Cilium Gateway API Load Balancer Egress Policy Bypass Vulnerability

A vulnerability in Cilium's handling of egress traffic through LoadBalancer resources configured via the Gateway API has been identified. This issue arises for Cilium users who implement Gateway API for Ingress, utilize LoadBalancer IP Address Management (LB-IPAM) or BGP for LoadBalancer services, and apply network policies to restrict egress traffic between workloads in different namespaces. Under these conditions, egress traffic from affected workloads to LoadBalancers managed by Gateway resources is incorrectly permitted, creating a potential oversight in traffic control. Notably, this issue does not impact LoadBalancer resources deployed outside of the Gateway API configuration.

Impact

Exploitation of this vulnerability allows for unauthorized egress traffic from workloads to LoadBalancers managed by the Gateway API, bypassing established network policies that restrict such inter-namespace traffic.

Reproduction

To reproduce this vulnerability, first, deploy Cilium in a Kubernetes cluster with Gateway API support. Create a LoadBalancer service using the Gateway API and ensure that network policies are in place to block egress traffic between namespaces. Then, observe that egress traffic to the Gateway-managed LoadBalancer is allowed, despite the network policy restrictions.

Remediation

Users can upgrade to Cilium versions 1.15.15, 1.16.8, or 1.17.2, where this issue has been fixed. For those unable to upgrade, a Clusterwide Cilium Network Policy can be applied as a workaround.