Fission: Environment Runtime.Container and Builder.Container SecurityContext bypass allows privileged pod creation

Summary

A follow-up bypass of the round-4 PodSpec hardening (GHSA-gx55-f84r-v3r7, GHSA-wmgg-3p4h-48x7, GHSA-v455-mv2v-5g92). Those advisories validate and sanitize the PodSpec (spec.runtime.podSpec / spec.builder.podSpec / function.spec.podSpec), but the Environment CRD also exposes spec.runtime.container and spec.builder.container — a standalone Container merged into the runtime/builder pod whose SecurityContext bypassed both layers.

Details

Admission-layer gap. Environment.Validate() calls ValidatePodSpecSafety() on Runtime.PodSpec and Builder.PodSpec only. That function takes a *PodSpec, so it never inspects the standalone Runtime.Container.SecurityContext or Builder.Container.SecurityContext.

Merge-layer gap. sanitizeContainerSecurityContext() ran only inside MergePodSpec(). The container field is merged via MergeContainer(), which did not sanitize. With only Runtime.Container set and Runtime.PodSpec nil, MergePodSpec is never invoked, so the sanitizer never ran.

Affected merge sites: poolmgr (gp_deployment.go), newdeploy (newdeploy.go), and buildermgr (envwatcher.go).

Proof of concept

apiVersion: fission.io/v1
kind: Environment
metadata:
  name: priv-escape-test
  namespace: default
spec:
  version: 3
  runtime:
    image: "ghcr.io/fission/python-env:latest"
    container:
      name: priv-escape-test
      securityContext:
        privileged: true
  poolsize: 1

The admission webhook accepts this Environment and the resulting pool pod runs with privileged: true. Equivalent bypasses: allowPrivilegeEscalation: true, capabilities.add: ["SYS_ADMIN"], capabilities.add: ["NET_ADMIN","SYS_PTRACE"]. The same attack applies to Builder.Container.

Impact

A tenant with environments.fission.io create/update RBAC can run privileged / allowPrivilegeEscalation / dangerous-capability containers in the Fission function or builder namespace, scheduled under the executor's high-privilege service account — enabling container-sandbox escape, host filesystem and network access, and potential node- and cluster-level compromise. Identical blast radius to GHSA-gx55-f84r-v3r7.

Fix

Fixed in #3406 and released in v1.24.0.

• Admission layer (primary defence): a new ValidateContainerSafety in pkg/apis/core/v1/podspec_safety.go applies the per-container SecurityContext denylist (privileged, allowPrivilegeEscalation, dangerous capabilities) to a standalone container, and is called from Environment.Validate() for Runtime.Container and Builder.Container.
• Merge layer (defence in depth): sanitizeContainerSecurityContext() is now invoked inside MergeContainer() itself, covering all three executor/builder call sites.

Workarounds

• Restrict Environment create/update RBAC to trusted administrators.
• Deploy a Kyverno / OPA Gatekeeper policy rejecting dangerous Container SecurityContext on Environment CRDs.
• Label the function/builder namespaces with pod-security.kubernetes.io/enforce: restricted.

References

• GHSA-gx55-f84r-v3r7, GHSA-wmgg-3p4h-48x7, GHSA-v455-mv2v-5g92 — the round-4 PodSpec fixes this advisory bypasses (#3391, e484df84).