Cilium CVE-2026-56742

Exploitation requires all of: (1) Cilium is configured with the Gateway API feature explicitly enabled - it is OFF by default, which is the single largest limiting factor; (2) the attacker holds Kubernetes RBAC permission to create or update namespaced HTTPRoute or GRPCRoute resources (i.e., an authenticated in-cluster tenant, not an anonymous outsider); and (3) the attacker crafts a RequestMirror filter whose BackendRef targets a Service in a namespace for which no ReferenceGrant has been issued. What limits exploitation: default deployments are not vulnerable because Gateway API is disabled; single-tenant clusters where the same trusted party already controls all namespaces gain no meaningful boundary to cross; and the attacker must have route-authoring privileges, so pure external unauthenticated exploitation is not possible.

Signals are consistent and point to a real-but-conditional risk rather than an urgent internet-facing emergency. The published CVSS 3.1 vector (AV:A/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:L, 8.9) reflects a low-privilege actor (namespaced RBAC to manage HTTPRoutes), no user interaction, and a scope change crossing the namespace/tenant boundary - all plausible. However, SSVC tempers this: CISA's framework rates Exploitation as 'none' (no observed or public exploitation), Automatable 'no', and Technical Impact only 'partial', and EPSS is just 0.20% (10th percentile). It is not KEV-listed and no POC is known. The practical prerequisite is significant: the attacker must already be an authenticated tenant with permission to create/update HTTPRoutes in a cluster that has explicitly enabled Gateway API (off by default). One tension worth flagging: the vector uses AV:A (adjacent) and I:H, but the attack is actually performed via the Kubernetes API (network-reachable) and its primary effect is confidential traffic duplication, so a network vector with high confidentiality and limited integrity impact is arguably a closer fit - see the independent assessment below. Net: a genuine multi-tenant isolation bug that operators of Gateway-API-enabled Cilium should patch promptly, but not a mass-exploitation priority.

In a multi-tenant cluster with Gateway API enabled, a tenant who only has RBAC to manage HTTPRoutes in their own namespace creates a route with a RequestMirror filter whose BackendRef points to a sensitive Service in another tenant's namespace, without any ReferenceGrant authorizing it. Cilium accepts the route and begins duplicating matching HTTP requests to the attacker-chosen destination, letting them exfiltrate or amplify traffic across the tenant boundary. No public POC is known and exploitation requires the attacker to already hold namespaced route-management permissions.