CVE-2026-65594: n8n: Member-Level Users Can Execute Other Users' MCP Server Trigger Workflows via Missing OAuth Authorization Check

n8n 2.27.0 added an OAuth 2.1 consent and token-issuance flow for MCP Server Trigger nodes set to n8n OAuth2 authentication. That flow does not check whether the signed-in user has access to the workflow named as the OAuth resource. A member can register an OAuth client, approve the consent screen for themselves, and receive a valid token for a workflow owned by someone else.

With the token, the attacker calls the workflow as an MCP tool. They set the tool inputs and read the outputs. The workflow runs in the owner's project, using the owner's stored credentials. If it connects to a mail account, CRM, database or cloud API, the attacker can drive those integrations and read what they return. This breaks user and project isolation.

The runs are logged under the owner's account, and the attacker cannot see them. That helps an attacker stay quiet and means the owner sees activity they did not start.

The precondition is a member account on the instance and at least one active workflow with an MCP Server Trigger using n8n OAuth2. Instances with only trusted users, or no such workflow, are not exposed.

The npm package n8n at version 2.27.0 up to, but not including, 2.29.8, and 2.30.0. The flaw needs the OAuth 2.1 flow that arrived in 2.27.0, so earlier versions lack it. It only applies where at least one active workflow uses an MCP Server Trigger node with n8n OAuth2 authentication.

npm ls n8n

Then list your active workflows and look for MCP Server Trigger nodes whose authentication is n8n OAuth2.

Upgrade to n8n 2.29.8 or 2.30.1, or later.

The advisory's temporary measures, which it says do not fully remove the risk:

• Limit instance access to fully trusted users.
• Audit active workflows that use an MCP Server Trigger with n8n OAuth2 authentication. Switch them to another authentication method, or deactivate them until you patch.

After upgrading, review which OAuth clients exist and remove any you do not recognize.

Look at the executions of every workflow that has an MCP Server Trigger with n8n OAuth2. Runs you did not start, or runs at odd times, are suspect. Remember that attacker-driven runs show up under the owner's account. Then check for OAuth clients registered by member users who have no reason to use MCP, and for consent approvals on workflows those users do not own.

For the integrations those workflows use, check the provider's audit log for actions that match the workflow's tools. Rotate the credentials stored in any workflow that looks abused.

Impact

The OAuth 2.1 consent and token-issuance flow introduced in n8n 2.27.0 does not verify that the authenticated user has access to the workflow referenced as the OAuth resource. A member-level user can register an OAuth client, self-approve consent for another user's n8n OAuth2-protected MCP Server Trigger workflow, and obtain a valid token for it.

The workflow runs in the owner's project context with the owner's stored credentials. The attacker sets the tool inputs and reads the outputs, which may include data from the owner's connected integrations, breaking user and project isolation. The resulting executions appear under the owner's account and are not visible to the attacker.

This issue only affects instances running n8n 2.27.0 or later where at least one active workflow uses an MCP Server Trigger node configured with n8n OAuth2 authentication.

Patches

The issue has been fixed in n8n versions 2.29.8 and 2.30.1. 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 n8n instance access to fully trusted users only.
• Audit active workflows using the MCP Server Trigger with n8n OAuth2 authentication and consider switching to a different authentication method or deactivating them until the patch is applied.

These workarounds do not fully remediate the risk and should only be used as short-term mitigation measures.