{"id":2792,"date":"2014-02-11T08:16:38","date_gmt":"2014-02-11T00:16:38","guid":{"rendered":"http:\/\/rmohan.com\/?p=2792"},"modified":"2014-02-11T08:23:16","modified_gmt":"2014-02-11T00:23:16","slug":"web-servers-ssl-ciphers","status":"publish","type":"post","link":"https:\/\/mohan.sg\/?p=2792","title":{"rendered":"Web Server\u2019s SSL Ciphers"},"content":{"rendered":"

How to Disable Weak SSL Protocols and Ciphers in IIS<\/h2>\n
\"\"\u00a0March 17th, 2011\u00a0\"\"\u00a0Wayne Zimmerman<\/div>\n
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I recently undertook the process of moving websites to different servers here at work. This required that university networking group scan the new webserver with a tool called Nessus. Unfortunately this turned up several errors, all of them had to do with Secure Sockets Layer or SSL which in Microsoft Windows Server 2003 \/ Internet Information Server 6 out of the box support both unsecure protocols and cipher suites. These problems would have to be solved before they would allow the new server though the firewalls. The report they university sent me was generated by Nessus generated errors like this:<\/p>\n

SSL Version 2 (v2) Protocol Detection\r\n\r\nSynopsis :\r\n\r\nThe remote service encrypts traffic using a protocol with known\r\nweaknesses.\r\n\r\nDescription :\r\n\r\nThe remote service accepts connections encrypted using SSL 2.0, which\r\nreportedly suffers from several cryptographic flaws and has been\r\ndeprecated for several years. An attacker may be able to exploit\r\nthese issues to conduct man-in-the-middle attacks or decrypt\r\ncommunications between the affected service and clients.\r\n\r\nSee also :\r\n\r\nhttp:\/\/www.schneier.com\/paper-ssl.pdf\r\n\r\nhttp:\/\/support.microsoft.com\/kb\/187498\r\n\r\nhttp:\/\/www.linux4beginners.info\/node\/disable-sslv2\r\n\r\nSolution :\r\n\r\nConsult the application's documentation to disable SSL 2.0 and use SSL\r\n3.0 or TLS 1.0 instead.\r\n\r\nRisk factor :\r\n\r\nMedium \/ CVSS Base Score : 5.0\r\n(CVSS2#AV:N\/AC:L\/Au:N\/C:P\/I:N\/A:N)\r\n\r\nNessus ID : 20007\r\n----------------------------------------------------------\r\nSSL Medium Strength Cipher Suites Supported\r\n\r\nSynopsis :\r\n\r\nThe remote service supports the use of medium strength SSL ciphers.\r\n\r\nDescription :\r\n\r\nThe remote host supports the use of SSL ciphers that offer medium\r\nstrength encryption, which we currently regard as those with key\r\nlengths at least 56 bits and less than 112 bits.\r\n\r\nNote: This is considerably easier to exploit if the attacker is on the\r\nsame physical network.\r\n\r\nSolution :\r\n\r\nReconfigure the affected application if possible to avoid use of\r\nmedium strength ciphers.\r\n\r\nRisk factor :\r\n\r\nMedium \/ CVSS Base Score : 4.3\r\n(CVSS2#AV:N\/AC:M\/Au:N\/C:P\/I:N\/A:N)\r\n\r\nPlugin output :\r\n\r\nHere are the medium strength SSL ciphers supported by the remote server :\r\n\r\nMedium Strength Ciphers (>= 56-bit and < 112-bit key)\r\nSSLv2\r\nDES-CBC-MD5 Kx=RSA Au=RSA Enc=DES(56) Mac=MD5\r\nSSLv3\r\nDES-CBC-SHA Kx=RSA Au=RSA Enc=DES(56) Mac=SHA1\r\nTLSv1\r\nEXP1024-DES-CBC-SHA Kx=RSA(1024) Au=RSA Enc=DES(56) Mac=SHA1 export\r\nEXP1024-RC4-SHA Kx=RSA(1024) Au=RSA Enc=RC4(56) Mac=SHA1 export\r\nDES-CBC-SHA Kx=RSA Au=RSA Enc=DES(56) Mac=SHA1\r\n\r\nThe fields above are :\r\n\r\n{OpenSSL ciphername}\r\nKx={key exchange}\r\nAu={authentication}\r\nEnc={symmetric encryption method}\r\nMac={message authentication code}\r\n{export flag}\r\n\r\nNessus ID : 42873\r\n--------------------------------------------------------------------\r\nSSL Weak Cipher Suites Supported\r\n\r\nSynopsis :\r\n\r\nThe remote service supports the use of weak SSL ciphers.\r\n\r\nDescription :\r\n\r\nThe remote host supports the use of SSL ciphers that offer either weak\r\nencryption or no encryption at all.\r\n\r\nNote: This is considerably easier to exploit if the attacker is on the\r\nsame physical network.\r\n\r\nSee also :\r\n\r\nhttp:\/\/www.openssl.org\/docs\/apps\/ciphers.html\r\n\r\nSolution :\r\n\r\nReconfigure the affected application if possible to avoid use of weak\r\nciphers.