Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS

### Summary The SSE event server bound to `0.0.0.0:5553` on Linux/macOS by default because the platform-dependent host default in `engine/flags.go:39-46` set `host = ""` for non-Windows, and `utils.JoinHostPort("", ":5553")` resolves to `":5553"` — a Go `http.Server.Addr` of `":5553"` listens on every interface. On Windows the same code chose `"localhost"`, binding loopback only. The result was a platform split where the OS Algernon's dev workflow is most often used on (Linux/macOS) got the network-exposed default, and only Windows users got the loopback-safe one. A LAN peer with no developer interaction could connect to `<dev-laptop-ip>:5553` and read the file-change stream. This advisory covers the bind-address default in isolation. The fix is independent of authentication (#2a) and CORS (#2b) — switching the default to loopback can be done without touching either. ### Details #### Root cause — platform-dependent `host` default in `handleFlags` ```go // engine/flags.go:39-46 (1.17.6) host := "" if runtime.GOOS == "windows" { host = "localhost" // Default Bolt database file ac.defaultBoltFilename = filepath.Join(serverTempDir, "algernon.db") // Default log file ac.defaultLogFile = filepath.Join(serverTempDir, "algernon.log") } ``` ```go // engine/config.go:388-391 (1.17.6, finalConfiguration) if ac.eventAddr == "" { ac.eventAddr = utils.JoinHostPort(host, ac.defaultEventColonPort) } ``` Result tabulated: | Platform | `host` | `eventAddr` after `JoinHostPort` | Effective bind | |---|---|---|---| | Linux | `""` | `":5553"` | `0.0.0.0:5553` (all interfaces) | | macOS | `""` | `":5553"` | `0.0.0.0:5553` (all interfaces) | | Windows | `"localhost"` | `"localhost:5553"` | `127.0.0.1:5553` (loopback) | The same `host` value also governs the main web server bind, so the platform split affects both ports. The web-server bind on Linux/macOS is a separate (defensible) design decision — a server intended to be reachable; the SSE port is *not* such a service and inherited the same default by accident. #### Why this is an independent finding The fix is a single line: change the default `host` value, or change the `eventAddr` default specifically, to `"localhost"` regardless of platform. No change to authentication or CORS is required to close the network-reach half of the original bundled advisory. A LAN peer can no longer connect — the listener is unreachable from another host — even if the SSE handler still has no authentication and still returns `Allow-Origin: *`. ### PoC (against 1.17.6 on Linux/macOS) ```bash # Operator's laptop on a hotel/cafe/office WiFi: algernon -a /path/to/project # => SSE listener bound to 0.0.0.0:5553 # Any peer on the same subnet: $ curl -sN http://<dev-laptop-ip>:5553/sse id: 0 data: /path/to/project/secret-notes.md id: 1 data: /path/to/project/.env.local ``` No interaction from the developer is required. The peer needs network reach and nothing else. ### Impact - **Confidentiality:** medium. LAN-bounded continuous information disclosure of filenames and edit timing. - **Integrity:** none. - **Availability:** none directly. The CVSS vector uses `AV:A` (adjacent network) to model the LAN-only reach. The vector for a misconfigured deployment behind a NAT-less or routed network would shift to `AV:N` and rise to 5.3. ### Suggestions to fix **Primary fix — pick `localhost` as the SSE default on every platform.** ```go // engine/flags.go -- platform-independent default for the event listener // (keep the existing platform split for the WEB server if desired, but // not for the event server) host := "localhost" ``` Or, more surgically: ```go // engine/config.go -- finalConfiguration if ac.eventAddr == "" { ac.eventAddr = utils.JoinHostPort("localhost", ac.defaultEventColonPort) } ``` An operator who genuinely wants LAN-reachable SSE can pass `--eventserver 0.0.0.0:5553` explicitly and accept the consequences. **Stronger fix — eliminate the second listener entirely.** Mount the SSE handler on the main mux at `/sse`. The bind address is then whatever the main server uses; there is no second listener and therefore no second bind-address default to get wrong. ### Live verification Audit-host bind check (Windows 10): ``` $ netstat -an | findstr 5553 TCP 127.0.0.1:5553 0.0.0.0:0 LISTENING ``` Confirms the Windows default is `localhost`. The Linux/macOS bind to `0.0.0.0:5553` is documented in the code path above; it was not exercised on the audit machine because the audit host was Windows. A maintainer reproducing on a Linux host would see `0.0.0.0:5553 LISTENING` from `ss -tlnp`.