How Cloudflare addressed a cross-tenant data exposure vulnerability in Containers

On September 4, 2026, Oren Yomtov, a security researcher from Accomplish, responsibly reported a vulnerability affecting Cloudflare Containers and Cloudflare Sandboxes (which is built on Containers), through Cloudflare’s bug bounty program. Cloudflare has fully remediated the vulnerability, and we have no evidence that customer data has been compromised. 

This post was prepared in collaboration with Oren Yomtov and the Accomplish security research team, whose detailed report and controlled testing helped us validate the issue and respond quickly.

Cloudflare Containers run workloads on multi-tenant infrastructure and automatically assign them to eligible servers; customers cannot select the underlying host. The researchers demonstrated that a customer with a Workers Paid account could recover residual disk blocks previously used by Containers on the same host. The technique could not target a particular customer, workload, host, or data, and residual data was not guaranteed to be present.

Cloudflare applied a fix across the Containers fleet, with no customer-side configuration changes required. Within the historical disk-I/O telemetry available to us, we identified no evidence of malicious exploitation. Activity we could attribute to the reported technique came from the researchers and Cloudflare engineers conducting authorized validation.

Here, we explain the underlying storage behavior, its potential impact, our investigation, and the actions we took in response.

How container storage allocation works 

Cloudflare Containers use Linux device mapper thin provisioning (dm-thin) to provide each container with a writable root disk. Each container lives inside a dedicated virtual machine powered by the Firecracker virtual machine monitor. Firecracker presents this disk to the virtual machine as /dev/vdc.