{"id":"CVE-2026-40937","aliases":["GHSA-pfcq-4gjr-6gjm"],"url":"https://o3.security/vulnerability/CVE-2026-40937","summary":"RustFS missing admin authorization on notification target endpoints, which allows unauthenticated configuration of event webhooks","details":"# Missing Admin Auth on Notification Target Endpoints in RustFS\n\n\n### Finding Summary\n\nAll four notification target admin API endpoints in `rustfs/src/admin/handlers/event.rs` use a `check_permissions` helper that validates authentication only (access key + session token), without performing any admin-action authorization via `validate_admin_request`. Every other admin handler in the codebase correctly calls `validate_admin_request` with a specific `AdminAction`. This is the only admin handler file that skips authorization.\n\nA non-admin user can overwrite a shared admin-defined notification target by name, causing subsequent bucket events to be delivered to an attacker-controlled endpoint. This enables cross-user event interception and audit evasion.\n\n### What Was Proven Live\n\n1. **Authorization bypass on all four endpoints** (03_readonly_user_bypass.py)\n   - PUT, GET list, GET arns, DELETE all return 200 for readonly-user\n   - Control routes (list-users, kms/status) correctly return 403\n   - Unauthenticated requests correctly rejected (403 Signature required)\n\n2. **SSRF via health probe** (04_ssrf_listener_landing.py)\n   - HEAD request from rustfs container to attacker-controlled listener\n   - No host validation: only scheme check (http/https)\n\n3. **Target hijacking and event exfiltration** (05_target_hijacking.py, 06_full_event_exfil.py)\n   - Readonly-user overwrites admin-configured target URL by name\n   - Subsequent S3 events delivered to attacker-controlled endpoint\n   - Captured event body includes object keys, bucket names, user identities, and request metadata\n\n4. **Audit evasion** (05_target_hijacking.py)\n   - Readonly-user can delete unbound targets\n   - Readonly-user can overwrite bound targets (silently redirecting events)\n\n### Escalation Vectors Tested But Not Viable\n\n1. **Self-referencing webhook to admin API** (13_self_referencing_test.py)\n   - Webhook sends unsigned POST with event JSON body\n   - Admin endpoints require SigV4 auth -- unsigned request rejected\n   - \"Confused deputy\" via self-referencing does NOT work\n\n2. **Protocol smuggling via non-HTTP targets**\n   - Only 2 target types implemented: webhook and MQTT (`event.rs:613` enforces this)\n   - No Redis, Kafka, AMQP, or other protocol targets exist\n   - CRLF injection in webhook config fields sanitized by reqwest\n   - MQTT uses rumqttc (pure Rust binary protocol client), no raw TCP injection\n\n3. **MQTT target for RCE**\n   - No unsafe code in MQTT handler\n   - rumqttc 0.29.0 has no known public CVEs\n   - No Command::new, template engines, or deserialization of broker responses\n\n4. **Unauth access**\n   - Endpoints correctly reject unauthenticated requests (403)\n   - Endpoints correctly reject invalid credentials (403)\n\n### Prior Art\n\nNo existing advisory covers notification target endpoints. 11 published GHSAs on rustfs/rustfs cover different handlers. Closest:\n- CVE-2026-22042 (ImportIam wrong action constant) -- same bug class, different file\n- CVE-2026-22043 (deny_only short-circuit) -- different bug class\n\n### Recommendation\n\nSubmit via GitHub PVR. The finding is well-supported with live PoC, code references, and clear root cause. The fix is straightforward (add `validate_admin_request` calls to event.rs handlers). Core submission should reference 2-3 focused PoC scripts (readonly bypass, target hijack, event exfil), not the full set of 13 exploratory scripts.\n\n\nKoda Reef\n\n### Patch\nThis issue has been patched in version https://github.com/rustfs/rustfs/releases/tag/1.0.0-alpha.94.","published":"2026-04-22T20:15:57.266Z","modified":"2026-10-28T02:30:40.909897209Z","cvss":{"score":8.3,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:L"},"epss":{"score":0.00293,"percentile":0.21638,"asOf":"2026-08-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"crates.io","name":"rustfs","fixedVersion":null}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/rustfs/rustfs/releases/tag/1.0.0-alpha.94"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/40xxx/CVE-2026-40937.json"},{"type":"ADVISORY","url":"https://github.com/rustfs/rustfs/security/advisories/GHSA-pfcq-4gjr-6gjm"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-40937"},{"type":"PACKAGE","url":"https://github.com/rustfs/rustfs"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:40.909897209Z"}}