\r\n\r\nRisk factor :\r\n\r\nMedium \/ CVSS Base Score : 4.3\r\n(CVSS2#AV:N\/AC:M\/Au:N\/C:P\/I:N\/A:N)\r\n\r\nPlugin output :\r\n\r\nHere is the list of weak SSL ciphers supported by the remote server :\r\n\r\nLow Strength Ciphers (< 56-bit key)\r\nSSLv2\r\nEXP-RC2-CBC-MD5 Kx=RSA(512) Au=RSA Enc=RC2(40) Mac=MD5 export\r\nEXP-RC4-MD5 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export\r\nSSLv3\r\nEXP-RC2-CBC-MD5 Kx=RSA(512) Au=RSA Enc=RC2(40) Mac=MD5 export\r\nEXP-RC4-MD5 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export\r\nTLSv1\r\nEXP-RC2-CBC-MD5 Kx=RSA(512) Au=RSA Enc=RC2(40) Mac=MD5 export\r\nEXP-RC4-MD5 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export\r\n\r\nThe fields above are :\r\n\r\n{OpenSSL ciphername}\r\nKx={key exchange}\r\nAu={authentication}\r\nEnc={symmetric encryption method}\r\nMac={message authentication code}\r\n{export flag}\r\n\r\nOther references : CWE:327, CWE:326, CWE:753, CWE:803, CWE:720\r\n\r\nNessus ID : 26928\r\n-----------------------------------------------------------------<\/pre>\n
These three error messages pretty much mean that you need to turn off SSL 2.0 due to exploits that were found after the standard was created. You need to turn off any encryption suites lower than 128bits. The third error message says we need to turn off anything for less than 56bits, but this will be accomplished by turning of anything less than 128bits. Basically your are modifying the settings that restrict the use of specific protocols and ciphers that are used by the schannel.dll. More detailed information can be found at Micorsoft\u2019s\u00a0KB187498<\/a>\u00a0or\u00a0KB245030<\/a><\/p>\n
How do we do this?<\/strong><\/p>\n
Disabling SSL 2.0 on IIS 6<\/strong><\/p>\n
    \n
  1. Open up \u201cregedit\u201d from the command line<\/li>\n
  2. Browse to the following key:
    \nHKLM\\SYSTEM\\CurrentControlSet\\Control\\SecurityProviders\\SCHANNEL\\Protocols\\SSL 2.0\\Server<\/li>\n
  3. Create a new REG_DWORD called \u201cEnabled\u201d and set the value to 0<\/li>\n
  4. You will need to restart the computer for this change to take effect. (you can wait on this if you also need to disable the ciphers)<\/li>\n<\/ol>\n
    Disable unsecure encryption ciphers less than 128bit<\/strong><\/p>\n
      \n
    1. Open up \u201cregedit\u201d from the command line<\/li>\n
    2. Browse to the following key:
      \nHKLM\\SYSTEM\\CurrentControlSet\\Control\\SecurityProviders\\SCHANNEL\\Ciphers\\DES 56\/56<\/li>\n
    3. Create a new REG_DWORD called \u201cEnabled\u201d and set the value to 0<\/li>\n
    4. Browse to the following key:
      \nHKLM\\SYSTEM\\CurrentControlSet\\Control\\SecurityProviders\\SCHANNEL\\Ciphers\\RC2 40\/128<\/li>\n
    5. Create a new REG_DWORD called \u201cEnabled\u201d and set the value to 0<\/li>\n
    6. Browse to the following key:
      \nHKLM\\SYSTEM\\CurrentControlSet\\Control\\SecurityProviders\\SCHANNEL\\Ciphers\\RC4 40\/128<\/li>\n
    7. Create a new REG_DWORD called \u201cEnabled\u201d and set the value to 0<\/li>\n
    8. Browse to the following key:
      \nHKLM\\SYSTEM\\CurrentControlSet\\Control\\SecurityProviders\\SCHANNEL\\Ciphers\\RC4 56\/128<\/li>\n
    9. Create a new REG_DWORD called \u201cEnabled\u201d and set the value to 0<\/li>\n
    10. You will need to restart the computer for this change to take effect.<\/li>\n<\/ol>\n
      How to verify the changes?<\/strong><\/p>\n
      Now that you have made these changes how can you be sure that they have taken place without having to go to your boss or higher authority just to find that you did them wrong. Well I found a nice tool called SSL-SCAN which you can download at\u00a0http:\/\/code.google.com\/p\/sslscan-win\/<\/a>\u00a0for the Windows port or you can download an compile for your favorite operating system at the original project SSL-SCAN site\u00a0http:\/\/sourceforge.net\/projects\/sslscan\/<\/a>. This tool provides some great detail about what is allows and not allows plus some analysis of the SSL certificate itself.<\/p>\n
      Below the screen shot shows that we have disabled any ciphers that attempt to use the SSL 2.0 protocol and we\u2019ve disabled all ciphers that less than 128bit.<\/p>\n
      \"sslscan1-595x1024\"<\/a><\/p>\n
      \n
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      <\/div>\n
      <\/div>\n
      \n
      Hardening Your Web Server\u2019s SSL\u00a0Ciphers<\/h2>\n
      \n