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

What is CVE-2026-55640?

CVE-2026-55640 is a critical missing authorization vulnerability in cbcoutinho nextcloud-mcp-server, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-306

Which versions are affected by CVE-2026-55640?

3 open source packages ship an affected version, including nextcloud-mcp-server.

How do I fix CVE-2026-55640?

Upgrade nextcloud-mcp-server to 0.117.2 or later.

Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.

Is CVE-2026-55640 being exploited?

There is no known exploitation of CVE-2026-55640 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-8vh3-g2qg-2h2c, PYSEC-2026-3866.