Reported by 8 outlets
How Cloudflare addressed a cross-tenant data exposure vulnerability in Containers
Why this score
- Breadth
- 2.00 / 2.0 · 8 sources
- Authority
- 1.80 / 2.0 · best outlet weight
- Severity
- 0.50 / 5.0 · CVSS + content signals
- Velocity
- 0.00 / 2.0 · updates in 48h
- Pickup speed
- 0.32 / 0.6 · time to second source
- Recency
- × 0.00 temporal multiplier
- Community
- + 0.00 / 1.5 · reddit signal
- Score
- 0.0 / 10 = base × recency + community
Desk analysis
Cloudflare addressed a cross-tenant data exposure vulnerability in Containers that allowed one sandbox to read other customers' files. The root cause was dm-thin pools configured with skip_block_zeroing and 64 KiB blocks, combined with Firecracker presenting container root disks as /dev/vdc. Researchers recovered residual material on 18 of 24 placements and 20 of 22 nodes across four continents, including 2,700 distinct foreign directory inodes.
Recovered data included directory listings, SQLite databases, Chromium profiles, .env files, and credential files. Remediation removed skip_block_zeroing fleet-wide, retired running container disks, and cleared cached snapshots by September 19, 2026. No exploitation evidence was found in retained telemetry. Customers should rotate secrets that may have been present in affected containers and verify snapshot cleanup status.
Timeline
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original reporting
How Cloudflare addressed a cross-tenant data exposure vulnerability in Containers
blog.cloudflare.com · 19d ago · September 24th, 2026
submitted by /u/natcoba [link] [comments]
via reddit r/cybersecurity: How Cloudflare addressed a cross-tenant data exposure vulnerability in Containers
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update from accomplish.ai
Escaping the Cloudflare sandbox
accomplish.ai · 20d ago · September 24th, 2026
Accomplish principal security researcher Oren Yomtov announced the disk isolation bug in Cloudflare Containers that let a sandbox read other customers' files, listing directory listings, SQLite databases, Chromium profiles, .env files, and credential files among the recovered material. The post confirms Cloudflare Sandboxes and Browser Run shared the affected disk implementation, that the team reported it to Cloudflare on September 4, 2026, and that this was the team's sixth published sandbox escape since July 2026, after SharedRoot (Claude Cowork), Beltdown (Claude Code), Beltdown2 (Cursor CLI), Docker's VMM, and OpenAI Codex's sandbox.
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update from blog.cloudflare.com
How Cloudflare addressed a cross-tenant data exposure vulnerability in Containers
blog.cloudflare.com · 19d ago · September 24th, 2026
Cloudflare's advisory names dm-thin pools configured with skip_block_zeroing and 64 KiB blocks as the root cause, with Firecracker presenting container root disks as /dev/vdc, and reports the researchers recovered residual material on 18 of 24 placements and 20 of 22 underlying nodes across four continents, including 2,700 distinct foreign directory inodes identified through ext4 directory block checksums. Remediation removed skip_block_zeroing fleet-wide, retired running container disks, and cleared pre-mitigation cached image snapshots, with rollout complete September 7, 2026 and snapshot cleanup complete September 19, 2026, and a review of retained disk-I/O telemetry found no exploitation. The issue was reported through HackerOne report 3997565 on September 4, 2026.
