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How to Process Secret Strings with Kyverno (Advanced Guide)

When operating Kubernetes clusters, you often need to store and reuse Secret values after applying specific string processing. For example, you might want to convert strings in Secrets injected by external systems to uppercase or replace certain patterns. This article explains two approaches to processing strings in Secrets at application time using Kyverno, a Kubernetes-native policy engine.

What is Kyverno?

Kyverno is a powerful policy engine that allows you to control and apply policies to Kubernetes resources using YAML-based definitions. Operating as an admission controller, Kyverno provides the following features:

• Validate: Verify conditions before resource application
• Mutate: Modify resources by applying patches during resource creation
• Generate: Automatically create new resources triggered by other resource events

String Processing Scenario

Consider a scenario where you want to take a Secret named "source-secret" when it's created, convert its data.username to uppercase, and reflect it in a new Secret called "target-secret". The initial approach might be to use Kyverno's generate feature to automatically create target-secret when source-secret is created.

However, Kyverno's generate feature has a constraint: "it may not always be possible to generate resources of the same Kind simultaneously". This can be particularly challenging when generating Secrets, as certain patterns might be difficult to implement due to the generate mechanism's limitations.

Constraints of Generate Rules

While Kyverno's generate feature works well for scenarios like creating a ConfigMap when another ConfigMap is created, attempting to "generate a Secret when a Secret arrives" will result in an error stating "generation kind and match resource kind should not be the same". This restriction appears to be in place to prevent an infinite loop where generated resources would trigger the generation of additional resources of the same kind.

In such situations, using mutate offers an alternative approach.

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Implementing Data Transformation Within the Same Secret Using Mutate

Using mutate rules, you can modify the Secret resource itself just before source-secret is applied. This means you can uppercase data.username in the same Secret resource before storing it in the cluster, avoiding the same-Kind generation issue.

String Processing Functions Available in Kyverno

Kyverno provides JMESPath extension functions with many useful string manipulation functions:

• to_upper(string): Convert string to uppercase
• to_lower(string): Convert string to lowercase
• replace(old, new, string), replaceAll(old, new, string): String replacement
• regex_replaceAll(pattern, replace, string): Regular expression replacement
• base64_decode(string) / base64_encode(string): Base64 encoding/decoding
• Trimming functions (trim, trim_prefix, trim_suffix) and more

Since Secret data values are Base64 encoded, processing data.username requires first using base64_decode() to restore the original string, then applying to_upper() for uppercase conversion, and finally using base64_encode() to re-encode the result.

Example of a Mutate Rule

Here's a policy example that uppercases data.username when source-secret is created in the default namespace:

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: mutate-secret-data
spec:
  rules:
    - name: mutate-secret-username
      match:
        resources:
          kinds:
            - Secret
          namespaces:
            - default
          names:
            - source-secret
      mutate:
        patchStrategicMerge:
          apiVersion: v1
          kind: Secret
          metadata:
            name: "{{ request.object.metadata.name }}"
            namespace: "{{ request.object.metadata.namespace }}"
          data:
            username: "{{ base64_encode(to_upper(base64_decode(request.object.data.username))) }}"

Verifying Policy Operation

1. Apply the above policy using kubectl apply -f
2. Apply the following Secret using kubectl apply -f:

apiVersion: v1
kind: Secret
metadata:
  name: source-secret
  namespace: default
type: Opaque
data:
  username: dXNlck5hbWU=  # Base64 encoded value of "userName"

1. After applying, check the contents using kubectl get secret source-secret -o yaml, and you'll see that the username value has been replaced with the Base64 encoded value of the uppercase "USERNAME".

This confirms that string transformation occurred within the same Secret during admission.

Summary

While we initially considered using the generate feature to "generate a new Secret with string processing triggered by another Secret," Kyverno's generate feature has limitations with same-Kind generation. Instead, when data transformation is needed within the same resource, using mutate allows us to transform the resource during admission.

With mutate, you can process data with any string manipulation just before the Secret is stored, and by combining Kyverno extension functions like base64_decode(), to_upper(), and base64_encode(), you can achieve complex string transformations.

Kyverno is a valuable tool for controlling and extending Kubernetes resources through policies. Consider the appropriate use of generate and mutate while managing Secrets, ConfigMaps, and other Kubernetes resources.