Cloudflare: Cloudflare Containers Vulnerability Could Leak Data Between Customer Workloads

Cloudflare Patches Cross-Tenant Data Exposure Flaw in Containers Platform

Cloudflare recently addressed a critical cross-tenant data exposure vulnerability in its Containers platform, which could have allowed one customer’s workload to access residual disk data from another tenant on the same physical host. The flaw also impacted Cloudflare Sandboxes, though the company found no evidence of malicious exploitation or customer data compromise in its telemetry logs.

The vulnerability was responsibly disclosed on September 4, 2026, by Oren Yomtov, a security researcher at Accomplish, via Cloudflare’s HackerOne bug bounty program. Exploitation required a Workers Paid account, and due to Cloudflare’s automatic workload placement, attackers could not target specific victims, hosts, or data though the flaw still breached tenant isolation in shared cloud infrastructure.

Root Cause & Exploitation Mechanism

The issue stemmed from Cloudflare’s storage layer, not a traditional container or VM escape. Each container runs in a Firecracker microVM, with its writable root disk exposed as /dev/vdc. Behind this, Linux’s dm-thin provisioning allocates 64 KiB physical storage blocks only when written to without zeroing recycled blocks due to the skip_block_zeroing option.

When a container’s disk was deleted, its physical blocks were returned to a shared pool. If a new container later wrote to an unmapped region, it could receive a recycled block containing residual data from a previous tenant. Researchers exploited this by:

• Writing 4 KiB to a 64 KiB-aligned region, forcing dm-thin to allocate a recycled block.
• Reading the remaining 60 KiB of untouched data, which could include fragments of another customer’s filesystem.

Impact & Findings

Across six production placements, researchers analyzed 5,614 testable directory blocks, finding 2,700 distinct foreign inodes none from their own proof-of-concept. Residual data appeared in:

• 18 of 24 placements and 20 of 22 underlying nodes across four continents.
• Formats including directory structures, database pages, and complete SQLite databases.

While the attack did not allow access to live disks, data modification, or workload disruption, exposed fragments could contain filesystem metadata, application data, or sensitive database content.

Remediation & Detection

Cloudflare’s response included:

1. Disabling skip_block_zeroing to enforce block clearing on new allocations.
2. Retiring existing container disks, draining hosts, restarting VMs, and clearing image caches to eliminate residual data.
3. Developing detection signatures for the attack’s unusual I/O pattern (small writes followed by large reads from new blocks).

A historical review found no unauthorized activity only traces from the researchers and Cloudflare’s validation. The fix required no customer action, though organizations may consider rotating secrets handled by affected workloads based on their risk policies.

Source: https://cybersecuritynews.com/cloudflare-containers-vulnerability/

Cloudflare cybersecurity rating report: https://www.rankiteo.com/company/cloudflare

"id": "CLO1790310238",
"linkid": "cloudflare",
"type": "Vulnerability",
"date": "9/2026",
"severity": "85",
"impact": "4",
"explanation": "Attack with significant impact with customers data leaks"

{'affected_entities': [{'customers_affected': 'Multiple (exact number unknown; '
                                              'residual data found in 18 of 24 '
                                              'placements)',
                        'industry': 'Technology/Internet Infrastructure',
                        'location': 'Global (affected nodes across four '
                                    'continents)',
                        'name': 'Cloudflare',
                        'size': 'Large',
                        'type': 'Cloud Service Provider'}],
 'attack_vector': 'Storage Layer Misconfiguration',
 'customer_advisories': 'No customer action required; recommended rotating '
                        'secrets if applicable',
 'data_breach': {'file_types_exposed': ['Directory structures',
                                        'Database files'],
                 'number_of_records_exposed': '2,700 distinct foreign inodes '
                                              '(residual data fragments)',
                 'personally_identifiable_information': 'Potential (not '
                                                        'confirmed)',
                 'sensitivity_of_data': 'Potentially sensitive (e.g., '
                                        'application data, PII)',
                 'type_of_data_compromised': ['Filesystem metadata',
                                              'Database pages',
                                              'SQLite databases',
                                              'Residual disk fragments']},
 'date_detected': '2026-09-04',
 'description': 'Cloudflare addressed a critical cross-tenant data exposure '
                'vulnerability in its Containers platform, which could have '
                'allowed one customer’s workload to access residual disk data '
                'from another tenant on the same physical host. The flaw also '
                'impacted Cloudflare Sandboxes, though no evidence of '
                'malicious exploitation or customer data compromise was found.',
 'impact': {'brand_reputation_impact': 'Potential reputational risk due to '
                                       'cross-tenant data exposure',
            'data_compromised': 'Residual disk data fragments (filesystem '
                                'metadata, database pages, SQLite databases)',
            'identity_theft_risk': 'Potential if residual data contained PII',
            'operational_impact': 'No workload disruption or live disk access; '
                                  'potential exposure of sensitive fragments',
            'payment_information_risk': 'Potential if residual data contained '
                                        'payment information',
            'systems_affected': 'Cloudflare Containers, Cloudflare Sandboxes'},
 'investigation_status': 'Completed (no evidence of malicious exploitation)',
 'lessons_learned': 'Importance of zeroing recycled storage blocks in '
                    'multi-tenant environments; need for robust detection of '
                    'unusual I/O patterns in cloud infrastructure.',
 'post_incident_analysis': {'corrective_actions': ['Disabled '
                                                   '`skip_block_zeroing` to '
                                                   'enforce block clearing',
                                                   'Retired and replaced '
                                                   'existing container disks',
                                                   'Developed detection '
                                                   'signatures for attack '
                                                   'patterns'],
                            'root_causes': 'Linux dm-thin provisioning '
                                           'misconfiguration '
                                           '(`skip_block_zeroing` enabled), '
                                           'allowing recycled disk blocks to '
                                           'retain residual data from previous '
                                           'tenants'},
 'recommendations': ['Rotate secrets handled by affected workloads based on '
                     'risk policies',
                     'Audit storage layer configurations for similar '
                     'misconfigurations',
                     'Implement automated detection for cross-tenant data '
                     'exposure risks'],
 'references': [{'source': 'Cloudflare Blog'},
                {'source': 'HackerOne Bug Bounty Report'},
                {'source': 'Oren Yomtov (Accomplish)'}],
 'response': {'containment_measures': ['Disabling `skip_block_zeroing` in '
                                       'dm-thin provisioning',
                                       'Retiring existing container disks',
                                       'Draining hosts and restarting VMs',
                                       'Clearing image caches'],
              'enhanced_monitoring': 'Detection signatures for attack patterns',
              'incident_response_plan_activated': True,
              'recovery_measures': 'No customer action required; recommended '
                                   'rotating secrets handled by affected '
                                   'workloads',
              'remediation_measures': ['Enforcing block clearing on new '
                                       'allocations',
                                       'Developing detection signatures for '
                                       'unusual I/O patterns']},
 'title': 'Cloudflare Patches Cross-Tenant Data Exposure Flaw in Containers '
          'Platform',
 'type': 'Data Exposure',
 'vulnerability_exploited': 'Cross-tenant data exposure due to unzeroed '
                            'recycled disk blocks in dm-thin provisioning'}