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
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.
Affected products
- Cbcoutinho Nextcloud-mcp-server — versions < 0.117.2
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_MISC)
- security-advisories@github.com (x_refsource_CONFIRM)
- security-advisories@github.com (x_refsource_MISC)
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.