CVE-2026-42236: n8n: Unauthenticated Denial of Service via MCP Client Registration
Impact The MCP OAuth client registration endpoint accepted unauthenticated requests and stored client data without adequate resource controls. An unauthenticated remote attacker could exhaust server memory resources by sending large registration payloads, rendering the n8n instance unavailable. The MCP enable/disable toggle gates MCP access but did not restrict client registrations, meaning the endpoint is reachable regardless of whether MCP access is enabled on the instance.
The patches address the unbound registration with an upper bound of registered clients and disabling creation when MCP is disabled on the instance. Mean to restrict the payload size of requests already exist and can be used to control additional risks.
Patches The issue has been fixed in n8n versions 1.123.32, 2.17.4, and 2.18.1. Users should upgrade to one of these versions or later to remediate the vulnerability.
Workarounds If upgrading is not immediately possible, administrators should consider the following temporary mitigations: - Restrict network access to the n8n instance to prevent requests from untrusted sources. - Reduce the maximum accepted payload size by lowering the N8NPAYLOADSIZEMAX environment variable from its default value.
These workarounds do not fully remediate the risk and should only be used as short-term mitigation measures.
Other sources
n8n is an open source workflow automation platform. Prior to versions 1.123.32, 2.17.4, and 2.18.1, the MCP OAuth client registration endpoint accepted unauthenticated requests and stored client data without adequate resource controls. An unauthenticated remote attacker could exhaust server memory resources by sending large registration payloads, rendering the n8n instance unavailable. The MCP enable/disable toggle gates MCP access but did not restrict client registrations, meaning the endpoint is reachable regardless of whether MCP access is enabled on the instance. This issue has been patched in versions 1.123.32, 2.17.4, and 2.18.1.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-42236?
CVE-2026-42236 has a high severity rating due to its potential to render the n8n instance unavailable through resource exhaustion.
How do I fix CVE-2026-42236?
To mitigate CVE-2026-42236, upgrade to n8n version 2.17.4, 2.18.1, or 1.123.32.
What kind of attack does CVE-2026-42236 enable?
CVE-2026-42236 allows unauthenticated remote attackers to exhaust server memory resources by sending large payloads.
Which versions of n8n are affected by CVE-2026-42236?
CVE-2026-42236 affects n8n versions from 2.0.0 up to 2.17.4 and versions from 2.18.0 up to 2.18.1 and all versions up to 1.123.32.
What is the impact of CVE-2026-42236 on n8n?
The impact of CVE-2026-42236 is that it can lead to denial of service by consuming server resources.