CVE-2026-55641: 9router: Unauthenticated `/v1` proxy access via `Host`-header spoofing → open AI relay + SSRF

9router’s request guard decides a request is “local” (and therefore exempt from API-key auth on the /v1 LLM proxy) by reading the client-controlled Host header. Because 9router binds 0.0.0.0 by default (and the CLI misleadingly prints “localhost”), a remote, unauthenticated attacker who can reach the port can send Host: localhost to be treated as local and obtain /v1 proxy access with no API key, no CLI token, and no dashboard login. In the default configuration (requireApiKey is absent from DEFAULT_SETTINGS, so the handler-side key check is skipped), this yields:

• Open AI relay — the proxy forwards the attacker’s requests to AI providers using the victim’s stored paid API keys (cost/quota theft, prompt-based data exfiltration through the victim’s accounts).

• Unauthenticated SSRF — /v1/search with the built-in noAuth searxng provider takes its outbound fetch URL from the request body (provider_options.baseUrl), so the attacker drives a server-side fetch to any internal/cloud-metadata host and gets the JSON response reflected back.

• Affected: 9router <= 0.4.80 (current), src/dashboardGuard.js (isLocalRequest), src/sse/handlers/{chat,search}.js, src/lib/db/repos/settingsRepo.js, cli/cli.js.

• Distinct from the existing advisories GHSA-fhh6-4qxv-rpqj (MCP-plugin RCE, patched) and GHSA-xrrh-p7f2-27vm (legacy <0.3.75 authz bypass).