A flaw in Cloudflare Containers let a paying customer read data earlier customers' containers had left behind on the same server. Reported on 4 September by Oren Yomtov of Accomplish through Cloudflare's bug bounty, the problem was in how shared disks were configured: each container gets a thin-provisioned disk allocated in 64 KB blocks, and when a container was deleted its blocks returned to a pool shared across accounts that had wiping disabled — the opposite of the usual default. Writing a small 4 KB block into unused space and reading the whole block back at raw disk level exposed roughly 60 KB of the previous tenant's bytes. In production tests the researchers recovered leftover material on 18 of 24 attempts, each on servers Cloudflare chose, and on 20 of 22 machines across four continents, including directory structures, database pages and structurally complete SQLite databases. The data came from retired disk space rather than live workloads and an attacker could not choose whose data they received. Cloudflare Sandboxes, marketed as a safe place to run untrusted code including AI-agent-written code, inherited the flaw; Cloudflare has fixed it service-wide and says customers need do nothing.

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