The vulnerability allows an attacker to register an OAuth client with any redirect_uri because the /mcp-oauth/register endpoint accepts registrations unauthenticated. When a user later denies the OAuth consent, the application redirects to the previously registered redirect_uri without validating it. This permits an open redirect to an attacker‑controlled site, enabling phishing or social engineering attacks against users who click "Deny" on the consent dialog. The weakness is a classic open‑redirect flaw (CWE‑601).

The flaw exists in n8n-io:n8n before the following releases: 1.123.32, 2.17.4, and 2.18.1. Users running any earlier version of n8n are affected.

The CVSS score of 5.1 indicates a moderate severity. The EPSS score is not available, so the current exploitation probability is unknown. The vulnerability is not listed in CISA’s KEV catalog. An attacker can exploit the flaw by creating an OAuth client with a malicious redirect_uri, sending a phishing link to a victim, and tricking the victim into clicking "Deny" on the consent form, after which the victim is silently redirected to the attacker‑controlled URL. The attack does not require any victim‑side software installation and can be performed purely through a web interaction.