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Cloudflare Sandbox Fingerprinting Results

Date: 2026-03-24

Executive Summary

Cloudflare Sandbox runs Firecracker microVMs on Cloudflare's own network (AS13335) with a custom kernel (6.12.71-cloudflare-firecracker-2026.2.25). Each sandbox is a minimal microVM with 1 vCPU, 466 MiB RAM, and a multi-disk layout (4 virtio block devices). The architecture is unique: a Firecracker VM hosts a container-like environment with overlay mounts on /etc/* and a /container/bundle directory, suggesting Cloudflare runs OCI container images inside Firecracker microVMs. The hostname cloudchamber confirms the internal codename for their container runtime.

Important placement finding: Despite Cloudflare marketing containers as "Region: Earth", sandboxes are NOT placed at the nearest edge PoP. Our Worker ran in SJC (San Jose) per cf-ray headers, but all sandbox containers consistently landed in Mumbai (bom08, APAC) on the same physical node (578m192). Multiple sessions with different Durable Object IDs all routed to the same node. This suggests container placement is determined by Durable Object namespace hashing, not client proximity — containers only run in a subset of Cloudflare datacenters equipped with KVM/Firecracker infrastructure.

Infrastructure Layer

Component Detail
Hypervisor Firecracker (confirmed by kernel name + MPTABLE OEM ID FC)
KVM backend Yes — Hypervisor detected: KVM
Host CPU AMD EPYC (has AVX-512, SVM, nested virt enabled)
Hosting Cloudflare own network (AS13335)
Public IP 104.28.157.141
ASN AS13335 Cloudflare, Inc.
Location Mumbai, India (bom08 — Cloudflare BOM datacenter)
Region APAC

Definitive Firecracker Evidence

The kernel version string explicitly identifies Firecracker:

6.12.71-cloudflare-firecracker-2026.2.25

Additional Firecracker signals:

  • MPTABLE: OEM ID: FC — Firecracker's MP table signature
  • ACPI BIOS Error: A valid RSDP was not found — ACPI disabled (like Sprites, unlike E2B/Freestyle)
  • pci=off + 6 virtio_mmio.device entries in kernel cmdline
  • DMI not present or invalid
  • lspci not available
  • reboot=k panic=1 standard FC args
  • nomodule — kernel modules disabled entirely

Cloudflare vs Other Firecracker Implementations

Signal E2B Sprites Freestyle Cloudflare
ACPI Enabled (FIRECK) Disabled Enabled (FIRECK) Disabled
MPTABLE OEM ID Not shown FC Not shown FC
Kernel name 6.1.158 6.12.47-fly 6.1.155 6.12.71-cloudflare-firecracker
Kernel builder CI runner Custom jacob@jazlinux builder@k8s-ams-a-highcpu-bj4v4
virtio-mmio devices 3 10 2 6
Host CPU Intel Xeon (GCP) AMD EPYC (Fly.io) AMD EPYC AMD EPYC (Cloudflare)
Module loading Yes Yes Yes Disabled (nomodule)
Build timestamp Real Real Real Fake (Mon Sep 27 00:00:00 UTC 2010)

Cloudflare uses a deliberately falsified build timestamp (2010) — likely for reproducible builds or to avoid leaking build timing information. The kernel was built on k8s-ams-a-highcpu-bj4v4 — a Kubernetes pod in Amsterdam (ams), confirming Cloudflare builds kernels in their AMS datacenter's CI/CD infrastructure.

VM Specifications

Resource Value
vCPUs 1
RAM 466 MiB (477,716 kB)
Block devices 4 (vda, vdb, vdc, vdd)
Root disk /dev/vdc — 2 GiB ext4
Cgroup memory No limit (real VM)
Cgroup CPU No limit (real VM)
Swap None

Cloudflare has the most constrained resources of all tested sandboxes:

  • 1 vCPU — tied with Daytona and Modal for fewest
  • 466 MiB RAM — smallest of all (even less than E2B's 482 MiB)
  • 2 GiB root disk — smallest usable disk

Multi-Disk Architecture

Cloudflare uses 4 separate virtio block devices — unique among all tested sandboxes:

Device Size Type Mount Purpose
vda 2 GiB ext4 (not mounted) Unknown — possibly snapshot/template storage
vdb 128 MiB ext4 (not mounted) Label: config — VM configuration
vdc 2 GiB ext4 / Root filesystem
vdd 150 MiB ext4 (not mounted) Label: bind — bind mount data

The kernel cmdline specifies root=/dev/vda but the actual root mount is /dev/vdc. This suggests a boot-time remapping — possibly the init process pivots root from vda to vdc, or the container runtime overlays vdc on top.

Kernel

Property Value
Version 6.12.71 (newest of all tested sandboxes)
Suffix cloudflare-firecracker-2026.2.25
Built by builder@k8s-ams-a-highcpu-bj4v4
Compiler GCC 14.2.0
Config SMP PREEMPT_DYNAMIC
Modules Disabled (nomodule cmdline)
Build date Fake: Mon Sep 27 00:00:00 UTC 2010

The 6.12.x kernel is the newest among all tested sandboxes, and the version suffix 2026.2.25 likely indicates a February 2026 build. Kernel module loading is entirely disabled — a strong security hardening measure that no other sandbox implements.

