) number
 2add(quantity|number, quantity|number) quantity (returns a quantity if any of the parameters is a quantity)
 3add(duration|number, duration|number) duration (returns a duration if any of the parameters is a duration)
 4subtract(number, number) number
 5subtract(quantity|number, quantity|number) quantity (returns a quantity if any of the parameters is a quantity)
 6subtract(duration|number, duration|number) duration (returns a duration if any of the parameters is a duration)
 7multiply(number, number) number
 8multiply(quantity|number, quantity|number) quantity (returns a quantity if any of the parameters is a quantity)
 9multiply(duration|number, duration|number) duration (returns a duration if any of the parameters is a duration)
10divide(quantity|number, quantity|number) quantity|number (returns a quantity if exactly one of the parameters is a quantity, else a number; the divisor must be non-zero)
11divide(duration|number, duration|number) duration|number (returns a duration if exactly one of the parameters is a duration, else a number; the divisor must be non-zero)
12modulo(number, number) number
13modulo(quantity|number, quantity|number) quantity (returns a quantity if any of the parameters is a quantity; the divisor must be non-zero)
14modulo(duration|number, duration|number) duration (returns a duration if any of the parameters is a duration; the divisor must be non-zero)

The special variable {{@}} may be used to refer to the current value in a given field, useful for source values.

To find examples of some of these functions in action, see the Kyverno policies library. And for more complete information along with samples for each custom filter, see the JMESPath page here.

Last modified October 21, 2022 at 10:39 AM PST: Fix/update language around background scans (#661) (aeaf922)