9router treats requests that originate from the loopback interface as trusted and does not require an API key for its /v1 API surface. When a reverse proxy forwards public traffic to the backend over 127.0.0.1, the request is misclassified as local, allowing an attacker to reach any /v1 endpoint without authentication. The exposed APIs provide model information and, if provider credentials are enabled, proxy access to upstream services, potentially exposing sensitive credentials. This flaw is an access‑control bypass (CWE‑287, CWE‑290, CWE‑306) coupled with improper origin verification (CWE‑441).

The vulnerable software is the open‑source 9router project by decolua. All releases prior to 0.5.2 are affected; any deployment that allows public traffic to be reverse‑proxied through 127.0.0.1 is at risk. The vulnerability exists in every instance of 9router running version 0.4.x or earlier.

The CVSS score of 8.3 indicates high severity while the EPSS score of less than 1% suggests a low exploitation probability. The vulnerability is not listed in the CISA KEV catalog. An attacker can exploit it remotely by configuring a reverse proxy that forwards external traffic to the loopback interface, enabling unauthenticated access to the /v1 API and potentially revealing upstream provider credentials, enabling further compromise of external services.