Securing your SSH Server

Secure your SSH server to prevent security risks.

Introduction

Keeping your data secure is critically important. Aspera strongly recommends taking additional steps to set up and configure your SSH server so that it's protected against common attacks. Most automated robots try to log into your SSH server on Port 22 as root 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 secure 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.

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 to secure your SSH server. The steps all require root access privileges.

  1. Locate and open your system's SSH configuration file.

    The SSH configuration file can be found in the following location:

    /etc/ssh/sshd_config

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

    The OpenSSH suite included in the installer uses TCP/22 as the default port for SSH connections. Aspera recommends opening TCP/33001 and 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. For details, see Configuring your Web UI Settings.

    To enable TCP/33001 while your organization is migrating from TCP/22, open port 33001 from your sshd_config file (where SSHD is listening on both ports). As demonstrated by this exercise, SSHD is capable of listening on multiple ports.

    ...
    Port 22
    Port 33001
    ...

    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 are shown below. To change the SSH port for Desktop 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.

    Client specifying your computer's SSH Port.

    To make an impromptu connection to TCP/33001 during an ascp session, specify the SSH port (33001) with the -P (capital P) flag. Note that this command does not alter ascp or your SSH server's configuration.

    $ ascp -P 33001 ...
  3. 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 is not available; 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 root 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 root
    AllowTcpForwarding yes

    Depending on your sshd_config file, you may have additional instances of AllowTCPForwarding that are set to the default Yes. Review your sshd_config file for other instances and disable as appropriate.

    Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. Review your user and file permissions, and see the instructions below on modifying shell access.

  4. 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 to advise account creators to use strong passwords. Be sure also to set PermitEmptyPasswords to "no".
    PermitEmptyPasswords no
  5. Disable Root Login

    OpenSSH defaults to allowing root logins; however disabling root access helps you to maintain a more secure server. Aspera recommends commenting out PermitRootLogin yes in the sshd_config file and adding PermitRootLogin No.

    ...
    #PermitRootLogin yes
    PermitRootLogin no
    ...

    Administrators can then utilize the su command if root privileges are needed.

  6. Restart the SSH server to apply new settings

    When you have finished updating your SSH server configuration, you must restart or reload the SSH service to apply your new settings. Note that restarting or reloading SSH does not impact currently connected users.

    To restart or reload your SSH server, run the following commands:

    OS Version Instructions
    RedHat (restart)
    $ sudo service sshd restart
    RedHat (reload)
    $ sudo service sshd reload
    Debian (restart)
    $ sudo /etc/init.d/ssh restart
    Debian (reload)
    $ sudo /etc/init.d/ssh reload
  7. Restrict user access

    Restricting user access is a critical component of securing your server. By default, all user accounts are allowed to browse and read all files on the server. To limit a user's access to a portion of the system, set the account's shell to the Aspera secured shell (aspshell) and create a document root (docroot) for that user. The aspshell permits only the following operations:

    • Run Aspera uploads and downloads to or from this computer.
    • Establish connections in the application and browse, create, delete, rename, or list contents.

    The following instructions describe how to change a user account so that it uses the aspshell. Keep in mind that this is an example, and there may be other ways to do so for your system. Open the following file with a text editor:

    /etc/passwd

    Add or replace the user's shell with /bin/aspshell. For example, to apply aspshell to the user aspera_user_1, use the following settings in the passwd file:

    ...
    aspera_user_1:x:501:501:...:/home/aspera_user_1:/bin/aspshell
    ...

    You can also restrict a user's file access by setting a document root (docroot). You can set a user's docroot by editing the aspera.conf file (/opt/aspera/etc/aspera.conf). The following template displays access options:

    <file_system>
       <access>
    	  <paths>
    		 <path>
    			<absolute>/sandbox/aspera_user_1</absolute>      <!-- Absolute Path -->
    			<read_allowed>true</read_allowed>          <!-- Read Allowed -->
    			<write_allowed>true</write_allowed>        <!-- Write Allowed -->
    			<dir_allowed>true</dir_allowed>            <!-- Browse Allowed -->
    		 </path>
    	  </paths>
       </access>
    ...
    </file_system>

    Once you've set the user's shell and docroot, you can further restrict access by disabling read, write, and/or browse using <path> settings in aspera.conf, as shown in the example above.

    Field Description Values
    Absolute Path The area of the file system (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

  8. Run the asp-check tool to check for potential user-security issues

    The asp-check tool performs the following secure checks:

    • Searches for full-access users and reports how many exist on the system. Note that the existence of full-access users does not necessarily indicate that your system is vulnerable; however, it is being brought to the attention of the system administrator to ensure that the existence of full-access users is intentional.
    • Searches for restricted users and potential misconfigurations, including incorrect login shell (one that is not restricted via aspshell); SSH tunnel access (which can be used to work around the restricted shell); and docroot settings that allow the users to access the home directory.

    Note: A docroot setting that allows access to the home directory does not necessarily indicate that your system is vulnerable; however, a user with this docroot can download or upload keys in .ssh, as well as upload .login scripts. These capabilities may be used to circumvent the intended, restricted-nature of the user. Aspera highly recommends setting the docroot under the user's home folder (such as /home/jane/data) or in an alternate location (for example, /data).

    To run the asp-check tool, run the following on the command line:

    $ sudo /opt/aspera/bin/asp-check.sh

    Search results are displayed as in the following example. If potential issues are identified, review your users' settings before proceeding.

    Users with full access: 22 (not considered insecure)
    Restricted users: 0
    Insecure users: 0
     - no restricted shell (aspshell): 0
     - docroot above home directory: 0
     - ssh tunneling enabled: 0
  9. Review your logs periodically for attacks.

    Aspera recommends reviewing your SSH log periodically for signs of a potential attack. Locate and open your syslog—for example, /var/log/auth.log or /var/log/secure. Depending on your system configuration, syslog's path and file name may vary.

    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 identify attacks, do the following:

    • Double-check the SSH security settings in this topic.
    • Report attackers to your ISP's email address for abuse reports (often abuse@your_isp.com).
  10. Set up transfer server authentication

    For transfers mediated by a web application (such as Faspex, Shares, or Console), 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:

    /opt/aspera/etc/aspera.conf

    Locate the <server> section, and add the <ssh_host_key_fingerprint> or the <ssh_host_key_path> option.

    • <ssh_host_key_fingerprint>

      Use this option to specify the fingerprint itself:

      <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, Isilon, 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>

    • <ssh_host_key_path>

      Use the key-path option to specify the transfer server's public or private key file and its location. The fingerprint is extracted automatically.

       <ssh_host_key_path>key_file</ssh_host_key_path> 

      On most Linux systems, the SSH keys are located in /etc/ssh. On OSX, the SSH keys are located in /etc. The following example uses a Linux server's public RSA key:

      <ssh_host_key_path>/etc/ssh/ssh_host_rsa_key.pub</ssh_host_key_path>

    After modifying aspera.conf, be sure to restart the node service by running asperanoded:

    # /etc/init.d/asperanoded restart