GHSA-67jx-r9pv-98rj: Traefik has Knative Ingress Rule Injection that Allows Host Restriction Bypass
SummaryThere is a potential vulnerability in Traefik's Kubernetes Knative, Ingress, and Ingress-NGINX providers related to rule injection.User-controlled values are interpolated into backtick-delimited Traefik router rule expressions without escaping or validation. A malicious value containing a backtick can terminate the literal and inject additional operators into Traefik's rule language, altering the parsed rule tree. In shared or multi-tenant deployments, this can bypass host and header routing constraints and redirect unauthorized traffic to victim services.Patcheshttps://github.com/traefik/traefik/releases/tag/v3.6.11https://github.com/traefik/traefik/releases/tag/v3.7.0-ea.2For more informationIf there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).SummaryTraefik's Knative provider builds router rules by interpolating user-controlled values into backtick-delimited rule expressions without escaping. In live cluster validation, Knative `rules[].hosts[]` was exploitable for host restriction bypass (for example `tenant.example.com`) || Host(`attacker.com`), producing a router that serves attacker-controlled hosts. Knative `headers[].exact` also allows rule-syntax injection and proves unsafe rule construction. In multi-tenant clusters, this can route unauthorized traffic to victim services and lead to cross-tenant traffic exposure. Severity is High in shared deployments.Tested on Traefik `v3.6.10`; the vulnerable pattern appears to have been present since the Knative provider was introduced. Earlier versions with Knative provider support are expected to be affected.DetailsThe issue is caused by unsafe rule-string construction using `fmt.Sprintf` with backtick-delimited literals.Incriminated code patterns:`pkg/provider/kubernetes/knative/kubernetes.go``fmt.Sprintf("Host(`%v`)", host)``fmt.Sprintf("Header(`%s`,`%s`)", key, headers[key].Exact)``fmt.Sprintf("PathPrefix(`%s`)", path)``pkg/provider/kubernetes/ingress/kubernetes.go``fmt.Sprintf("Host(`%s`)", host)``fmt.Sprintf("(Path(`%[1]s`) || PathPrefix(`%[1]s/`))", path)``pkg/provider/kubernetes/ingress-nginx/kubernetes.go` (hardening candidate; not the primary confirmed vector in this report)`fmt.Sprintf("Header(`%s`, `%s`)", c.Header, c.HeaderValue)`related host/path/header concatenations with backticksBecause inputs are inserted directly into rule expressions, a malicious value containing a backtick can terminate the literal and inject additional operators/tokens in Traefik's rule language. Example payload:`x`) || Host(`attacker.com`When used as a header value in Knative rule construction, the resulting rule contains:`Header(`X-Poc`,`x`) || Host(`attacker.com`)`This alters rule semantics and enables injection into Traefik's rule language. Depending on the field used (`hosts[]` vs `headers[].exact`) this can become a direct routing bypass.Important scope note:Gateway API code path (`pkg/provider/kubernetes/gateway/httproute.go`) already uses safer `%q` formatting for header/query rules and is not affected by this exact pattern.For standard Kubernetes Ingress, `spec.rules.host` is validated as DNS-1123 by the API server, which rejects backticks (so this specific host-injection payload is typically blocked).For Knative Ingress, `rules[].hosts[]` and `headers[].exact` are typed as `string` in CRD schema with no pattern constraint.In this validation environment, `rules[].hosts[]` was accepted and produced a practical host bypass. `headers[].exact` was also accepted and produced rule-syntax injection in generated routers.Ingress-NGINX patterns are included as follow-up hardening targets and are not claimed as independently exploitable here.Exploitability depends on admission/validation policy and who can create these resources.PoCLocal deterministic PoC (no cluster required):Run:Save the inline PoC below as `poc_build_rule.go`Run `go run poc_build_rule.go`Observe output:Legitimate rule: `(Host(`tenant.example.com`)) && (Header(`X-API-Key`,`secret123`)) && PathPrefix(`/`)`Malicious rule: `(Host(`tenant.example.com`)) && (Header(`X-API-Key`,`x`) || Host(`attacker.com`)) && PathPrefix(`/`)`This proves syntax injection in current string-construction logic.Inline PoC code (self-contained):Cluster PoC (Knative host injection, primary / practical bypass):Preconditions:Kubernetes test cluster with Knative Serving.Traefik configured with Knative provider.Apply manifest:`kubectl apply -f - <<'YAML'`(If API version mismatch, adjust between `networking.internal.knative.dev/v1alpha1` and `networking.knative.dev/v1alpha1`.)Verify:Check Traefik router rule contains: `(Host(`tenant.example.com`) || Host(`attacker.com`)) && PathPrefix(`/`)`.Request with `Host: attacker.com` returns backend 200.This demonstrates host restriction bypass in practice.Cluster PoC (Knative header injection, confirms rule-syntax injection):Apply:`kubectl apply -f - <<'YAML'`Verify:Inspect generated Traefik dynamic router rule (API/dashboard/logs).Confirm injected fragment `|| Host(`attacker.com`)` is present.Send request with `Host: attacker.com` and no expected tenant header (expected: 404 for this payload shape, because leading `Host(tenant)` still applies).Send request with `Host: tenant.example.com` and `X-Poc: x` (expected: 200 from backend).Optional Ingress PoC (scope check):Apply:`kubectl apply -f - <<'YAML'`Expected in most clusters: API server rejects this payload because Ingress `host` must satisfy DNS-1123.Keep this step only as a negative control to demonstrate the distinction between native Ingress validation and Knative CRD behavior.Validation executed in this report:Local deterministic PoC executed with `go run` and output matched expected injected rule.Live cluster test executed on local `kind` cluster (`kind-traefik-poc`) with Traefik `v3.6.10` and Knative Serving CRDs.Annotation key confirmed in this environment: `networking.knative.dev/ingress.class` (dot). The hyphen variant was not used by the successful processing path.Traefik API/logs confirmed generated routers included injected expressions.Live HTTP request with `Host: attacker.com` reached backend (`200`) for Knative host-injection payload.Impact**Vulnerability type:** Rule injection / authorization bypass at routing layer.**Primary impact:** Bypass of intended routing predicates (host/header/path), enabling unauthorized routing to protected services.**Who is impacted:** Primarily deployments using Traefik Knative provider where untrusted or semi-trusted actors can create/update Knative Ingress resources (typical in multi-tenant clusters, shared namespaces, or weak admission controls). Standard Kubernetes Ingress host injection is usually blocked by API validation.**Security consequences:** Cross-tenant traffic access, internal service exposure, policy bypass, and potential chaining with app-level vulnerabilities.
CVSS v4.0: 6.1