VolumeServer.FetchAndWriteNeedle fetches a caller-supplied remote endpoint and writes the response into a needle. Before 4.24 this RPC performed no authentication and no validation of the target, so anyone able to reach a volume server's gRPC port could coerce the server into issuing requests to arbitrary hosts — including loopback, link-local, RFC 1918, and cloud metadata endpoints such as 169.254.169.254 — and read the response back. On cloud deployments this discloses instance metadata and IAM credentials, and can be used to reach otherwise-unexposed internal services (SSRF with response read-back).

The volume server gRPC plane is unauthenticated on a default deployment, so no credentials are required. Configuring the documented JWT signing keys does not close it, because that hardening does not apply to this RPC.

Fixed in 4.24. FetchAndWriteNeedle now requires admin authorization and refuses loopback / link-local / RFC 1918 / IMDS destinations through a guarded dialer that resolves the host itself and pins the resolved address for the duration of the request, defeating DNS-rebinding. The Rust volume server carries the equivalent endpoint validation.

Restrict volume server gRPC ports to trusted hosts via firewall / network policy, and enable mTLS via security.toml.