Unauthenticated Open Redirect, Arbitrary HTTP Response Header Injection, Missing CSRF, and Invisible-Mode Bypass in goshs `/?redirect` endpoint

Summary

### Summary The `GET /?redirect` endpoint in `goshs` v2.0.0-beta.6 performs an HTTP redirect to any attacker-supplied `url=` value and writes any attacker-supplied `header=Name: Value` pair into the response, without scheme/host validation, without a header-name allow-list, without authentication in the default deployment, and without the `checkCSRF()` guard that GHSA-jrq5-hg6x-j6g3 added to the other state-changing GET routes (`?mkdir`, `?delete`). The same dispatcher also lacks an `fs.Invisible` branch, so the endpoint stays responsive in `-I` stealth mode and reliably fingerprints an "invisible" goshs deployment with a single request.

### Details `httpserver/handler.go:222-228` — the dispatcher gates `?redirect` only with `denyForTokenAccess` (which only blocks share-token callers). It does not check `fs.Invisible` and does not call `checkCSRF`:

```go if _, ok := req.URL.Query()["redirect"]; ok { if denyForTokenAccess(w, req) { return true } fs.handleRedirect(w, req) return true } ```

`httpserver/handler.go:753-787` — `handleRedirect`:

```go func (fs *FileServer) handleRedirect(w http.ResponseWriter, req *http.Request) { q := req.URL.Query()

target := q.Get("url") // (1) no scheme/host validation if target == "" { /* 400 */ }

status := http.StatusFound if s := q.Get("status"); s != "" { // (2) only constrained to 3xx code, err := strconv.Atoi(s) if err != nil || code < 300 || code > 399 { /* 400 */ } status = code }

for _, h := range q["header"] { // (3) arbitrary header set parts := strings.SplitN(h, ": ", 2) if len(parts) != 2 || strings.TrimSpace(parts[0]) == "" { /* 400 */ } w.Header().Set(strings.TrimSpace(parts[0]), parts[1]) }

http.Redirect(w, req, target, status) // (4) attacker Location

body := fs.emitCollabEvent(req, status) logger.LogRequest(req, status, fs.Verbose, fs.Webhook, body) } ```

`httpserver/server.go:85-100` — `BasicAuthMiddleware` is registered only when `fs.User != "" || fs.Pass != ""`; the default `goshs` invocation has neither, so `?redirect` is open to anyone on the network._Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer._

### PoC [poc.zip](https://github.com/user-attachments/files/26673401/poc.zip) Please extract the uploaded compressed file before proceeding

1. docker build -t goshs-poc . 2. sh poc.sh

<img width="1379" height="197" alt="스크린샷 2026-04-13 오후 8 04 20" src="https://github.com/user-attachments/assets/a557846f-47c7-4640-9fc5-34aa099d1a57" />

### Impact - Cross-subdomain session fixation — `Set-Cookie: …; Domain=.corp.com` lands a fixed session on every sibling app on the parent domain. - TLS downgrade — `Strict-Transport-Security: max-age=0` invalidates prior HSTS state for the origin, enabling MITM on subsequent visits.