CVE-2026-46430: Algernon Auto-refresh SSE Event Server Binds Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gj84-924c-48fx",
  "modified" : "2026-05-20T15:33:56Z",
  "published" : "2026-05-20T15:33:56Z",
  "aliases" : [ "CVE-2026-46430" ],
  "summary" : "Algernon: Auto-refresh SSE event server binds to all interfaces by default on Linux/macOS",
  "details" : "### Summary\n\nThe 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.\n\nThe 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.\n\nThis 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.\n\n### Details\n\n#### Root cause — platform-dependent `host` default in `handleFlags`\n\n```go\n// engine/flags.go:39-46  (1.17.6)\nhost := \"\"\nif runtime.GOOS == \"windows\" {\n    host = \"localhost\"\n    // Default Bolt database file\n    ac.defaultBoltFilename = filepath.Join(serverTempDir, \"algernon.db\")\n    // Default log file\n    ac.defaultLogFile = filepath.Join(serverTempDir, \"algernon.log\")\n}\n```\n\n```go\n// engine/config.go:388-391  (1.17.6, finalConfiguration)\nif ac.eventAddr == \"\" {\n    ac.eventAddr = utils.JoinHostPort(host, ac.defaultEventColonPort)\n}\n```\n\nResult tabulated:\n\n| Platform | `host` | `eventAddr` after `JoinHostPort` | Effective bind |\n|---|---|---|---|\n| Linux | `\"\"` | `\":5553\"` | `0.0.0.0:5553` (all interfaces) |\n| macOS | `\"\"` | `\":5553\"` | `0.0.0.0:5553` (all interfaces) |\n| Windows | `\"localhost\"` | `\"localhost:5553\"` | `127.0.0.1:5553` (loopback) |\n\nThe 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.\n\n#### Why this is an independent finding\n\nThe 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: *`.\n\n### PoC (against 1.17.6 on Linux/macOS)\n\n```bash\n# Operator's laptop on a hotel/cafe/office WiFi:\nalgernon -a /path/to/project\n# => SSE listener bound to 0.0.0.0:5553\n\n# Any peer on the same subnet:\n$ curl -sN http://<dev-laptop-ip>:5553/sse\nid: 0\ndata: /path/to/project/secret-notes.md\n\nid: 1\ndata: /path/to/project/.env.local\n```\n\nNo interaction from the developer is required. The peer needs network reach and nothing else.\n\n### Impact\n\n- **Confidentiality:** medium. LAN-bounded continuous information disclosure of filenames and edit timing.\n- **Integrity:** none.\n- **Availability:** none directly.\n\nThe 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.\n\n### Suggestions to fix\n\n**Primary fix — pick `localhost` as the SSE default on every platform.**\n\n```go\n// engine/flags.go -- platform-independent default for the event listener\n// (keep the existing platform split for the WEB server if desired, but\n// not for the event server)\nhost := \"localhost\"\n```\n\nOr, more surgically:\n\n```go\n// engine/config.go -- finalConfiguration\nif ac.eventAddr == \"\" {\n    ac.eventAddr = utils.JoinHostPort(\"localhost\", ac.defaultEventColonPort)\n}\n```\n\nAn operator who genuinely wants LAN-reachable SSE can pass `--eventserver 0.0.0.0:5553` explicitly and accept the consequences.\n\n**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.\n\n### Live verification\n\nAudit-host bind check (Windows 10):\n\n```\n$ netstat -an | findstr 5553\n  TCP    127.0.0.1:5553         0.0.0.0:0              LISTENING\n```\n\nConfirms 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`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/xyproto/algernon"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.17.7"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.17.6"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/xyproto/algernon/security/advisories/GHSA-gj84-924c-48fx"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/xyproto/algernon"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1188", "CWE-668" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-20T15:33:56Z",
    "nvd_published_at" : null
  }
}