Cloudflare Fixes Cross-Tenant Residual Disk Flaw in Containers

Cloudflare patched a Containers disk-reuse flaw that could expose up to 60 KiB of another tenant’s leftover data, with no malicious exploitation found.

On September 4, 2026, Oren Yomtov of Accomplish responsibly reported a cross-tenant data exposure in Cloudflare Containers, which also underlies Cloudflare Sandboxes; GBHackers additionally said Browser Rendering shared the same disk implementation. Containers run in Firecracker VMs whose writable disks used Linux device-mapper thin provisioning with skip_block_zeroing, so a newly assigned 64 KiB block was not wiped and a 4 KiB write could leave up to 60 KiB of a previous tenant’s data readable. A Workers Paid customer could not choose the host or target a specific tenant or live disk. Researchers recovered residual data on 18 of 24 production placements; Cloudflare’s account cited directory, database, and SQLite data, while other reports also described filesystem metadata, Chromium profiles, .env files, and credential files. Cloudflare said it restored wiping, retired or cleared existing disks—by September 19, according to The Hacker News—found no malicious exploitation, and required no customer action.

• Oren Yomtov of Accomplish reported the issue on September 4, 2026; one outlet said the disclosure was via HackerOne. No CVE id was stated.
• The flaw affects Cloudflare Containers and also underlies Sandboxes; GBHackers additionally said Browser Rendering used the same disk implementation.
• Writable disks in Firecracker VMs used Linux dm-thin with skip_block_zeroing, so a reused 64 KiB block was not wiped and a 4 KiB write could leave up to 60 KiB of a prior tenant readable.
• Residual foreign data was recovered on 18 of 24 production placements or attempts.
• Cloudflare described directory, database, and SQLite leftovers; other reports also cited filesystem metadata, SQLite pages, Chromium profiles, .env files, and credential files.
• A Workers Paid customer could read leftovers but could not choose a host or target a specific tenant, live disk, or chosen data.
• Cloudflare restored block wiping and retired or cleared existing disks; The Hacker News said cleanup finished on September 19, and customers need no changes.
• Telemetry showed no malicious exploitation; The Hacker News said retained logs showed only authorized researcher and Cloudflare testing.