Auth bypass in Cbcoutinho Nextcloud-Mcp-Server

CVE-2026-55640

Nextcloud MCP Server is a production-ready MCP server that connects AI assistants to a Nextcloud instance. Prior to 0.117.2, the POST /webhooks/nextcloud endpoint in nextcloud_mcp_server/vector/webhook_receiver.py has no authentication by default because WEBHOOK_SECRET defaults to None and startup validation does not require it. When WEBHOOK_SECRET is unset, handle_nextcloud_webhook() accepts unauthenticated requests. The payload["user"]["uid"] field parsed in nextcloud_mcp_server/vector/webhook_parser.py is attacker-controlled and is used without an authenticated-session cross-check for Qdrant operations, allowing a network attacker to delete or trigger re-indexing of vector embeddings for any user and to destroy the semantic search index by sending forged deletion events. This issue is fixed in version 0.117.2.

Vulnerability class: Broken Authentication

Published · last modified .

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.

EPSS exploit prediction

EPSS: 0.007 (52.9th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 53rd percentile — 52.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-55640: held from 0.005 to 0.007.

EPSS over last 30 days · oldest: 0.005 · newest: 0.007 · change: +0.002

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-55640?
CVE-2026-55640 is a critical-severity vulnerability in Cbcoutinho Nextcloud-Mcp-Server, classified under Missing Authentication for Critical Function. CVSS score: 9.1/10. Published 2026-08-25.
How severe is CVE-2026-55640?
Critical severity. CVSS v3 base score is 9.1 out of 10.