Envoy proxy (ext_authz filter): When Envoy's ext_authz filter is configured with failure_mode_allow set to true

Impact

When Envoy's ext_authz filter is configured with failure_mode_allow set to true, a downstream client can force an invalid gRPC request that circumvents the external-authorization check entirely — meaning traffic that should have been checked against an auth service (a common pattern for gating access to inference or cluster-management endpoints) sails through unauthenticated.

Who can reach it

Remote — a downstream client crafts a malformed gRPC request against an Envoy instance where ext_authz is set to fail open.

What to do

Software upgrade to Envoy 1.29.1, 1.28.1, 1.27.3, 1.26.7, or later. No config workaround exists (the vendor advisory states none), so this is a binary/image upgrade and restart across every Envoy instance doing ext_authz-based access control in the cluster ingress path — a rolling restart avoids a hard outage if you run multiple replicas.

References