CVE-2026-55640: Nextcloud MCP Server: Unauthenticated `POST /webhooks/nextcloud` allows arbitrary vector data deletion when `WEBHOOK_SECRET` is unset ( default )

Summary The POST /webhooks/nextcloud endpoint has no authentication by default: WEBHOOKSECRET defaults to None and is never required by startup validation. When unset, the receiver accepts any unauthenticated POST. The userid is taken directly from the attacker-supplied payload and passed to Qdrant, allowing an unauthenticated attacker to delete or corrupt vector embeddings for any user.

Details Vulnerable file: nextcloudmcpserver/vector/webhookreceiver.py, function handlenextcloudwebhook(), lines 55-67

Root cause 1: Auth check is guarded by if secret: - skipped entirely when WEBHOOKSECRET is unset.

Root cause 2: webhooksecret: str | None = None in config - no startup validator enforces it, even when vector sync is enabled.

Trusted field: payload["user"]["uid"] in webhookparser.py is used as-is for all Qdrant operations - no cross-check against an authenticated session.

webhookreceiver.py, lines 55-67: python secret = getsettings().webhooksecret # None by default if secret: # skipped entirely when unset ... validate Bearer header ... else: warnmissingsecretonce() # just logs, still processes webhookparser.py, line 57: python userid = payload["user"]["uid"] # attacker-controlled PoC No credentials required. Works on any deployment where WEBHOOKSECRET is not explicitly set (the default). json POST /webhooks/nextcloud Content-Type: application/json

{ "event": { "class": "OCP\\Files\\Events\\Node\\BeforeNodeDeletedEvent", "node": { "path": "/victim/files/Notes/any.md", "id": 12345 } }, "user": { "uid": "victim" }, "time": 0 } Result: Qdrant deletes all vector embeddings for victim doc 12345 with no authentication. Attacker can loop over doc IDs for mass deletion. All user targets accepted.

Impact + Anyone on the network with access to port 8000 - no credentials needed. + Attacker can delete or trigger re-index of any user's vector embeddings in Qdrant by spoofing user.uid in the payload. + Mass-sending delete events for all doc IDs destroys the entire semantic search index for all users, requiring a full re-scan to recover.

Recommend Fix 1. Enforce WEBHOOKSECRET at startup ( file configvalidators.py ) python if vectorsyncenabled and not settings.webhooksecret: raise ConfigurationError( "WEBHOOKSECRET must be set when vector sync is enabled" ) 2. Reject requests when secret is unset ( file webhookreceiver.py ) python secret = getsettings().webhooksecret if not secret: return JSONResponse({"status": "unavailable"}, statuscode=503) provided = request.headers.get("authorization", "").encode() if not hmac.comparedigest(provided, f"Bearer {secret}".encode()): return JSONResponse({"status": "unauthorized"}, statuscode=401)