CVE-2026-33726: Cilium Auth Bypass Vulnerability

CVE-2026-33726 Overview

CVE-2026-33726 is an Improper Access Control vulnerability in Cilium, a networking, observability, and security solution with an eBPF-based dataplane. The vulnerability allows Ingress Network Policies to be bypassed for traffic from pods to L7 Services (Envoy, GAMMA) with a local backend on the same node when Per-Endpoint Routing is enabled and BPF Host Routing is disabled. This creates a policy enforcement gap that could allow unauthorized network communication between pods.

Critical Impact

Network policies designed to restrict pod-to-pod traffic may not be enforced in affected configurations, potentially allowing unauthorized lateral movement within Kubernetes clusters, particularly in cloud IPAM deployments like Amazon EKS with Cilium ENI mode.

Affected Products

• Cilium versions prior to 1.17.14
• Cilium versions prior to 1.18.8
• Cilium versions prior to 1.19.2

Discovery Timeline

• 2026-03-27 - CVE-2026-33726 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-33726

Vulnerability Analysis

This vulnerability represents an Improper Access Control issue (CWE-284) in Cilium's network policy enforcement layer. The core problem lies in the interaction between Per-Endpoint Routing and BPF Host Routing configurations when handling traffic destined for L7 Services.

When Per-Endpoint Routing is enabled and BPF Host Routing is disabled, there is a specific code path where Ingress Network Policies are not properly evaluated for traffic flowing from pods to L7 proxy services (Envoy or GAMMA) that have local backends residing on the same node. This creates a policy bypass condition where traffic that should be denied by network policies is instead allowed to reach its destination.

The vulnerability requires adjacent network access, meaning an attacker would need to have a pod running within the affected cluster. While the attack complexity is low and requires no privileges or user interaction, the impact is limited to integrity concerns without confidentiality or availability implications.

Root Cause

The root cause stems from a logic flaw in Cilium's packet processing pipeline when handling traffic between pods and L7 Services with local backends. Under the specific conditions of Per-Endpoint Routing being enabled and BPF Host Routing being disabled, the eBPF dataplane fails to properly invoke the network policy evaluation logic for ingress traffic to L7 proxy services.

Per-Endpoint Routing, while disabled by default, is automatically enabled in several cloud IPAM deployment scenarios including:

• Cilium ENI on Amazon EKS (eni.enabled)
• AlibabaCloud ENI (alibabacloud.enabled)
• Azure IPAM (azure.enabled, excluding AKS BYOCNI)
• Some GKE deployments (gke.enabled)

Attack Vector

The vulnerability is exploitable from the adjacent network layer, specifically within the Kubernetes cluster network. An attacker with the ability to deploy or control a pod within the cluster could leverage this vulnerability to communicate with L7 Services that should be restricted by Ingress Network Policies.

The attack scenario involves a malicious or compromised pod attempting to access an L7 Service (such as an Envoy-proxied service or GAMMA endpoint) that has a backend pod running on the same node. When Ingress Network Policies are configured to deny this traffic, the policies may not be enforced due to the vulnerability, allowing the traffic to succeed.

In practice, Amazon EKS environments running Cilium in ENI mode represent the most common affected deployment configuration. Tunneled deployments and chaining deployments are not affected by this vulnerability.

Detection Methods for CVE-2026-33726

Indicators of Compromise

• Unexpected successful connections between pods and L7 Services that should be blocked by Ingress Network Policies
• Network flow logs showing traffic to Envoy or GAMMA services from pods where such traffic should be denied
• Audit logs indicating pod-to-service communication that violates configured CiliumNetworkPolicy or NetworkPolicy resources

Detection Strategies

• Review Cilium configuration to identify if Per-Endpoint Routing is enabled (bpf.endpointRoutes=true or cloud IPAM modes)
• Verify BPF Host Routing status using cilium config view and check for bpf-host-routing setting
• Implement network policy audit logging to detect policy violations or unexpected allowed traffic
• Monitor Hubble flow data for traffic patterns that should be blocked by configured network policies

Monitoring Recommendations

• Enable Cilium Hubble for comprehensive network flow visibility and policy verdict monitoring
• Configure alerts for traffic flows that receive POLICY_DENIED verdicts but successfully complete
• Implement periodic network policy testing using tools like netpol-analyzer to validate expected enforcement behavior
• Monitor for anomalous pod-to-service traffic patterns, especially to L7 proxied endpoints

How to Mitigate CVE-2026-33726

Immediate Actions Required

• Upgrade Cilium to patched versions: 1.17.14, 1.18.8, or 1.19.2 or later
• Review current Cilium deployment configuration to determine if Per-Endpoint Routing is enabled
• Audit existing Ingress Network Policies to identify which policies may not be enforced
• Conduct network policy testing to verify enforcement status in your specific deployment

Patch Information

Cilium has released security patches addressing this vulnerability in versions 1.17.14, 1.18.8, and 1.19.2. The fix is tracked in GitHub PR #44693 and documented in GitHub Security Advisory GHSA-hxv8-4j4r-cqgv.

Organizations should upgrade to the patched version corresponding to their current Cilium release branch:

• For Cilium 1.17.x: Upgrade to 1.17.14 or later
• For Cilium 1.18.x: Upgrade to 1.18.8 or later
• For Cilium 1.19.x: Upgrade to 1.19.2 or later

Workarounds

• There is currently no officially verified or comprehensive workaround for this vulnerability
• Disabling Per-Endpoint Routes may be possible but will likely cause disruptions to ongoing connections
• Disabling Per-Endpoint Routes may cause potential conflicts in cloud provider environments
• The recommended approach is to upgrade to a patched Cilium version as soon as possible

# Check current Cilium version and configuration
kubectl exec -n kube-system ds/cilium -- cilium version
kubectl exec -n kube-system ds/cilium -- cilium config view | grep -E "bpf-host-routing|endpoint-routes"

# Upgrade Cilium using Helm (example for 1.19.2)
helm repo update cilium
helm upgrade cilium cilium/cilium --version 1.19.2 \
  --namespace kube-system \
  --reuse-values