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update from cyberpress.org
Cloudflare Sandbox Escape Flaw Exposes Other Customers’ Files and Credentials
cyberpress.org · 19d ago · September 25th, 2026
Cloudflare has remediated a cross-tenant data exposure vulnerability in its Containers platform that could have allowed one customer’s workload to recover residual disk data belonging to other customers sharing the same physical host. Cloudflare Sandbox Escape Flaw Cloudflare said it found no evidence of malicious exploitation or customer data compromise after reviewing retained disk-I/O telemetry, and it completed fleet-wide remediation without requiring customer configuration changes. The issue was not a conventional container breakout or Firecracker microVM escape . Instead, it originated in the way Cloudflare provisioned storage for container root disks. Cloudflare runs every Container in a dedicated Firecracker microVM and presents its writable root disk as /dev/vdc . The platform uses Linux device mapper thin provisioning, known as dm-thin, to assign physical storage only when a virtual disk writes to an unmapped region. A configuration setting, skip_block_zeroing , prevented dm-thin from clearing a newly allocated physical block before exposing it to a new container. Once a container disk was deleted, the physical blocks backing it r
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update from gbhackers.com
Cloudflare Containers Flaw Could Expose Data From Other Customers’ Workloads
gbhackers.com · 18d ago · September 25th, 2026
This flaw also affected Cloudflare Sandboxes and the Browser Run service within Browser Rendering, both of which utilize the same underlying disk implementation. Cloudflare […] The post Cloudflare Containers Flaw Could Expose Data From Other Customers’ Workloads appeared first on GBHackers Security | #1 Globally Trusted Cyber Security News Platform .
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update from bleepingcomputer.com
Cloudflare fixes Containers cross-tenant flaw exposing customer data
bleepingcomputer.com · 16d ago · September 27th, 2026
Cloudflare Containers is a service available on the Workers Paid plan that lets developers run containerized applications on Cloudflare’s infrastructure, alongside Cloudflare Workers. Developers and companies building applications on Cloudflare typically use it, including those running backend services, processing jobs, and code execution environments. “When the thin volume backing a container's root disk was deleted, its physical blocks were returned to a pool that served workloads belonging to multiple customer accounts,” Cloudflare explains . Without the zeroing operation, only the 4 KiB write would overwrite the block, leaving in a readable state the remaining 60 KiB that may contain data from a previous customer. They
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mentioned by www.bleepingcomputer.com
Cloudflare fixes Containers cross-tenant flaw exposing customer data https:// bleepingcomputer.com/news/security/ cloudflare-fixes-container
www.bleepingcomputer.com · 16d ago · September 27th, 2026
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mentioned by blog.cloudflare.com
🤖 Cloudflare Containers/Sandboxes cross-tenant data exposure: dm-thin storage pools ran with skip_block_zeroing, so a Workers Paid tenant co
blog.cloudflare.com · 15d ago · September 28th, 2026
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update from beyondmachines.net
Cloudflare Patches Cross-Tenant Data Exposure Flaw in Containers Service Cloudflare resolved a cross-tenant data exposure vulnerability in i
beyondmachines.net · 15d ago · September 28th, 2026
Cloudflare Patches Cross-Tenant Data Exposure Flaw in Containers Service Cloudflare resolved a cross-tenant data exposure vulnerability in i
via Mastodon https://beyondmachines.net/event_details/cloudflare-patches-cross-tenant-data-exposure-flaw-in-containers-service-1-n-k-9-p/gD2P6Ple2L
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update from meterpreter.org
Discover how a Cloudflare Containers data flaw exposed cross-tenant fragments. Learn about the skip_block_zeroing thin provisioning vulnerab
meterpreter.org · 15d ago · September 28th, 2026
Learn about the skip_block_zeroing thin provisioning vulnerab
via Mastodon https://meterpreter.org/cloudflare-containers-cross-tenant-data-flaw/?utm_source=mastodon&utm_medium=jetpack_social
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mentioned by www.bleepingcomputer.com
# Cloudflare fixes # Containers cross-tenant flaw exposing customer data https://www. bleepingcomputer.com/news/secu rity/cloudflare-fixes-c
www.bleepingcomputer.com · 14d ago · September 29th, 2026
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mentioned by blog.cloudflare.com
Cloudflare fixes Flaw that let one Container read another Customer's Leftover Disk Data. The data came from disk space that earlier containe
blog.cloudflare.com · 12d ago · October 1st, 2026
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update from blog.cloudflare.com
Multi-tenant security requires absolute isolation down to the storage layer. Cloudflare recently shared details on how they addressed a cros
blog.cloudflare.com · 9d ago · October 4th, 2026
Multi-tenant security requires absolute isolation down to the storage layer.