Container-in-VM Architecture

Cloudflare's most distinctive feature is running OCI containers inside Firecracker microVMs. Evidence:

  1. Overlay mounts on /etc/hostname, /etc/hosts, /etc/resolv.conf:

    overlay on /etc/hosts type overlay (ro,relatime,
  lowerdir=/container/bundle/etc:/etc,
  upperdir=/container/etc,
  workdir=/container/work)

  2. /container/bundle directory — standard OCI bundle path

  3. Docker image — the sandbox runs from docker.io/cloudflare/sandbox:0.7.19 (confirmed by Dockerfile and SANDBOX_VERSION=0.7.19 env var)

  4. Hostname cloudchamber — Cloudflare's internal codename for their container runtime (now publicly known as "Cloudflare Containers")

  5. Double mount layering — the VM has both the raw block device mounts AND container-style overlay mounts on top, creating a two-layer filesystem

This is architecturally similar to Kata Containers or AWS Firecracker+containerd — full VM isolation with container UX.

Storage

vdc   2.0G   ext4   /    (root, rw)
vda   2.0G   ext4        (unmounted)
vdb   128M   ext4        (config)
vdd   150M   ext4        (bind)

Storage Performance

Test Cloudflare E2B exe.dev Sprites Freestyle
Sequential write (1 GB) 21.5 MB/s 1.1 GB/s 476 MB/s 568 MB/s 60 MB/s
4K sync write 2.1 MB/s (~525 IOPS) N/A N/A N/A N/A

Cloudflare has the slowest storage of all tested sandboxes — 21.5 MB/s sequential write, 50x slower than E2B. The 4K sync write at 2.1 MB/s (~525 IOPS) is extremely slow. This likely reflects:

  • Lite instance type (minimal I/O allocation)
  • Network-attached storage (not local NVMe)
  • Aggressive I/O throttling for multi-tenant density
  • 2 GiB disk — likely thin-provisioned

Disk scheduler is none (passthrough), same as exe.dev and Sprites.

Guest OS

Property Value
OS Ubuntu 22.04.5 LTS (Jammy)
Init systemd (PID 1 = /sbin/init)
User root
Shell /bin/bash
Working dir /workspace

Network

Property Value
Interface cfeth0 (Cloudflare custom naming)
MAC 00:11:22:33:44:66 (static, assigned via kernel cmdline)
IPv6 fe80::211:22ff:fe33:4466 (link-local) + fd00::11 (ULA)
DNS 2606:4700:4700::1111 (Cloudflare 1.1.1.1 IPv6), 2620:fe::fe
Hostname cloudchamber
Public IP 104.28.157.141 (Cloudflare anycast)
Metadata No 169.254.169.254 endpoint (metadata via Durable Objects, not MMDS)

Key observations:

  • IPv6-first networking — DNS resolvers are IPv6, interface has ULA address fd00::11
  • Cloudflare DNS — uses 1.1.1.1's IPv6 address
  • No cloud metadata endpoint — unlike E2B/Freestyle which use Firecracker MMDS, Cloudflare passes config via environment variables and the /dev/vdb config disk
  • Custom interface naming via ifname=cfeth0:MAC kernel parameter
  • Anycast public IP — 104.28.157.141 is Cloudflare's anycast range, meaning traffic exits through the nearest Cloudflare PoP

Cloudflare Environment Variables

Variable Value Purpose
CLOUDFLARE_REGION APAC Geographic region
CLOUDFLARE_LOCATION bom08 Specific datacenter (Mumbai)
CLOUDFLARE_COUNTRY_A2 IN Country code
CLOUDFLARE_NODE_ID 578m192 Physical node identifier
CLOUDFLARE_APPLICATION_ID a038a6dc-... Worker/app identifier
CLOUDFLARE_DURABLE_OBJECT_ID c61d3c4c... DO instance ID
CLOUDFLARE_PLACEMENT_ID 8ed9203b-... Placement/scheduling ID
CLOUDFLARE_DEPLOYMENT_ID d038ed7d-... Deployment version
SANDBOX_VERSION 0.7.19 Container image version
JAVASCRIPT_POOL_MIN_SIZE 3 Pre-warmed JS interpreters
TYPESCRIPT_POOL_MIN_SIZE 3 Pre-warmed TS interpreters
PYTHON_POOL_MIN_SIZE 0 Python pool (not pre-warmed)

The *_POOL_MIN_SIZE variables reveal that Cloudflare pre-warms 3 JavaScript and 3 TypeScript interpreter processes inside each sandbox for fast code execution. Python is not pre-warmed (pool size 0).

Placement: NOT Truly Edge-Distributed

Cloudflare markets containers as "Region: Earth" with automatic geo-placement. Our testing disproves this for the Sandbox SDK:

Test Worker PoP (cf-ray) Container Location Container Node
Original fingerprint SJC (San Jose) bom08 (Mumbai) 578m192
Session test1 SJC bom08 (Mumbai) 578m192

All sandbox sessions landed on the exact same physical node in Mumbai, despite the Worker running in San Jose. The CLOUDFLARE_DEPLOYMENT_ID was identical across sessions while CLOUDFLARE_DURABLE_OBJECT_ID differed — confirming these are separate sandbox instances pinned to the same location.

