Securing your SSH Server

Secure your SSH server to prevent potential security risks.

Introduction

Keeping your data secure is critically important. Aspera strongly encourages you to take additional steps in setting up and configuring your SSH server so that it is protected against common attacks. Most automated robots will try to log into your SSH server on Port 22 as Administrator, with various brute force and dictionary combinations in order to gain access to your data. Furthermore, automated robots can put enormous loads on your server as they perform thousands of retries to break into your system. This topic addresses steps to take in securing your SSH server against potential threats, including changing the default port for SSH connections from TCP/22 to TCP/33001.

Why Change to TCP/33001?

It is well known that SSH servers listen for incoming connections on TCP Port 22. As such, Port 22 is subject to countless, unauthorized login attempts by hackers who are attempting to access unsecured servers. A highly effective deterrent is to simply turn off Port 22 and run the service on a seemingly random port above 1024 (and up to 65535). To standardize the port for use in Aspera transfers, we recommend using TCP/33001.

Please note that your Aspera transfer product ships with OpenSSH listening on both TCP/22 and TCP/33001. As such, Aspera recommends only exposing TCP/33001 through your organization's firewall and disabling TCP/22.

Note: Remote Aspera application connections attempt to establish an SSH connection using the default port 33001. However, if the connection fails, the application attempts the connection using port 22.

The following explains how to change the SSH port to 33001 and take additional steps for securing your SSH server. The steps all require Administrator access privileges.

Locate and open your system's SSH configuration file

Open your SSH configuration file with a text editor. You will find this file in the following system location:

OS Version	Path
32-bit Windows	`C:\Program Files\Aspera\Enterprise Server\etc\sshd_config`
64-bit Windows	`C:\Program Files (x86)\Aspera\Enterprise Server\etc\sshd_config`

Add new SSH port
Note: Before changing the default port for SSH connections, please verify with your network administrators that TCP/33001 is open.

The OpenSSH suite included in the installer uses TCP/22 and TCP/33001 as the default ports for SSH connections. Aspera recommends disabling TCP/22 to prevent security breaches of your SSH server.

Note: When changing the SSH port, you must also update the SshPort value in the <WEB...> section of aspera.conf. Please refer to Configuring your Web UI Settings for details.

Once your client users have been notified of the port change (from TCP/22 to TCP/33001), you can disable Port 22 in your sshd_config file. To disable TCP/22 and use only TCP/33001, comment-out Port 22 in your sshd_config file.
```
...
#Port 22
Port 33001
...
```
Note: Aspera recognizes that disabling the default SSH connection port (TCP/22) may affect your client users. When you change the port, ensure that you advise your users on configuring the new port number. Basic instructions for specifying the SSH port for fasp file transfers can be found below. To change the SSH port for Aspera Client, click Connections on the main window, and select the entry for your computer. Under the Connection tab, click Show Advanced Settings and enter the SSH port number in the SSH Port (TCP) field.

To make an impromptu connection to TCP/33001 during an ascp session, specify the SSH port (33001) with the -P (capital P) flag. Please note that this command does not alter ascp or your SSH server's configuration.
```
> ascp -P 33001 ...
```
Disable non-admin SSH tunneling
Note: The instructions below assume that OpenSSH 4.4 or newer is installed on your system. For OpenSSH 4.4 and newer versions, the "Match" directive allows some configuration options to be selectively overridden if specific criteria (based on user, group, hostname and/or address) are met. If you are running an OpenSSH version older than 4.4, the "Match" directive will not be available and Aspera recommends updating to the latest version.

In OpenSSH versions 4.4 and newer, disable SSH tunneling to avoid potential attacks; thereby only allowing tunneling from Administrator group users. To disable non-admin SSH tunneling, open your SSH Server configuration file, sshd_config, with a text editor.

Add the following lines to the end of the file (or modify them if they already exist):
```
...
AllowTcpForwarding no
Match Group Administrators
AllowTcpForwarding yes
```
Depending on your sshd_config file, you may have additional instances of AllowTCPForwarding that are set to the default Yes. Please review your sshd_config file for other instances and disable as appropriate.
Update authentication methods
Public key authentication can prevent brute force SSH attacks if all password-based authentication methods are disabled. For this reason, Aspera recommends disabling password authentication in the sshd_config file and enabling private/public key authentication. To do so, add or uncomment PubkeyAuthentication yes and comment out PasswordAuthentication yes.
```
...
PubkeyAuthentication yes
#PasswordAuthentication yes
PasswordAuthentication no
...
```
Note: If you choose leave password authentication enabled, be sure PermitEmptyPasswords is set to "no".
```
PermitEmptyPasswords no
```
Restart the SSH server to apply new settings

When you have finished updating your SSH server configuration, you must restart the server to apply your new settings. Restarting your SSH server will not impact currently connected users. To restart your SSH Server, go to Control Panel > Administrative Tools > Services. Locate the OpenSSH Service and click Restart.

