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

// 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")
}

// engine/config.go:388-391  (1.17.6, finalConfiguration)
if ac.eventAddr == "" {
    ac.eventAddr = utils.JoinHostPort(host, ac.defaultEventColonPort)
}

Result tabulated:

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)

# 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.

// 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:

// 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.

References

• GHSA-gj84-924c-48fx