The flaw in 9router allows a remote attacker to overwrite the Host header to localhost, causing the application to treat the request as local and skip API‑key authentication. As a result, the attacker can use the /v1 LLM proxy endpoint to forward requests to upstream AI providers with stored credentials, effectively gaining unauthenticated access to those services. The vulnerability which, when combined with a malicious provider_options.baseUrl value such as a cloud‑metadata endpoint, can trigger server‑side requests to internal hosts, creating a server‑side request forgery (SSRF) vector. These behaviors correspond to the CWE identifiers listed: improper input validation, authentication bypass, and SSRF.

The software affected is decolua's 9router product. Versions released prior to 0.5.2 are vulnerable under the default configuration, where the /v1 proxy route processes the Host header without proper validation. Any instance running those earlier releases, especially with the default token‑saver and AI routing configuration, is at risk.

The CVSS rating of 8.2 signals a high severity condition. An attacker can simply craft an HTTP request with a forged Host header; no authentication is required. While the EPSS score is <1%, the lack of a KEV listing does not diminish the risk, as the attack path is straightforward and the impact includes a high potential from the Internet.