A sandbox without a network boundary is only half a sandbox

This post details Vercel's approach to network isolation for sandboxes, emphasizing the need for egress control in addition to compute isolation. It introduces the Vercel Sandbox firewall, which polices outbound TCP connections and DNS queries on the host outside the microVM. The firewall inspects TLS handshakes (SNI) and checks against domain and CIDR policies. It also supports selective TLS termination for header injection and request forwarding, allowing for credential injection at the network boundary and programmable policy enforcement via custom proxies. The post highlights that credentials are never stored within the microVM and are disposed of when the sandbox stops. It also emphasizes that these security features are a baseline product, available on all Sandbox plans.