Full Report
A flaw in Cloudflare Containers let a paying customer read data that other customers' containers had left behind on the same server, Cloudflare and the researchers who found it said on Thursday. The data came from disk space that earlier containers had used and given up, not from any live workload, and an attacker could not choose whose data they got, according to Cloudflare. The company
Analysis Summary
# Morning News Roll-up September 25, 2026
## Overview
Cloudflare has addressed a significant cross-tenant vulnerability in its Cloudflare Containers and Sandboxes platforms. The flaw allowed customers to read leftover data from previously deleted containers due to a disk provisioning misconfiguration. While the issue was confirmed via research, Cloudflare reports no evidence of active exploitation by malicious actors.
## Top Stories
### Cloudflare Fixes Flaw That Let One Container Read Another Customer's Leftover Disk Data
- Summary: A vulnerability in the way Cloudflare Containers handled thin-provisioned disks allowed paying customers to access 64KB blocks of data belonging to previous tenants. The issue arose because the system skipped the default "wiping" process when returning blocks to the shared pool. Researchers successfully recovered directory structures, SQLite databases, and credential files from these leftover blocks.
- Source: hxxps://thehackernews[.]com/2026/09/cloudflare-fixes-flaw-that-let-one.html
# Main Topic
Cross-tenant data leakage vulnerability in Cloudflare Containers and Sandboxes due to improper disk block scrubbing.
## Key Points
- **Discovery:** Reported on September 4, 2026, by Oren Yomtov of Accomplish via Cloudflare’s bug bounty program.
- **Technical Root Cause:** The Linux "thin provisioning" feature used 64KB blocks. A configuration error caused the system to skip wiping these blocks before reallocating them to new containers.
- **Data Exposure:** If a new container wrote less than 64KB (e.g., 4KB) to a block, the remaining space (e.g., 60KB) retained data from the previous owner.
- **Impact:** Researchers confirmed data recovery on 18 of 24 attempts across multiple global regions. Recovered data included SQLite databases, environment (.env) files, and browser profiles.
- **Scope:** Affected Cloudflare Containers and Sandboxes (used for untrusted AI agent code).
## Threat Actors
- **Attribution:** No malicious actors were identified.
- **Researchers:** Oren Yomtov of security firm **Accomplish**.
- **Historical Context:** This is part of a series of sandbox escapes identified by Accomplish affecting platforms like OpenAI, Docker, and Anthropic.
## TTPs
- **Disk Residual Data Extraction:** Accessing raw disk levels to read uninitialized or non-zeroed storage blocks.
- **Cross-Tenant Data Access:** Exploiting shared infrastructure where resource isolation fails at the hardware/storage layer.
- **Information Gathering:** Using analysis scripts to check for specific file formats (Chromium profiles, credentials) within recovered raw data blocks.
## Affected Systems
- **Cloudflare Containers:** Shared multi-tenant container environment.
- **Cloudflare Sandboxes:** Environment for running untrusted code/AI agents.
- **Cloudflare Browser Run:** Identified by researchers as potentially affected by the same disk setup.
## Mitigations
- **System-Wide Fix:** Cloudflare re-enabled block wiping (zeroing) for all newly allocated blocks as of September 14.
- **Cache Cleansing:** Cloudflare retired every running container disk and cleared server image caches to ensure no "dirty" blocks remained in use.
- **Server Restarts:** Cloudflare performed controlled server drains and restarts to finalize the cleanup by September 19.
- **Detection:** The company developed signatures based on the researcher's proof of concept (PoC) to scan historical disk-activity records for unauthorized access.
- **User Action:** No action is required by Cloudflare customers as the fix was implemented at the infrastructure level.
## Conclusion
This incident highlights the inherent risks of "thin provisioning" in multi-tenant cloud environments. While Cloudflare responded rapidly and confirmed no malicious exploitation through forensic disk-activity logs, the vulnerability underscores the necessity for cloud providers to ensure that storage isolation includes mandatory data destruction between tenant handovers. Organizations using shared container environments should monitor for similar cross-tenant architectural flaws.