Replit rebuilt its sandbox engine with microVMs

Replit rebuilt its sandbox engine to support more autonomous agents, persistent workspaces, higher concurrency, and deployment in customers’ own cloud environments. Instead of adapting its earlier Docker-based runtime, the company built a new system around KVM microVMs, each with its own Linux kernel, using Cloud Hypervisor to run them. A minimal guest process manages containers inside each VM, while a host supervisor configures the VM, networking, and services. The same general-purpose environment can run coding tools or agents handling tasks such as incident investigation and support triage. Hot-plugged disks and memory snapshots let customer storage attach to warm VMs, supporting persistent workspaces and fast startup. Replit says it migrated hundreds of millions of projects without data loss.

The main security benefit is a stronger boundary: a compromised guest kernel must still cross the virtualization layer to reach the host, unlike containers that share the host kernel. Replit further limits risk with role-specific permissions, an unprivileged virtual-machine monitor, and a pared-down guest kernel that excludes unneeded hardware drivers. In an audit of 6,211 Linux CVEs published from June through November 2026, the company says 5,969 were avoided through prior fixes or excluded code: 4,375 fixes were incorporated before disclosure, and configuration excluded 676 additional vulnerabilities. eBPF and Linux Security Modules provide monitoring and enforcement, with supervised security components that can be updated rapidly across running sandboxes. Separate network namespaces and routing tables also isolate overlapping sandbox IP ranges.