Kyverno Controller Denial of Service via forEach Mutation Panic

A denial-of-service vulnerability exists in the forEach mutation handler of Kyverno, a Kubernetes policy engine. Specifically, Kyverno versions v1.13.0 through v1.17.1 are susceptible to a flaw where an unchecked type assertion within the ForEach function in pkg/engine/mutate/mutation.go can be triggered by a specially crafted Policy or ClusterPolicy. Any user with the ability to create these policy types can exploit this vulnerability. When a patchesJson6902 field contains 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. This results in the background controller entering a persistent CrashLoopBackOff state, effectively halting background processing. The admission controller will also drop connections and block matching resource operations. CEL-based policies are unaffected.

Attack Chain

1. An attacker crafts a malicious Policy or ClusterPolicy YAML manifest containing a forEach rule.
2. The crafted rule includes a patchesJson6902 field with a variable substitution, such as {{ element.nonexistent }}, designed to resolve to nil at runtime.
3. The attacker applies the malicious policy to the Kubernetes cluster. This requires appropriate permissions to create Policy or ClusterPolicy resources.
4. When a resource matching the policy’s match criteria is created or updated, the Kyverno admission controller attempts to apply the policy.
5. The ForEach function in pkg/engine/mutate/mutation.go is invoked, processing the patchesJson6902 field.
6. The variable substitution resolves to nil, leading to a bare type assertion failure: fe["patchesJson6902"].(string).
7. This triggers an unrecoverable Go panic, causing either the background controller (if triggered by mutateExisting rules) or the admission controller to terminate the connection.
8. The background controller enters a CrashLoopBackOff state due to the persistent UpdateRequest resources that re-trigger the panic on every restart, achieving a denial-of-service.

Impact

Successful exploitation of this vulnerability leads to a denial of service affecting Kyverno’s core functionalities within the Kubernetes cluster. An attacker can crash the background controller, halting critical background tasks such as generate rules, mutateExisting rules, and cleanup processes. The admission controller can also be affected, dropping connections and blocking resource operations that match the malicious policy’s criteria. If a ClusterPolicy is used, this block extends cluster-wide. This vulnerability allows even users with limited, namespace-scoped permissions (via Policy creation) to impact the entire cluster, thus escalating privileges.

Recommendation

• Upgrade to Kyverno version v1.17.2 or later to patch the vulnerability (see Overview).
• Deploy the Sigma rule Detect Kyverno Policy with Suspicious forEach to identify potentially malicious policies containing forEach loops with patchesJson6902 fields that could trigger the vulnerability.
• Monitor Kyverno controller logs for “panic: interface conversion: interface {} is nil, not string” errors, indicating a potential exploitation attempt (see Attack Chain, step 7).
• Implement strict RBAC policies to limit the ability to create or modify Kyverno Policy and ClusterPolicy resources (see Impact).