CVE-2026-84360: Nginx UI: OTP secure-session step-up can be bypassed via MCP configuration tools

### Summary An OTP-enabled authenticated user who cannot use the HTTP configuration mutation endpoints without a valid secure session can still mutate Nginx configuration through `/mcp` and `/mcp_message` using only ordinary authentication. ### Details The HTTP configuration mutation surface is explicitly grouped behind `middleware.RequireSecureSession()`. By contrast, the MCP endpoints are exposed with only `middleware.AuthRequired()`. The MCP server exposes configuration mutation tools including `nginx_config_add` and `nginx_config_modify`, so OTP-enabled users gain an alternate path around the intended step-up requirement. Relevant code paths: - [api/config/router.go](/home/kali/Desktop/bounty/nginx-ui/api/config/router.go:14) - [internal/middleware/secure_session.go](/home/kali/Desktop/bounty/nginx-ui/internal/middleware/secure_session.go:52) - [mcp/router.go](/home/kali/Desktop/bounty/nginx-ui/mcp/router.go:9) - [mcp/config/config_add.go](/home/kali/Desktop/bounty/nginx-ui/mcp/config/config_add.go:34) - [mcp/config/config_modify.go](/home/kali/Desktop/bounty/nginx-ui/mcp/config/config_modify.go:55) This is a security-control bypass rather than a missing feature. The product already established that OTP-enabled users should complete a secure session before high-impact configuration mutations are accepted on the HTTP path. ### PoC I verified this locally in an isolated environment with a temporary end-to-end harness that exercised the real `/mcp` and `/mcp_message` routes. What the executed test did: 1. Created a temporary config directory and configured Nginx control commands to harmless local values (`true`) to avoid touching the host. 2. Created a real OTP-enabled user in a temporary SQLite database. 3. Generated a real JWT for that OTP-enabled user. 4. Started a local test server exposing the real `/mcp` and `/mcp_message` routes. 5. Opened an authenticated SSE connection to `/mcp` with the JWT and no `X-Secure-Session-ID`. 6. Initialized the MCP session over `/mcp_message`. 7. Called the real `nginx_config_add` tool over `/mcp_message`. 8. Verified that the tool succeeded and created `mcp-bypass.conf` in the managed config directory without any secure-session step-up. Observed result from the actual local verification: - The MCP session initialized successfully without any `X-Secure-Session-ID`. - The config file was written through the MCP tool path using only JWT authentication. ### Impact OTP-enabled accounts lose the intended step-up protection for high-impact Nginx configuration changes when MCP is enabled. A user who should be blocked until re-authenticating can still create or modify configuration files and trigger reload behavior through MCP.