GHSA-5587-2x54-jj6h: Medium severity go/github.com/zalando/skipper vulnerability

Description

The routesrv component exposes the full cluster route topology (Ingress/RouteGroup configurations, backend URLs, filter chains, OAuth/OIDC callback paths) and cache-cluster topology (Redis/Valkey shard addresses) over plain HTTP with zero authentication. Any pod in the Kubernetes cluster can reach routesrv via its predictable DNS name and retrieve sensitive cluster-wide routing and cache infrastructure data.

Vulnerable Code

routesrv/routesrv.go:87-99,114-137 — all handler registrations on the main mux: go mux.Handle("/routes", b) // eskipBytes.ServeHTTP — all route data mux.Handle("/routes/{zone}", b) // zone-scoped route data mux.Handle("/swarm/redis/shards", rh) // Redis cluster addresses mux.Handle("/swarm/valkey/shards", vh) // Valkey cluster addresses

routesrv/eskipbytes.go:134-196 — eskipBytes.ServeHTTP: go func (e eskipBytes) ServeHTTP(rw http.ResponseWriter, r http.Request) { // ... only checks GET/HEAD method, NO auth check if r.Method != "GET" && r.Method != "HEAD" { w.WriteHeader(http.StatusMethodNotAllowed) return } // ... serves all route data immediately }

routesrv/redishandler.go:28-41 — RedisHandler.ServeHTTP: go func (rh RedisHandler) ServeHTTP(w http.ResponseWriter, r http.Request) { if r.Method != "GET" { w.WriteHeader(http.StatusMethodNotAllowed) return } // ... serves Redis cluster addresses immediately, NO auth check }

routesrv/valkeyhandler.go:28-41 — ValkeyHandler.ServeHTTP: go func (vh ValkeyHandler) ServeHTTP(w http.ResponseWriter, r http.Request) { if r.Method != "GET" { w.WriteHeader(http.StatusMethodNotAllowed) return } // ... serves Valkey cluster addresses immediately, NO auth check }

Attack Path

1. Initial Compromise: Attacker compromises any pod in the Kubernetes cluster (via application CVE, supply-chain attack, malicious container image, etc.) 2. Discovery: Attacker discovers routesrv via predictable Kubernetes DNS name: skipper-ingress-routesrv.kube-system.svc.cluster.local:9090 (documented at docs/tutorials/operations.md:108, docs/tutorials/ratelimit.md:137,197) 3. Data Extraction without Auth: - GET http://<routesrv>:9090/routes → All Ingress/RouteGroup configurations across ALL namespaces - GET http://<routesrv>:9090/swarm/redis/shards → Redis cache cluster node addresses - GET http://<routesrv>:9090/swarm/valkey/shards → Valkey cache cluster node addresses 4. Subsequent Attacks: With cache cluster topology, attacker can perform direct cache-level attacks (ratelimit data manipulation, session data exfiltration)

Permission Boundary Analysis

The routesrv uses a ServiceAccount with cluster-wide RBAC to list Ingress (networking.k8s.io), RouteGroup (zalando.org), Endpoints, and Services across all namespaces (see clusterclient.go:648-653 fetchClusterState). The kube-apiserver requires proper ServiceAccount token + RBAC authorization for the Kubernetes API itself, but routesrv exposes the aggregated data over HTTP with zero authentication.

A compromised pod with limited RBAC (restricted to its own namespace) can bypass Kubernetes RBAC entirely by reading routesrv. This crosses the boundary from "namespace-scoped Kubernetes workload with restricted RBAC" to "full cluster route topology across all namespaces".

No NetworkPolicy manifests exist in the deploy/ directory. The default Kubernetes flat network model allows any pod to reach any service, further widening the attack surface.

Exposed Data

| Endpoint | Data Exposed | Impact | |----------|-------------|--------| | GET /routes | All ingress/routegroup backends: internal service URLs, filter chains (auth, rate limiting, OAuth, JWT, OPA policies), load balancer group membership | Cluster-wide reconnaissance, targeted backend attacks | | GET /routes/{zone} | Zone-scoped subset of above route data | Same, scoped | | GET /swarm/redis/shards | Redis cluster internal IP:port pairs | Direct cache-level attacks, ratelimit data manipulation | | GET /swarm/valkey/shards | Valkey cluster internal IP:port pairs | Same |

Additionally, the data-plane client (eskipfile/remote.go:190-219) also performs plain HTTP GET with no credentials — only an ETag header is sent — confirming that no auth capability exists in the architecture at all.

Mitigation

1. Add authentication to all routesrv HTTP endpoints (basic auth, bearer token, mTLS, or shared secret) via flag -route-server-filters="" 2. Deploy Kubernetes NetworkPolicies restricting ingress to routesrv to only the data-plane skipper pod selectors 3. Consider using mutual TLS authentication between data-plane and control-plane components

NetworkPolicy does not remove the missing-auth condition Restrictive NetworkPolicies are a valid mitigation, but they are not an application-layer authentication mechanism. The security-relevant defect remains that routesrv serves control-plane-derived data to unauthenticated callers whenever network reachability exists.

Impact framing This report does not rely on claiming direct integrity or availability impact. The verified issue is a confidentiality-focused control-plane exposure: route definitions, backend topology, filter-chain details, and Redis/Valkey shard addresses become readable to any reachable in-cluster client.

Resources

- routesrv/routesrv.go:87-99 — handler registration (zero auth) - routesrv/eskipbytes.go:134-196 — route data handler (no auth) - routesrv/redishandler.go:28-41 — Redis shard handler (no auth) - routesrv/valkeyhandler.go:28-41 — Valkey shard handler (no auth) - dataclients/kubernetes/clusterclient.go:648-653 — fetchClusterState() — shows cluster-wide RBAC - eskipfile/remote.go:190-219 — data-plane client also has no auth capability - docs/tutorials/operations.md:108, docs/tutorials/ratelimit.md:137,197 — documented routesrv DNS name