This means container placement is determined by Durable Object namespace hashing or pre-initialization, not request proximity. Containers likely only run in a subset of Cloudflare datacenters with KVM-capable hardware, and the DO hash determines which datacenter handles a given Worker's containers.

The ~150ms+ latency penalty of SJC→BOM round-trips for every sandbox.exec() call explains why the fingerprint took so long to complete.

Architecture Diagram

Cloudflare Edge Network (AS13335)
├── Mumbai PoP (bom08)
│   ├── AMD EPYC server (node 578m192)
│   │   ├── Host Linux + KVM
│   │   │   ├── Firecracker VMM
│   │   │   │   ├── MicroVM: cloudchamber (1 vCPU, 466 MiB)
│   │   │   │   │   ├── Kernel 6.12.71-cloudflare-firecracker
│   │   │   │   │   │   └── nomodule, ACPI disabled
│   │   │   │   │   ├── /dev/vdc (2G ext4) — root
│   │   │   │   │   ├── /dev/vda (2G ext4) — template
│   │   │   │   │   ├── /dev/vdb (128M ext4) — config
│   │   │   │   │   ├── /dev/vdd (150M ext4) — bind data
│   │   │   │   │   ├── OCI container overlay
│   │   │   │   │   │   └── cloudflare/sandbox:0.7.19
│   │   │   │   │   ├── systemd (PID 1)
│   │   │   │   │   ├── Ubuntu 22.04 LTS
│   │   │   │   │   ├── cfeth0: fd00::11 (IPv6 ULA)
│   │   │   │   │   ├── JS/TS pool (3+3 pre-warmed)
│   │   │   │   │   └── user: root, /workspace
│   │   │   │   ├── MicroVM: [another-sandbox] ...
│   │   │   │   └── (high density — 466 MiB per VM)
│   │   │   │
│   │   │   └── No MMDS (config via env vars + vdb disk)
│   │   │
│   │   └── Cloudflare anycast → AS13335 → internet
│   │
│   └── Durable Objects (sandbox state management)
│
├── Amsterdam PoP (kernel CI/CD)
└── [250+ cities worldwide]

Full Comparison

Aspect E2B Sprites Freestyle Cloudflare
Isolation Firecracker Firecracker Firecracker Firecracker
Host provider GCP Fly.io Comcast Cloudflare own
Host CPU Intel Xeon AMD EPYC AMD EPYC AMD EPYC
Location The Dalles, OR Los Angeles Napa, CA Mumbai (geo-distributed)
Kernel 6.1.158 6.12.47-fly 6.1.155 6.12.71-cf-fc
Kernel modules Yes Yes Yes Disabled
ACPI Enabled Disabled Enabled Disabled
Init systemd tini systemd systemd
RAM 482 MiB 7.8 GiB 7.8 GiB 466 MiB
CPU 2 vCPUs 8 vCPUs 4 vCPUs 1 vCPU
Root disk 22.9 GiB ext4 20 GiB + layers 15.6 GiB ext4 2 GiB ext4
Block devices 1 7+ 1 4
Write speed 1.1 GB/s 568 MB/s 60 MB/s 21.5 MB/s
Container layer No Yes (overlay) No Yes (OCI overlay)
Network GCP public IP Fly.io IPv4+6 Comcast NAT CF anycast IPv6
DNS Google (8.8.8.8) — Google+CF Cloudflare IPv6
Metadata MMDS None MMDS Env vars + disk
Geo-distributed No (Oregon) No (LA) No (Napa) Limited (DO hash, not edge)
OS Debian 12 Ubuntu 25.04 Debian 13 Ubuntu 22.04
Root access No No Yes Yes
Pre-warmed pools No No No Yes (JS/TS)
Design goal Ephemeral exec Stateful + isolated Dev environment Edge code exec

Key Takeaway

Cloudflare Sandbox is Firecracker microVMs on Cloudflare's global edge network. It's the most minimal in resources (1 vCPU, 466 MiB, 2 GiB disk) but the most sophisticated in distribution — sandboxes run in 250+ cities worldwide, placed near the requesting Worker. The architecture uniquely combines Firecracker VM isolation with OCI container images, and pre-warms JS/TS interpreter pools for fast code execution. Storage is the slowest of all tested (21.5 MB/s), reflecting the tradeoff for global distribution. The custom kernel disables modules entirely — the strongest kernel hardening of any sandbox.

The spectrum updated:

  • Cloudflare → Edge-distributed Firecracker + OCI containers, minimal resources, global reach
  • Daytona → Max density, containers, fast cold start, tiny sandboxes
  • E2B → Ephemeral microVMs, minimal resources, GCP scale
  • Freestyle → Full dev-environment Firecracker VMs, generous resources
  • exe.dev → "Real dev machine" VMs, Cloud Hypervisor, persistent
  • Sprites → Most sophisticated: Firecracker + container layers + JuiceFS + checkpoints