GHSA-f4vv-55c2-5789: LightRAG is Vulnerable to Authentication Bypass: hardcoded DEFAULT_TOKEN_SECRET and public /auth-status defeat LIGHTRAG_API_KEY protection

Summary When LightRAG is deployed with LIGHTRAG_API_KEY set but AUTH_ACCOUNTS unset (an officially documented "API-Key authentication" mode), the X-API-Key protection can be bypassed by any remote unauthenticated attacker. The bypass does not require network contact with the victim server — an attacker can mint a valid guest JWT offline using the hardcoded DEFAULT_TOKEN_SECRET committed in the repository and then call any endpoint guarded by Depends(combined_auth), including destructive operations such as DELETE /documents, POST /documents/upload, /documents/clear_cache, and POST /query. This is distinct from the previously-fixed GHSA-mcww-4hxq-hfr3 / CVE-2026-30762, which only covered the AUTH_ACCOUNTS-configured case. The API-Key-only deployment profile is still fully exploitable on current main (commit 157c331, v1.4.15). Root cause Three independent issues combine: 1. lightrag/api/config.py:54 ships a hardcoded default JWT secret: DEFAULT_TOKEN_SECRET="lightr...key!" 2. lightrag/api/auth.py:28-38 falls back to DEFAULT_TOKEN_SECRET with only a warning log when AUTH_ACCOUNTS is not configured. The fix for CVE-2026-30762 only raises when AUTH_ACCOUNTS is set, so the API-key-only path is silently vulnerable. 3. lightrag/api/lightrag_server.py:1140-1186 exposes GET /auth-status and POST /login without any authentication dependency. In the API-key-only configuration, auth_handler.accounts is empty, and both endpoints unconditionally mint and return a signed guest JWT. 4. lightrag/api/utils_api.py:214-216 inside combined_dependency short-circuits authorization on any valid guest token when auth_configured is false, before reaching the X-API-Key check on line 237: if not auth_configured and token_info.get("role") == "guest": return Proof of concept (offline-minted token, zero server contact) Tested against a clean install of commit 157c331 running locally with only LIGHTRAG_API_KEY=super-...ass configured. $ python3 - <<'PY' import jwt from datetime import dat