Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.27.13, the routesrv component serves cluster-wide control-plane data without application-layer authentication through /routes, /routes/{zone}, /swarm/redis/shards, and /swarm/valkey/shards. The handlers registered in routesrv/routesrv.go, eskipBytes.ServeHTTP in routesrv/eskipbytes.go, RedisHandler.ServeHTTP in routesrv/redishandler.go, and ValkeyHandler.ServeHTTP in routesrv/valkeyhandler.go only restrict HTTP methods and do not authenticate callers. A compromised or otherwise attacker-controlled pod that can reach routesrv on the Kubernetes cluster network can bypass its namespace-scoped RBAC and read Ingress and RouteGroup backend URLs, filter-chain and OAuth or OIDC path details, and Redis or Valkey shard addresses across namespaces. NetworkPolicy can restrict reachability but does not remove the missing-authentication condition, and the established impact is confidentiality exposure rather than direct integrity or availability loss. This issue is fixed in version 0.27.13.

Affected: Go: github.com/zalando/skipper

## 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 chec…