Firecracker vs gVisor: Which isolation technology should you use?

Traditional Docker containers share the host kernel, which creates a fundamental security problem: if an attacker breaks out of a container through a kernel exploit, they gain access to the host system and potentially every other container running on that host.

This is more critical now because:

• AI agents generate and execute code automatically without human review
• Developers install thousands of npm packages they've never audited
• Malicious packages get published to registries regularly
• Supply chain attacks spread laterally across shared infrastructure
• Running untrusted code in production containers is increasingly common

Firecracker is a Virtual Machine Monitor written in Rust by AWS. It creates microVMs, which are lightweight virtual machines with minimal device emulation.

Each Firecracker microVM runs its own Linux kernel inside a KVM virtual machine. This provides hardware-enforced isolation. An attacker breaking out of your application still needs to escape a full VM boundary, which is significantly harder than escaping a container.

Firecracker implements only five devices:

• virtio-net for networking
• virtio-block for storage
• virtio-vsock for host-guest communication
• Serial console for debugging
• Minimal keyboard controller

This minimalism is intentional. Fewer devices mean less code, which means fewer potential vulnerabilities. Firecracker's entire codebase is roughly 50,000 lines of Rust, compared to QEMU's nearly 2 million lines of C.

gVisor takes a completely different approach. Instead of running VMs, it implements a user-space kernel written in Go.

When your container makes a system call, gVisor intercepts it. Rather than passing the syscall directly to the host kernel, gVisor handles it in a sandboxed process called the Sentry. This drastically reduces the kernel attack surface because your application never directly interacts with the host kernel.

gVisor supports multiple execution modes: Systrap uses seccomp to intercept syscalls with better performance, KVM uses virtualization for syscall isolation (fastest on bare-metal), and storage modes include Directfs for high-performance file operations.

Here's the comparison table summarizing the key technical differences between Firecracker and gVisor across isolation, performance, and operational complexity.

Firecracker delivers stronger isolation. Hardware virtualization creates a hard boundary that's significantly more difficult to breach than a software-based sandbox.

Each Firecracker microVM has a dedicated kernel completely isolated from the host, hardware-enforced memory isolation via KVM, and minimal attack surface with only 5 devices and 50k lines of Rust code. Breaking out requires escaping both the guest kernel AND the hypervisor layer.

gVisor provides strong isolation but not VM-level. It reduces kernel attack surface dramatically by intercepting syscalls, but your workload still shares some host resources. The Sentry process runs on the host, and while it's sandboxed, it's not a full hardware boundary.

For untrusted multi-tenant workloads where customers are actively adversarial, Firecracker's hardware isolation provides the strongest security guarantees.

Performance differences between Firecracker and gVisor depend on whether you're measuring startup time or runtime overhead.

• Startup time: Firecracker boots microVMs in approximately 125 milliseconds. gVisor starts faster since there's no VM boot process, typically just milliseconds. For workloads that spin up frequently, gVisor has the edge.
• Runtime performance: Firecracker provides near-native performance with minimal VM boundary overhead. gVisor's syscall interception adds latency, sometimes 10-30% slower on I/O-heavy workloads. For CPU-bound workloads, both perform reasonably well. For I/O-intensive applications, Firecracker maintains better throughput.

The operational complexity differs significantly: gVisor works with existing container tooling, while Firecracker requires VM orchestration infrastructure.

• gVisor integrates directly with container runtimes. You install runsc and configure Docker or containerd to use it. For Kubernetes, create a RuntimeClass pointing to gVisor. Your existing container images work without modification. The trade-off: not every syscall is implemented.
• Firecracker requires preparing kernel images, creating root filesystems, configuring networking (TAP devices, routing), implementing the jailer for security hardening, and building VM lifecycle management. Running one microVM is straightforward. Running thousands requires significant engineering investment.

This is why projects like Kata Containers exist. Kata integrates Firecracker (and other VMMs) with Kubernetes, handling the complexity so you can use microVMs through standard container APIs.

Use Firecracker when you need hardware-level isolation for untrusted code. Running customer workloads, AI-generated code, or scenarios where code might be actively malicious requires the strongest possible isolation.

It's ideal for building serverless platforms (fast boot times, minimal memory overhead), running multi-tenant SaaS where tenant isolation is critical, and I/O workloads requiring consistent, predictable performance.

Use gVisor when you want enhanced isolation without managing VMs. It provides syscall-level sandboxing with minimal operational overhead. If your threat model is "defense in depth" rather than "actively adversarial tenants," gVisor is sufficient.

It works well for existing container workflows you don't want to rewrite, compute-heavy workloads with limited syscalls, and infrastructure where nested virtualization isn't available.

• Choose Firecracker if you need the strongest possible isolation for untrusted code, are building serverless infrastructure, security is your top priority, or I/O performance matters.
• Choose gVisor if you want enhanced security over standard containers without full VMs, have existing container workflows, or need fast deployment with minimal infrastructure changes.

Firecracker creates lightweight VMs with hardware-enforced isolation through KVM. Each workload runs in its own virtual machine with a dedicated kernel. gVisor implements a user-space kernel that intercepts syscalls, providing isolation without full VMs. Firecracker offers stronger security, gVisor offers easier integration.

Firecracker provides stronger isolation through hardware virtualization. Each microVM has a dedicated kernel and hardware-enforced memory boundaries. gVisor reduces kernel attack surface significantly through syscall interception but doesn't provide full VM-level isolation. For actively adversarial workloads, Firecracker is more secure.

Yes. gVisor integrates with Docker through the runsc runtime. You configure Docker to use gVisor and your existing containers work with enhanced isolation. Most applications run without modification, though some syscalls aren't supported.

Firecracker microVMs boot to userspace in approximately 125 milliseconds. This makes them suitable for serverless functions that need to scale from zero quickly. Traditional VMs take seconds to boot, containers boot faster but without VM-level isolation.

Kata Containers is an orchestration project that integrates multiple VMMs (including Firecracker, Cloud Hypervisor, and QEMU) with Kubernetes. It handles the complexity of running microVMs through container APIs. Kata makes Firecracker usable in Kubernetes without building custom infrastructure.

Choose gVisor when you want enhanced container security without the complexity of managing VMs. gVisor integrates with existing container workflows, requires no guest OS management, and works on infrastructure where nested virtualization isn't available. If your threat model doesn't require hardware-level isolation, gVisor is simpler to operate.