vLLM documentation: Security (docs/usage/security.md)

Upstream's own statement of its threat model: ALL inter-node communication is insecure by default and must be protected by network isolation - PyTorch Distributed, KV cache transfer, and tensor/pipeline/data parallel communications. PyTorch Distributed has no authorization protocol, sends messages unencrypted, and accepts connections from anywhere. --allowed-media-domains is the documented SSRF control and it applies to the batch runner as well as online serving.