GCP Professional Cloud Security Engineer Practice Question

A security assessment of several public-facing Compute Engine VMs shows that the instances still allow access to the legacy metadata endpoints /computeMetadata/v0.1 and /computeMetadata/v1beta1. Firewalls already block all inbound traffic except TCP 443 to the web application. Why does keeping these legacy endpoints enabled remain a serious security risk?

They disable automatic rotation of customer-managed encryption keys for attached persistent disks, increasing the chance of cryptographic compromise.
They respond to requests from processes inside the VM without requiring the protective X-Google-Metadata-Request (Metadata-Flavor: Google) header, letting an attacker exploit an SSRF-vulnerable application to steal the VM's service-account access token.
Anyone on the internet can reach the metadata server directly if a public firewall rule allows HTTPS, so attackers can download the entire instance metadata.
The legacy endpoints store all imported SSH public keys in plaintext files that are world-readable on the boot disk, exposing administrator access.

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Answer Description

The primary danger comes from server-side request forgery (SSRF) or remote-code-execution flaws in software running inside the VM. If attacker-supplied input can make the application issue HTTP requests, the attacker can direct the code to query the metadata server. The legacy endpoints reply without requiring the protective header (Metadata-Flavor: Google), so an attacker can retrieve the VM's OAuth access token for its attached service account and use that token to access other Google Cloud resources. Inbound firewall rules do not mitigate this because the metadata server is reached over the VM's loopback interface. The other options describe issues that are either incorrect (user-level SSH keys are not stored on disk in plaintext), already mitigated by the firewall, or unrelated to the metadata server (disk-encryption key rotation).

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