GHSA-gj84-924c-48fx: Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS
SummaryThe 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.DetailsRoot cause — platform-dependent `host` default in `handleFlags`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 findingThe 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)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.**Or, more surgically: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 verificationAudit-host bind check (Windows 10):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`.
CVSS v3.1: 4.3