Executive Summary

This vulnerability exists in the n8n workflow automation platform prior to versions 1.123.32, 2.17.4, and 2.18.1. The /mcp-oauth/register endpoint allowed OAuth client registrations without requiring authentication, which meant that arbitrary redirect_uri values could be registered by anyone.

When a user denies the MCP OAuth consent dialog, the system's handleDeny handler redirects the user to the registered redirect_uri without validating it. This behavior enables an open redirect to an attacker-controlled URL.

An attacker can exploit this by crafting a phishing link and sending it to a victim. If the victim clicks "Deny" on the consent page, they are silently redirected to an external malicious site controlled by the attacker.

This issue has been fixed in versions 1.123.32, 2.17.4, and 2.18.1 of n8n.

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Impact Analysis

This vulnerability can be exploited to perform phishing attacks by redirecting users to malicious external websites without their knowledge after they deny an OAuth consent.

Users may be tricked into visiting attacker-controlled sites, potentially leading to credential theft, malware installation, or other malicious activities.

Because the redirect occurs silently upon denial of consent, users might not suspect the redirection is malicious, increasing the risk of successful attacks.

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Mitigation Strategies

To mitigate this vulnerability, upgrade n8n to version 1.123.32, 2.17.4, or 2.18.1 or later, where the issue has been patched.

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