CVE-2020-8559: Kubernetes Privilege Escalation Vulnerability

CVE-2020-8559 Overview

CVE-2020-8559 is an unvalidated redirect vulnerability in the Kubernetes kube-apiserver that affects multiple versions of Kubernetes. The vulnerability exists in proxied upgrade requests, where an attacker who has already compromised a node can exploit the unvalidated redirect to escalate privileges from node-level access to full cluster compromise.

Critical Impact
An attacker with node-level access can leverage this vulnerability to escalate privileges and gain complete control over the entire Kubernetes cluster, potentially compromising all workloads and sensitive data across the cluster.

Affected Products

Kubernetes versions v1.6 through v1.15 (all releases)
Kubernetes versions prior to v1.16.13
Kubernetes versions prior to v1.17.9
Kubernetes versions prior to v1.18.6

Discovery Timeline

2020-07-22 - CVE-2020-8559 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-8559

Vulnerability Analysis

This vulnerability stems from an unvalidated redirect condition (CWE-601) in the Kubernetes kube-apiserver's handling of proxied upgrade requests. When the kube-apiserver proxies certain requests to backend services or nodes, it fails to properly validate redirect responses. This allows an attacker who has compromised a node to intercept and manipulate these redirect responses, ultimately tricking the kube-apiserver into making requests to arbitrary destinations.

The attack scenario requires the attacker to first gain control of a node within the Kubernetes cluster. Once node-level access is achieved, the attacker can intercept proxied requests from the kube-apiserver and respond with malicious redirect responses. Because the kube-apiserver does not validate these redirects, it follows them blindly, potentially sending authenticated requests to attacker-controlled endpoints or other cluster components.

Root Cause

The root cause of this vulnerability is the lack of proper validation of redirect responses in the kube-apiserver's proxy handling code. When processing upgrade requests (such as those used for kubectl exec, kubectl attach, or other streaming connections), the kube-apiserver trusts redirect responses from nodes without verifying that the redirect target is a legitimate and expected destination. This trust relationship allows a compromised node to abuse the redirect mechanism.

Attack Vector

The attack requires network access and involves the following sequence:

The attacker first compromises a Kubernetes node through a separate vulnerability or misconfiguration
With node-level access, the attacker positions themselves to intercept communications from the kube-apiserver
When a legitimate user or service initiates a proxied upgrade request (e.g., kubectl exec), the attacker intercepts the request at the node level
The attacker responds with a crafted redirect response pointing to a sensitive cluster endpoint or another node
The kube-apiserver follows the redirect, sending authenticated requests to the attacker-specified target
This allows the attacker to access resources and execute commands beyond their initial node-level scope, achieving full cluster compromise

The vulnerability requires high privileges (node compromise) and user interaction (a legitimate user must trigger a proxied request), but the potential impact is severe as it enables complete cluster takeover.

Detection Methods for CVE-2020-8559

Indicators of Compromise

Unusual redirect responses in kube-apiserver proxy logs indicating unexpected destination targets
Anomalous network traffic patterns from the kube-apiserver to unexpected endpoints within the cluster
Evidence of node compromise such as unauthorized processes, modified kubelet configurations, or suspicious container activity
Unexpected kubectl exec or kubectl attach session behaviors or failures

Detection Strategies

Monitor kube-apiserver audit logs for proxied requests that result in redirects to unexpected destinations
Implement network monitoring to detect unusual traffic patterns from the control plane to worker nodes
Deploy runtime security monitoring on all nodes to detect signs of node compromise
Review Kubernetes audit logs for suspicious exec, attach, and portforward operations

Monitoring Recommendations

Enable comprehensive Kubernetes audit logging with focus on proxy-related events and exec/attach operations
Implement alerting on kube-apiserver errors or warnings related to redirect handling
Deploy network segmentation and monitor east-west traffic within the cluster for anomalies
Use SentinelOne Singularity for Cloud to continuously monitor Kubernetes workloads and detect suspicious behavior indicative of compromise

How to Mitigate CVE-2020-8559

Immediate Actions Required

Upgrade Kubernetes to version v1.16.13, v1.17.9, v1.18.6, or later immediately
Review all nodes for signs of compromise before and after patching
Implement network policies to restrict communication paths within the cluster
Audit cluster access and ensure least-privilege principles are enforced

Patch Information

Kubernetes has released patched versions that address this vulnerability. Organizations should upgrade to the following minimum versions:

Kubernetes v1.16.x users: Upgrade to v1.16.13 or later
Kubernetes v1.17.x users: Upgrade to v1.17.9 or later
Kubernetes v1.18.x users: Upgrade to v1.18.6 or later

For detailed patch information, refer to the Kubernetes GitHub Issue #92914 and the Kubernetes Security Announcement.

Workarounds

Implement strict network policies to limit which endpoints the kube-apiserver can reach
Restrict access to kubectl exec, kubectl attach, and similar privileged operations using RBAC
Deploy additional monitoring on nodes to detect and respond to compromise attempts quickly
Consider using admission controllers to limit the scope and frequency of proxied requests

bash

# Example: Restrict exec/attach permissions via RBAC
# Create a ClusterRole that denies exec access
kubectl create clusterrole no-exec \
  --verb=create \
  --resource=pods/exec \
  --dry-run=client -o yaml | \
  sed 's/verbs:/verbs: []/g' > restricted-exec-role.yaml

# Apply network policy to restrict kube-apiserver egress
kubectl apply -f - <<EOF
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: restrict-apiserver-egress
  namespace: kube-system
spec:
  podSelector:
    matchLabels:
      component: kube-apiserver
  policyTypes:
  - Egress
  egress:
  - to:
    - namespaceSelector: {}
    ports:
    - protocol: TCP
      port: 10250
EOF