CVE-2026-40937

Description

# Missing Admin Auth on Notification Target Endpoints in RustFS ### Finding Summary All 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. A 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. ### What Was Proven Live 1. **Authorization bypass on all four endpoints** (03_readonly_user_bypass.py) - PUT, GET list, GET arns, DELETE all return 200 for readonly-user - Control routes (list-users, kms/status) correctly return 403 - Unauthenticated requests correctly rejected (403 Signature required) 2. **SSRF via health probe** (04_ssrf_listener_landing.py) - HEAD request from rustfs container to attacker-controlled listener - No host validation: only scheme check (http/https) 3. **Target hijacking and event exfiltration** (05_target_hijacking.py, 06_full_event_exfil.py) - Readonly-user overwrites admin-configured target URL by name - Subsequent S3 events delivered to attacker-controlled endpoint - Captured event body includes object keys, bucket names, user identities, and request metadata 4. **Audit evasion** (05_target_hijacking.py) - Readonly-user can delete unbound targets - Readonly-user can overwrite bound targets (silently redirecting events) ### Escalation Vectors Tested But Not Viable 1. **Self-referencing webhook to admin API** (13_self_referencing_test.py) - Webhook sends unsigned POST with event JSON body - Admin endpoints require SigV4 auth -- unsigned request rejected - "Confused deputy" via self-referencing does NOT work 2. **Protocol smuggling via non-HTTP targets** - Only 2 target types implemented: webhook and MQTT (`event.rs:613` enforces this) - No Redis, Kafka, AMQP, or other protocol targets exist - CRLF injection in webhook config fields sanitized by reqwest - MQTT uses rumqttc (pure Rust binary protocol client), no raw TCP injection 3. **MQTT target for RCE** - No unsafe code in MQTT handler - rumqttc 0.29.0 has no known public CVEs - No Command::new, template engines, or deserialization of broker responses 4. **Unauth access** - Endpoints correctly reject unauthenticated requests (403) - Endpoints correctly reject invalid credentials (403) ### Prior Art No existing advisory covers notification target endpoints. 11 published GHSAs on rustfs/rustfs cover different handlers. Closest: - CVE-2026-22042 (ImportIam wrong action constant) -- same bug class, different file - CVE-2026-22043 (deny_only short-circuit) -- different bug class ### Recommendation Submit 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. Koda Reef ### Patch This issue has been patched in version https://github.com/rustfs/rustfs/releases/tag/1.0.0-alpha.94.

Affected packages (1)

• crates.io/rustfsfrom 0, <= 0.0.2

CVSS scores

References (4)

• ADVISORYhttps://nvd.nist.gov/vuln/detail/CVE-2026-40937
• PATCHhttps://github.com/rustfs/rustfs
• WEBhttps://github.com/rustfs/rustfs/releases/tag/1.0.0-alpha.94
• WEBhttps://github.com/rustfs/rustfs/security/advisories/GHSA-pfcq-4gjr-6gjm