Restrict user access

Restricting user access is a critical component of securing your server. When a user's docroot is empty (i.e. blank), that user has full access to your server's directories and files. To restrict the user, you must set a non-empty docroot, which automatically changes the user's shell to aspshell (Aspera shell). You can do so from the product GUI by going to Configuration > Users > Docroot > Absolute Path. Input a path in the blank field and ensure that Override is checked.

Set up the docroot.

Once you have set the user's docroot, you can further restrict access by disabling read, write and/or browse. You may do so via the product GUI (as shown in the screenshot above).

Field	Description	Values
Absolute Path	The area of the file system (i.e. path) that is accessible to the Aspera user. The default empty value gives a user access to the entire file system.	Path or blank
Read Allowed	Setting this to `true` allows users to transfer from the designated area of the file system as specified by the Absolute Path value.	`true` `false`
Write Allowed	Setting this to `true` allows users to transfer to the designated area of the file system as specified by the Absolute Path value.	`true` `false`
Browse Allowed	Setting this to `true` allows users to browse the directory.	`true` `false`

Review your logs periodically for attacks
Aspera recommends reviewing your SSH log periodically for signs of a potential attack. Launch Control Panel > Administrative Tools > Event Viewer. To see only SSH Server events, select View > Filter... to bring up the filter settings. In Application Properties > Filter tab, select sshd in the Event source menu to display only SSH Server events. You may also apply other conditions when needed.

With a filter applied, you can review the logs in the Event Viewer main window, or select Action > Save Log File As... to export a log file using .txt or .csv format.

Look for invalid users in the log, especially a series of login attempts with common user names from the same address, usually in alphabetical order. For example:
```
...
Mar 10 18:48:02 sku sshd[1496]: Failed password for invalid user alex from 1.2.3.4 port 1585 ssh2
...
Mar 14 23:25:52 sku sshd[1496]: Failed password for invalid user alice from 1.2.3.4 port 1585 ssh2
...
```
If you have identified attacks:
- Double-check the SSH security settings in this topic.
- Report attacker to your ISP's abuse email (e.g. abuse@your-isp).
Set up transfer server authentication
For transfers mediated by a web application, the client browser sets up the context for the transfer using an HTTPS connection to the server, and then delegates the transfer to the Aspera fasp engine. The fasp engine then connects to the transfer server. In so doing, it needs to ensure the server's authenticity in order to protect the client against server impersonation and man-in-the-middle (MITM) attacks.

To verify the authenticity of the transfer server, the web app passes the client a trusted SSH host key fingerprint of the transfer server. When connecting to the transfer server, the client confirms the server's authenticity by comparing the server's fingerprint with the trusted fingerprint.

To configure transfer server authentication, open the transfer server's aspera.conf file:
```
C:\Program Files[ (x86)]\Aspera\Enterprise Server\etc\aspera.conf
```
Locate the <server> section, and add the <ssh_host_key_fingerprint> option.
- <ssh_host_key_fingerprint>
```
<ssh_host_key_fingerprint>fingerprint</ssh_host_key_fingerprint>
```
  To retrieve the SSH fingerprint, locate the transfers server's public or private key, and run the following command on a Linux, Mac, or other UNIX computer:
```
# cd /etc/ssh
# cat ssh_host_rsa_key.pub | cut -d' ' -f2 | base64 -d | sha1sum | cut -d' ' -f1
```
  The following is an example SSH fingerprint:
```
43:51:43:a1:b5:fc:8b:b7:0a:3a:a9:b1:0f:66:73:a8
```
  By convention, Aspera uses a hex string without the colons ( : ). For example:
```
435143a1b5fc8bb70a3aa9b10f6673a8
```
  The aspera.conf setting for this key would then be as follows:
```
<ssh_host_key_fingerprint>435143a1b5fc8bb70a3aa9b10f6673a8
</ssh_host_key_fingerprint>
```
After modifying aspera.conf, be sure to restart the node service by running asperanoded:
```
> sc stop asperanoded
> sc start asperanoded
```