CVE-2026-66005 describes a CORS misconfiguration vulnerability in the Jan local API server affecting versions up to 0.8.4. The server replaces user-configured trusted hosts with a wildcard that reflects arbitrary origins along with credentials, enabling network-adjacent attackers or those using DNS rebinding to bypass trusted host restrictions. This allows unauthorized access to the OpenAI-compatible API without authentication, enabling actions such as model enumeration, inference execution, invocation of MCP tools, and reading of cross-origin responses. The vulnerability is addressed in a fix committed as 3e1c1e7.

An attacker on the local network or leveraging DNS rebinding can bypass trusted host restrictions and access the unauthenticated API. This can lead to unauthorized inference requests, model enumeration, execution of MCP tools, and exposure of cross-origin API responses. The vulnerability impacts confidentiality and integrity of the API interactions but does not require prior authentication or privileges.

A fix is available in the commit identified as 3e1c1e7 that corrects the CORS misconfiguration. Users should upgrade to a version that includes this fix. Since the vendor advisory does not explicitly state the availability of an official patch version, verify the vendor repository or release notes for the fixed version. Until patched, restrict network access to the local API server to trusted hosts only and consider network-level controls to prevent DNS rebinding attacks.