# What is Firecracker?

Firecracker is an open-source virtual machine monitor from AWS that uses Linux KVM to run lightweight microVMs, each with its own kernel.

**Runtime runs every sandbox in its own Firecracker microVM**, on dedicated
servers Runtime operates. A new sandbox answered its first Python command
351 ms after the create request at the median, measured from a laptop over the
public internet on 24 September 2026 ([speed](/docs/speed)).

## How does Firecracker work?

Firecracker is one process per microVM. It asks the Linux kernel's KVM to
create a virtual machine, then boots a guest Linux kernel inside it. The guest
sees a deliberately short list of devices; the project's site counts "only 5
emulated devices": virtio-net, virtio-block, virtio-vsock, a serial console
and a minimal keyboard controller used only to stop the microVM.

Two more layers sit around the monitor. Seccomp filters "limit the host system
calls Firecracker can use" to "the bare minimum", and a jailer process sets up
cgroups and namespaces and drops privileges before Firecracker starts. The
project calls the jailer "a second line of defense in case the virtualization
barrier is ever compromised." Its design document treats every guest vCPU as
"running malicious code as soon as they have been started."

## Facts

| Property             | Firecracker, as the project states it                        |
| -------------------- | ------------------------------------------------------------ |
| What it is           | A virtual machine monitor (VMM) on Linux KVM                 |
| Language and licence | Rust, Apache 2.0                                             |
| Start time           | Up to 125 ms from the start call to the guest's `/sbin/init` |
| Memory overhead      | Up to 5 MiB for the VMM threads, on a 1 vCPU, 128 MiB guest  |
| Creation rate        | Up to 150 microVMs per second per host                       |
| CPU performance      | Over 95% of bare metal for compute-only work                 |
| Built for            | AWS Lambda and AWS Fargate                                   |
| Integrated in        | Kata Containers, Flintlock                                   |

The creation rate is from the project's site; the rest come from its
specification, measured on AWS M5D.metal and M6G.metal hosts with a minimal
guest kernel and root filesystem. They measure the monitor alone, not a
sandbox with a full operating system and network.

## Is Firecracker a container runtime?

No. It runs virtual machines. Container tools can use it underneath: Kata
Containers can boot each pod in a Firecracker microVM. See
[Kata Containers vs Firecracker](/compare/kata-containers-vs-firecracker).

## How Runtime uses Firecracker

Each Runtime sandbox is a Firecracker microVM with its own Linux kernel and
disk, running Ubuntu 24.04 with Python, Node.js, Bun, git and a compiler.
`sudo` works inside, and root in the guest still cannot change the network
rules, CPU, memory or cost, which the host enforces
([security](/docs/security)). Custom image builds run in their own Firecracker
guest too ([images](/docs/images)).

```bash no-run
npx withruntime sandbox run --trial -- uname -a
```

Related: [microVM](/glossary/microvm), [gVisor](/glossary/gvisor),
[Firecracker vs gVisor](/compare/firecracker-vs-gvisor),
[Firecracker vs Docker](/compare/firecracker-vs-docker).

## Sources

Checked 25 September 2026.

- [Firecracker](https://firecracker-microvm.github.io/)
- [Firecracker README](https://github.com/firecracker-microvm/firecracker)
- [Firecracker specification](https://github.com/firecracker-microvm/firecracker/blob/main/SPECIFICATION.md)
- [Firecracker design](https://github.com/firecracker-microvm/firecracker/blob/main/docs/design.md)

Facts on this page were checked on 25 September 2026.