Cloudflare fixes flaw that let Containers customers read other tenants’ leftover data
A researcher found reused disk blocks in Cloudflare Containers and Sandboxes held residual data from other customers; Cloudflare says it saw no malicious exploitation.
By Tech CEO Daily Staff, Newsroom
· 2 min read
The news
Cloudflare disclosed on September 24 that a storage configuration in Cloudflare Containers and Cloudflare Sandboxes could expose residual data from one customer’s workload to another. The issue was reported through its bug bounty programme on September 4 by Oren Yomtov of Accomplish.
According to Cloudflare, the root cause was a Linux storage setting that skipped zeroing disk blocks before reusing them, so fragments of a previous container’s files could remain readable by the next tenant on the same host. The researcher recovered residual material on 18 of 24 container placements. BleepingComputer reported that the leftovers could include database pages, browser profiles, .env files and credential files.
Cloudflare said exploitation required a Workers Paid account, an attacker could not choose a target or guarantee what data they would find, and active disks could not be read or modified. Its telemetry review found no evidence of malicious exploitation beyond the researcher’s authorised testing.
The company said it disabled the setting fleet-wide, retired container disks created before the fix, cleared cached image snapshots and completed remediation on September 19, 15 days after the report. Cloudflare said customers do not need to take any action.
The numbers
- Reported
- Sept 4, 2026
- Fully remediated
- Sept 19, 2026
- Placements with residual data in testing
- 18 of 24
Why CEOs should care
Multi-tenant cloud services promise isolation between customers, and this case shows that promise can fail at low levels of the stack. Cloudflare’s response was quick and transparent, but leaders should ask their engineering teams whether workloads write secrets, API keys or customer data to local disk, and whether those secrets are rotated regularly.
For vendor risk, add questions to cloud and SaaS reviews: how does the provider sanitise shared storage between tenants, how does it detect cross-tenant access, and how fast has it fixed past isolation bugs? Cloudflare’s 15-day fix-to-fleet timeline is a useful benchmark.
The bigger picture
Serverless containers and AI agent sandboxes are growing fast, often running untrusted or customer-supplied code. Isolation flaws in these environments are likely to draw more researcher attention.
Sources
- PrimaryCloudflare Containers cross-tenant vulnerability— Cloudflare
- ReportCloudflare fixes Containers cross-tenant flaw exposing customer data— BleepingComputer
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