Protecting your SFTPPlus configuration against SWEET32

Fri 02 March 2018 | security

Details of attacks on DES (Data Encryption Standard) and Triple DES, Birthday attacks on 64-bit block ciphers were released with the CVE ID of CVE-2016-2183. Read more about the CVE details here).

DES and Triple DES ciphers, used in TLS and SSH protocols and in subsequent relation also used in file transfer products, have a birthday bound of approximately four billion blocks. This makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, as demonstrated by an HTTPS session using Triple DES in CBC mode. This attack is otherwise known as the "SWEET32" attack.

If your SFTPPlus configuration is vulnerable to these attacks, you will need to ensure that you are on the latest SFTPPlus MFT version 3.xx.xx series and use the following custom configurations outlined below.

From SFTPPlus version 3.31.0, you can use the secure cipher option for the ssl_cipher_list and ssh_cipher_list configurations. The special keyword secure contains all the algorithms that we currently consider secure.

From version 3.32.0, the default secure ssl_cipher_list configuration was updated to HIGH:!PSK:!RSP:!eNULL:!aNULL:!RC4:!MD5:!DES:!3DES:!aDH:!kDH:!DSS where HIGH is defined by upstream OpenSSL.

In this way, when updating the OpenSSL library, you will automatically get an update in the list of secure ciphers without the need to update SFTPPlus.

Exclude DES and Triple-DES from the SSL Cipher List

SFTPPlus relies on OS crypto for the SSL/TLS portion. There is no need to run this configuration if your operating system has already deprecated the use of DES/3DES.

To check if your OpenSSL has DES and 3DES ciphers enabled, check the output of openssl ciphers -v 'DES:3DES'

If DES/3DES are still available in OpenSSL, disable them for SFTPPlus by ensuring that the ssl_cipher_list configuration for the service has the value :!DES:!3DES: added in the list. This will exclude the use of DES and Triple DES as indicated by the ! mark.

For more details about this configuration, please refer to the ssl cipher list configuration option here.

Exclude DES and Triple-DES from the SSH Cipher List

In the ssh_cipher_list configuration for the service, add the value :!DES:!3DES: to exclude the use of DES and Triple DES. There is no need to run this configuration if your operating system has already deprecated the use of DES/3DES.

More details about this configuration for the ssh cipher list here.

Only use TLS 1.2 (for services utilizing TLS/SSL)

In the ssl_allowed_methods option for the associate service, add the value tlsv1.2 to use TLS v1.2.

Ensure that tlsv1.2 is the only value in ssl_allowed_methods so that this component will use TLS v1.2 during the secure communication handshake.

DES ciphers are used in TLS 1.0 and TLS 1.1, but they are not available in TLS 1.2 as noted in RFC 5469.

In the event that you still require communication with services still utilizing TLS 1.1 and/or TLS 1.0, you may add tlsv1.1 and/or tlsv1.0 providing that the DES and Triple-DES ciphers are excluded in the first instance.

More details about this configuration are available in our documentation here.

Confirming the SFTP configurations using openssl s_client

You can use openssl s-client, a diagnostic tool.

Connect via openssl s_client -cipher 3DES -connect site:port to check that 3DES is disabled.

To check the TLS version, the output will also reveal which TLS version is being used:

SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : 0000
    Session-ID:
    Session-ID-ctx:
    Master-Key:
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1519733544
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)

Confirming the FTPS configurations using curl

If you wish to use curl instead, there are two tests that you can run once these options are saved in your development lab.

Run curl with the excluded ciphers DES-CBC3-SHA and check that you cannot complete the operation.

curl -v --ftp-ssl -k ftp://user@domain:port --ciphers DES-CBC3-SHA

Run curl with the allowed TLS version and check that you are able to complete the operation.

curl -v --ftp-ssl -k ftp://user@domain:port --tlsv1.2

Using --tlsv1.0 should fail the operation.

Confirming the configuration from the log

SFTPPlus configurations can also be confirmed via the log. For this issue, it should be done more as a supplementary step rather than as a sole point of confirmation of your configuration changes.

From the client-shell:

| SFTPPlus (3.30.0) file transfer client shell
| > connect
| 20140 2018-02-04 10:23:03 uuid Process 0.0.0.0:0 Connecting resource "ftpse".
| 10102 2018-02-04 10:23:03 uuid user localhost:10021 Location ftpse connected
  to the FTP server.
| 10106 2018-02-04 10:23:03 uuid user localhost:10021 Connection to FTP/FTPS
  was authenticated for location ftpse. Protected using TLSv1.2
  ECDHE-RSA-AES256-GCM-SHA384.
  Server certificate: (4) C=UK/O=ACME/CN=myserver.

From the server-side log:

| 10067 2018-02-04 10:23:03 Process Unknown 127.0.0.1:55555 Client initiating
  authentication as "user".Command protected using TLSv1.2
  ECDHE-RSA-AES256-GCM-SHA384.
  Client certificate: (2) C=GB/O=ACME/CN=myclient

The logs above indicate that TLSv1.2 was used alongside the list of ciphers ECDHE-RSA-AES256-GCM-SHA384. Please note that just because TLSv1.2 is used, it does not indicate that other TLS versions are disabled.

Evaluating SFTPPlus MFT

The features listed in this article are just a select few out of many integration and configuration options that is available to teams today to ensure data loss prevention. Feel free to talk to the Support team with your DLP requirements with file transfer software.

SFTPPlus MFT Server supports FTP, Explicit FTPS, Implicit FTPS, SFTP, SCP, HTTP and HTTPS.

SFTPPlus MFT is available as an on-premise solution supported on Windows, Linux, AIX, MacOS, Solaris, HP-UX, and FreeBSD.

It is also available on the cloud as Docker containers, AWS or Azure instances and many other cloud providers.

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