Cloudflare Containers / Sandboxes (24 Sep): cross-tenant residual disk exposure via dm-thin skip_block_zeroing — patched fleet-wide
Cloudflare Blog (published 24 September 2026 15:00 UTC; joint write-up with Accomplish) and Accomplish (24 Sep) document a cross-tenant data-exposure flaw in Cloudflare Containers (also affecting Sandboxes / Browser Run that share the disk stack). Oren Yomtov (Accomplish) reported via HackerOne on 4 September 2026. Root cause: Linux device-mapper thin provisioning (dm-thin) pools used skip_block_zeroing; when a container’s writable /dev/vdc thin disk was deleted, 64 KiB physical blocks returned to a multi-tenant pool uncleared. A Workers Paid tenant could force allocation of a recycled block with a 4 KiB write into free ext4 space then raw-read residual bytes (directory structures, DB pages, SQLite) left by prior tenants on the same host — opportunistic (no victim/host targeting; placement automatic). PoC saw residual material on 18/24 placements across four continents. No CVE / no vendor CVSS in the posts. Cloudflare removed skip_block_zeroing fleet-wide, retired running container disks and pre-mitigation OCI image snapshot caches, drained/restarted hosts, and built disk-I/O detection for the 4 KiB-write / large-read pattern; retained telemetry showed only researcher and authorized validation activity — no evidence of malicious exploitation. No customer-side config change required. Primary: Cloudflare Blog; researcher: Accomplish.
- Product
- Cloudflare Containers; Cloudflare Sandboxes / Browser Run (shared disk implementation)
- Versions
- Affected until Cloudflare’s Sep 2026 fleet remediation (skip_block_zeroing removed; disks/image caches retired). No CVE ID published. Workers Paid account was a prerequisite for the PoC path.
- Exploited in Australia?
- unknown
- Patch to
- Cloudflare-side fix already applied fleet-wide — no customer config required. Tenants who stored secrets on Container/Sandbox disks before the fix should rotate credentials/API keys/.env material as defense-in-depth; review Cloudflare status/blog for any follow-on guidance
Primary: Cloudflare Blog — cross-tenant data exposure in Containers (24 Sep 2026) · Vendor: Cloudflare (vendor) · Accomplish — Escaping the Cloudflare sandbox (24 Sep 2026)