Unnamed "Subscription" operation results in Denial-of-Service in apollographql/router

Summary

The Apollo Router is a configurable, high-performance graph router written in Rust to run a federated supergraph that uses Apollo Federation 2. Affected versions are subject to a Denial-of-Service (DoS) type vulnerability which causes the Router to panic and terminate when GraphQL Subscriptions are enabled. It can be triggered when **all of the following conditions are met**: 1. Running Apollo Router v1.28.0, v1.28.1 or v1.29.0 ("impacted versions"); **and** 2. The Supergraph schema provided to the Router (either via Apollo Uplink or explicitly via other configuration) **has a `subscription` type** with root-fields defined; **and** 3. The YAML configuration provided to the Router **has subscriptions enabled** (they are _disabled_ by default), either by setting `enabled: true` _or_ by setting a valid `mode` within the `subscriptions` object (as seen in [subscriptions' documentation](https://www.apollographql.com/docs/router/executing-operations/subscription-support/#router-setup)); **and** 4. An [anonymous](https://spec.graphql.org/draft/#sec-Anonymous-Operation-Definitions) (i.e., un-named) `subscription` operation (e.g., `subscription { ... }`) is received by the Router If **all four** of these criteria are met, the impacted versions will panic and terminate. There is no data-privacy risk or sensitive-information exposure aspect to this vulnerability. This is fixed in Apollo Router v1.29.1. Users are advised to upgrade. Updating to v1.29.1 should be a clear and simple upgrade path for those running impacted versions. However, if Subscriptions are **not** necessary for your Graph – but are enabled via configuration — then disabling subscriptions is another option to mitigate the risk.