Authentication Bypass in Nextcloud MCP Server
CVE-2026-55640

9.1CRITICAL

Key Information:

Vendor

Cbcoutinho

Status
Nextcloud-mcp-server
Vendor
CVE Published:
25 August 2026

What is CVE-2026-55640?

The Nextcloud MCP Server lacks necessary authentication for the POST /webhooks/nextcloud endpoint prior to version 0.117.2. The default configuration results in the WEBHOOK_SECRET being unset, allowing unauthenticated requests to be processed. This vulnerability permits an attacker to exploit the handle_nextcloud_webhook() function, enabling them to delete or manipulate vector embeddings for any user without an authentication check. Consequently, attackers can destroy the semantic search index by forging deletion events. A fix is available in version 0.117.2.

Affected Version(s)

nextcloud-mcp-server < 0.117.2

References

https://github.com/cbcoutinho/nextcloud-mcp-server/securi...
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https://github.com/cbcoutinho/nextcloud-mcp-server/commit...
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https://github.com/cbcoutinho/nextcloud-mcp-server/tree/v...
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CVSS V3.1

Score:
9.1
Severity:
CRITICAL
Confidentiality:
None
Integrity:
High
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

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