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CVE-2012-4929

    Question

  • We have Exchange 2013 CU5 servers in our environment on which identified CVE-2012-4929.Shall we implement the below solution

    SSL/TLS Compression   Algorithm Information Leakage Vulnerability

    SSL/TLS protocols   support and optional compression algorithm. When used  compression can ease data transfer   significantly.
         
        An information leakage was discovered related to compression algorithm use   in SSL/TLS protocols. The attacker needs to have ability to submit any plain   text to compression and encryption process and observe the output to be able   to exploit this vulnerability.
         
        The attack works like this:
        the attacker who has control over a web browser that is communicating to a   web site that uses SSL/TLS can send a HTTP POST request that looks like   this:
        POST /login.php HTTP/1.1
        Cookie: XYZ
         
        Cookie: 
         
        The first Cookie is in the HTTP header and the second one is in the body of   the request.
         
        If compression algorithm is used it will replace the second occurrence of   the string 'Cookie: ' by a reference to the first one and thus decrease the   length of the string to be encrypted and eventually the output length of SSL   packet. This can be observed on the network.   
        The attacker can then prepare another request that contains a guess as to   what the first character of the cookie is. That HTTP request looks like this:  
        POST /login.php HTTP/1.1
        Cookie: XYZ
         
        Cookie: A
        If the guess was correct then the length of the output of compression +   encryption will decrease more than if the guess was incorrect.
        Using this approach the attacker can verify their guesses and completely   recover the value of the cookie. 

    Solution:

    Compression   algorithms should be disabled. The method of disabling it varies depending on   the application you're running.
        If you're using a hardware device or software not listed here, you'll need   to check the manual or vendor support options.
         
        For IIS SSL Compression is referred to as HTTP compression. It can be   disabled from IIS configuration->Web Site->Properties->Service   (tab).HTTP Compression checkboxes need to be turned off.
          For Redhat systems with Zlib   Compression.
        - Set the OPENSSL_NO_DEFAULT_ZLIB environment variable can be used to   disable zlib compression support.   
        - Further details can be found under Bugzilla Redhat 857051.   (https://bugzilla.redhat.com/show_bug.cgi?id=857051c5 )
         
        For other HTTP servers please check the vendors documentation on how to   disable SSL compression. 
        Best practices for SSL/TLS Deployment can be found at QUALYS SSL Labs.   (https://www.ssllabs.com/)

    SAH

    Wednesday, August 12, 2015 11:18 AM

Answers

  • Hi,

    It looks great.

    We probably should disable TLS compression on the web server, if you use SSL on a highly security-sensitive site.
    For most of your user base, this is not strictly necessary.  Turning off TLS compression in the web server is useful only to protect the small fraction of users who are running older, vulnerable browsers.


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    Allen Wang
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    Thursday, August 13, 2015 3:14 AM
    Moderator