Full Report
External security researchers at Accomplish identified a vulnerability in Cloudflare Containers that could expose residual disk data from previous workloads. We explain how the issue worked, how we investigated it, and the steps we took to remediate it.
Analysis Summary
# Vulnerability: Cloudflare Containers Cross-Tenant Data Exposure
## CVE Details
- **CVE ID**: Not explicitly assigned in the report (Internal Cloudflare Incident).
- **CVSS Score**: Not provided (Estimated: Medium/High due to multi-tenant data exposure).
- **CWE**: CWE-226: Sensitive Information Uncleared Before Reuse.
## Affected Systems
- **Products**: Cloudflare Containers; Cloudflare Sandboxes (built on Containers).
- **Versions**: All versions prior to the September 7, 2026, fleet-wide patch.
- **Configurations**: Multi-tenant infrastructure using Linux device mapper thin provisioning (**dm-thin**) with the `skip_block_zeroing` option enabled.
## Vulnerability Description
The flaw resides in the storage allocation logic of Cloudflare’s container infrastructure. Cloudflare Containers use **dm-thin** to provide writable root disks. Because the `skip_block_zeroing` configuration was active, the system did not wipe physical storage blocks when they were returned to the pool or reassigned to new containers.
When a new container performed a "partial write" (e.g., a 4 KiB write to a new 64 KiB thin-block), the system allocated a physical block but only overrode the specific 4 KiB section. The remaining 60 KiB of that block could contain residual raw data belonging to the previous tenant who owned that physical block.
## Exploitation
- **Status**: PoC available (developed by Accomplish researchers); No evidence of exploitation in the wild.
- **Complexity**: Medium (Requires a Workers Paid account and specific disk I/O manipulation).
- **Attack Vector**: Local (Attacker must run a workload/container on the same physical host as the victim).
## Impact
- **Confidentiality**: **High** (Potential exposure of residual disk data from other tenants, including potentially sensitive files or credentials).
- **Integrity**: **None** (The attacker can read residual data but cannot modify the active data of other tenants).
- **Availability**: **None**.
## Remediation
### Patches
- **Cloudflare Runtime Fix**: Applied by Cloudflare internally between September 4 and September 7, 2026.
- **Infrastructure Update**: `skip_block_zeroing` has been disabled; all new blocks are now zeroed before reassignment.
### Workarounds
- No customer-side workarounds are required as the fix was implemented at the infrastructure level by the vendor.
## Detection
- **Indicators of Compromise**: High volumes of small, 64 KiB-aligned writes to `/dev/vdc` followed by raw disk reads of unallocated filesystem space.
- **Detection Methods**: Cloudflare monitored disk-I/O telemetry to verify that only authorized researchers utilized the technique. Customers generally lack the low-level visibility to detect this at the host level.
## References
- **Vendor Advisory**: [https://blog.cloudflare.com/containers-cross-tenant-vulnerability/](https://blog.cloudflare.com/containers-cross-tenant-vulnerability/)
- **Researcher Site**: [https://accomplish.ai/](https://accomplish.ai/)
- **HackerOne Program**: [https://hackerone.com/cloudflare](https://hackerone.com/cloudflare)