fix(security): remove Docker socket mount from LocalStack and enforce non-root user in signer images

[mertcano]

Summary of Changes

Addresses the Blocker and Medium container security audit findings in arc-remote-signer (Cat 7: Dependency & supply-chain security, Cat 21: IaC & container hardening)[cite: 44, 51]:

1. Docker Daemon Socket Removal (deployments/docker-compose.yaml - Blocker): Removed /var/run/docker.sock volume mount and unused DOCKER_HOST environment variable from the LocalStack service, eliminating a critical host-level root privilege escalation vulnerability.
2. Non-Root User Enforcement (docker/Dockerfile, docker/Dockerfile.enclave - Medium): Created dedicated system user circle:circle (UID/GID 1000) and applied USER circle across both signer and enclave Dockerfiles to prevent root execution when handling signing keys.
3. Loopback Exposure Tightening (deployments/docker-compose.yaml): Bound enclave port 10350 to 127.0.0.1 to restrict access strictly to localhost.

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Vulnerability Analysis

• Docker Socket Host Takeover (Blocker - Cat 7):[cite: 44]

  • /var/run/docker.sock grants arbitrary container creation and root execution over the host daemon[cite: 44].
  • LocalStack runs repository-supplied init scripts (create-kms-keys.sh, create-secret-manager-key.sh) at startup, turning any malicious alteration of these scripts into an immediate host compromise[cite: 44].
  • KMS and Secrets Manager mock services do not require Docker socket access[cite: 44].

• Root Privilege Execution (Medium - Cat 21):

  • Processes holding encrypted signing keys and interacting with AWS KMS ran as UID 0 (root)[cite: 51].
  • A compromise in the gRPC server or KMS client would yield root access inside the container[cite: 51].

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Key Remediations

• deployments/docker-compose.yaml:

  • Removed Docker socket mount from LocalStack service volumes[cite: 44].
  • Removed DOCKER_HOST: unix:///var/run/docker.sock environment setting.
  • Bound enclave service port to 127.0.0.1:10350:10350.

• docker/Dockerfile & docker/Dockerfile.enclave:

  • Provisioned system group/user circle with fixed UID/GID 1000[cite: 51].
  • Chowned /usr/local/circle to circle:circle to ensure 0750 executables (app, run_proxy.sh, run_enclave.sh) remain executable[cite: 51].
  • Set USER circle as the execution user for both images[cite: 51].

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Verification

• Parsed docker-compose.yaml via YAML schema validation: successfully parsed with 0 errors; confirmed no docker.sock volume mounts exist.
• Verified that reproducible build steps in Dockerfile.enclave (touch -hcd "@0") are preserved alongside the circle user creation[cite: 51].

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Follow-up Action Required (Section 10 - Blocker)

• Unauthenticated gRPC Signer (internal/app/public/public.go:New): As noted in Section 8 and Section 10 of the audit report, the gRPC signing service returns Ed25519 signatures without authentication and TLS defaults to disabled[cite: 55, 57]. Once the Go toolchain is installed in the target environment, implement a peer-allowlist or mTLS interceptor and default TLS to enabled[cite: 55, 57].