CVE-2026-56742: 
Cilium vulnerability analysis and mitigation

CVE-2026-56742 is an authorization bypass vulnerability in Cilium's Gateway API implementation that allows users with permissions to create or update namespaced HTTPRoutes to mirror HTTP traffic to any Service in any namespace, circumventing the ReferenceGrant authorization mechanism. It affects all Cilium versions prior to 1.17.17, versions 1.18.0–1.18.10, and versions 1.19.0–1.19.4. The vulnerability was published on July 15, 2026, and patched versions were released on July 17, 2026. It carries a CVSS v3.1 base score of 8.9 (High) per NVD, though the GitHub Security Advisory rates it as Moderate (5.9) (GitHub Advisory).

The root cause is a missing authorization check (CWE-862) in Cilium's Gateway API route reconciliation logic: when processing HTTPRouteFilterRequestMirror and GRPCRouteFilterRequestMirror filters, the code did not invoke helpers.IsBackendReferenceAllowed() to validate cross-namespace backend references against existing ReferenceGrant objects. This means a user with RBAC permissions to create or update HTTPRoute or GRPCRoute objects in their own namespace could specify a RequestMirror filter pointing to a Service in a different namespace, and Cilium would honor the mirror without checking for a corresponding ReferenceGrant. The attack requires adjacent network access and low privileges (HTTPRoute create/update permissions), but no user interaction. The fix adds the missing IsBackendReferenceAllowed check in operator/pkg/model/ingestion/gateway.go for both HTTP and GRPC route mirror filters (GitHub Advisory, Patch Commit).

A malicious or compromised user with HTTPRoute or GRPCRoute creation permissions can silently mirror HTTP/gRPC traffic destined for services in their namespace to arbitrary services in other namespaces, including sensitive or privileged namespaces. This results in unauthorized access to data in transit (confidentiality impact), potential for data manipulation or injection into mirrored services (integrity impact), and limited availability impact on the mirrored target services. The scope change means the impact extends beyond the attacker's own namespace to the broader cluster, enabling cross-tenant data exfiltration in multi-tenant Kubernetes environments (GitHub Advisory).

No public proof-of-concept exploit code is known, and there is no evidence of in-the-wild exploitation at this time (Feedly). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The EPSS score is approximately 0.002 (0.2%), indicating a low probability of exploitation in the near term. Exploitation requires that the attacker already holds Kubernetes RBAC permissions to create or update HTTPRoute objects and that the target cluster has Gateway API functionality enabled (it is disabled by default in Cilium) (GitHub Advisory).

1. Identify target cluster: Confirm the Cilium cluster has Gateway API enabled (non-default configuration) and identify a namespace where the attacker has create or update permissions on HTTPRoute objects.
2. Enumerate target services: List services in other namespaces that the attacker wishes to intercept traffic from (e.g., kubectl get services --all-namespaces).
3. Craft malicious HTTPRoute: Create an HTTPRoute manifest in the attacker-controlled namespace with a RequestMirror filter referencing a Service in a different namespace, without a corresponding ReferenceGrant authorizing the cross-namespace reference.
4. Apply the HTTPRoute: Submit the manifest (kubectl apply -f malicious-httproute.yaml). On vulnerable Cilium versions, the reconciliation logic will not check for a ReferenceGrant and will configure the mirror.
5. Receive mirrored traffic: Direct a receiving service (in any accessible namespace or external endpoint) to collect the mirrored HTTP requests, capturing sensitive data such as authentication tokens, API payloads, or PII from the target namespace's traffic (GitHub Advisory, Patch Commit).

• Kubernetes API Audit Logs: Audit log entries showing create or update operations on HTTPRoute or GRPCRoute resources by non-admin users, particularly where the RequestMirror filter references a namespace different from the route's own namespace.
• Network: Unexpected HTTP traffic mirrored to services in namespaces that have no legitimate cross-namespace routing configured; unusual traffic volume increases on services in sensitive namespaces.
• Configuration: Presence of HTTPRoute or GRPCRoute objects with filters.requestMirror.backendRef.namespace set to a namespace other than the route's own namespace, without a corresponding ReferenceGrant object authorizing the reference.
• Cilium Operator Logs: Absence of ReferenceGrant validation errors for cross-namespace mirror backends in Cilium operator logs on affected versions (the check was missing and would not produce warnings).

Upgrade Cilium to the patched versions: 1.17.17, 1.18.11, or 1.19.5, which add the missing ReferenceGrant check for RequestMirror backends in both HTTPRoute and GRPCRoute reconciliation (GitHub Advisory). There is no complete workaround; however, organizations unable to upgrade immediately should restrict Kubernetes RBAC permissions so that only cluster administrators can create or update HTTPRoute and GRPCRoute objects. Additionally, if Gateway API functionality is not required, it should remain disabled (it is off by default) to eliminate the attack surface entirely (GitHub Advisory).

The vulnerability was reported by community member @galanko and triaged and remediated by Cilium maintainer @mhofstetter, with the Cilium community working together with members of Isovalent to prepare the fix. The GitHub Security Advisory was published by @ferozsalam on July 6, 2026, and rated as Moderate severity by the project. No significant broader media coverage or notable external researcher commentary has been identified at this time (GitHub Advisory).