Showing posts with label TLS. Show all posts
Showing posts with label TLS. Show all posts

Sunday, May 24, 2015

Secure your Apache server against LOGJAM

Some time ago I wrote a post about the dismaying history of US government attempts to regulate encryption out of existence. I had to omit quite a bit; it was a post and not a book after all. One of the details left out of the story was the DHE_EXPORT cipher suites. During the 90's, developers were forced by the US government to us deliberately insecure ciphers when communicating with entities in foreign countries (readers will remember from the last post that law makers were convinced that encryption should fall under the same rules as weapons technology, and thus could not be shared with anyone outside the Father Land). These insecure ciphers became DHE_EXPORT. The DH stands for Diffie-Hellman; the key exchange system that bears their name was first published in 1976.

Along with the cipher suite was a mechanism to force a normal encrypted transaction to downshift to a lower-bit DHE_EXPORT cipher. As so many short-sighted technology regulations have done in the past, this silly bit of Washington DC-brand programming has come back to haunt us in the form of the LOGJAM vulnerability. Until just a few days ago, all major browsers continued to support these deprecated DHE_EXPORT ciphers, as have a variety of applications as fundamental to web infrastructure as OpenSSL.

The exploit is described in detail on a website hosted by the researchers responsible for its discovery - which also hosts their paper on the same subject (PDF).

Meanwhile, patching your Apache server is simple: Apache HTTP Server (mod_ssl)
SSL parameters can globally be set in httpd.conf or within specific virtual hosts.
Cipher Suites
Disable support for SSLv2 and SSLv3 and enable support for TLS, explicitly allow/disallow specific ciphers in the given order :
SSLProtocol             all -SSLv2 -SSLv3


SSLHonorCipherOrder     on
DH Parameters
In newer versions of Apache (2.4.8 and newer) and OpenSSL 1.0.2 or later, you can directly specify your DH params file as follows:
SSLOpenSSLConfCmd DHParameters "{path to dhparams.pem}"
If you are using Apache with LibreSSL, or Apache 2.4.7 and OpenSSL 0.9.8a or later, you can append the DHparams you generated earlier to the end of your certificate file.
Reload configuration
sudo service apache2 reload

Thursday, May 7, 2015

Amazon Finally Ditches SSLv3

Amazon S3 subscribers recently received a form letter like this one:

Dear AWS Customer,

This message explains some security improvements in our services. Your security is important to us. Please review the entire message carefully to determine whether your use of the services will be affected, and if so what you need to do.

As of 12:00 AM PDT May 20, 2015, AWS will discontinue support of SSLv3 for securing connections to S3 buckets. Security research published late last year demonstrated that SSLv3 contained weaknesses in its ability to protect and secure communications. These weaknesses have been addressed in Transport Layer Security (TLS), which is the replacement for SSL. Consistent with our top priority to protect AWS customers, AWS will only support versions of the more modern TLS rather than SSLv3.

You are receiving this email because some of your users are accessing Amazon S3 using a browser configured to use SSLv3, or some of your existing applications that use Amazon S3 are configured to use SSLv3. These requests will fail once AWS disables support for SSLv3 for the Amazon S3 service.

The following bucket(s) are currently accepting requests from clients (e.g. mobile devices, browsers, and applications) that specify SSLv3 to connect to Amazon S3 HTTPS endpoints.


For your applications to continue running on Amazon S3, your end users need to access S3 from clients configured to use TLS. As any necessary changes would need to be made in your application, we recommend that you review your applications that are accessing the specified S3 buckets to determine what changes may be required. If you need assistance (e.g. to help identify clients connecting to S3 using SSLv3), please contact our AWS Technical Support or AWS Customer Service.

For further reading on SSLv3 security concerns and why it is important to disable support for this nearly 18 year old protocol, we suggest the following articles:

Thank you for your prompt attention.

The Amazon Web Services Team

Amazon Web Services, Inc. is a subsidiary of, Inc. is a registered trademark of, Inc. This message was produced and distributed by Amazon Web Services Inc., 410 Terry Ave. North, Seattle, WA 98109-5210

Wednesday, September 19, 2012

More Fun With PCI

I received a notification from a large security auditing firm that of the ciphers currently available, only RC4 ciphers will be considered PCI compliant.

My assumption based on the notification is that this move is intended as a rejection of CBC (Cipher Block Chaining). Well, that's fine as far as I am concerned. CBC has some serious issues as implemented in SSL v3 / TLS v1.0. In a nutshell, you can time responses for applications using the block cipher to get ranges of possible data in SSLv3 and partial payload decryption in TLS. So-called "stream" ciphers like RC4 are immune to this particular attack vector. You don't get private keys from the attack, its by no means a fast attack (minimum of three hours), and you need access to monitor the session. Further, patches for CBC exist to over-ride the timing exploit (for example the NSS libraries used by Mozilla have been patched).

I will save debunking the man in the middle hysteria for a later post. What frustrates me about the requirement of RC4 stream ciphers for PCI compliance is not that CBC ciphers are no good - they are weak - it is the notion that somehow RC4 is somehow sufficient. Some points to consider:

-RC4 exploit using SSH with null password prevention enabled

-RC4 is frequently implemented poorly within applications other than sshd, for example by using poor to no random number generation

-Successful attack vectors exist, but they have yet to be put into a helpful graphical interface for use by your neighborhood teenager (as the BEAST framework did for CBC). Paul and Maitra published on RC4 key reconstruction techniques in 2007 (Permutation after RC4 Key Scheduling Reveals the Secret Key. SAC 2007), based on keystream byte assignment biases first published by Roos in 1995. This means, unlike CBC, there is a published algorithmic approach to full private key decryption of RC4. 

It cant be stressed enough that all of these vectors assume an attacker on your wireless or local network. If that is the case than SSL is the very least of your problems. While theoretical dismissal of this or that cipher based on penetration ability is sound and valuable, the PCI standard suggests a holistic approach to security. The various levels of PCI-DSS compliance suggest admission of the reality that the goals of securing a system will differ based on the purpose of that system. Banning the use of CBC will not serve to get less sites hacked, but it will keep administrators preoccupied with yet more busy work, switching from one cipher with published flaws to another cipher with published flaws.

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