Server-side workaround for BEAST SSL vulnerability on IIS

Recently, a vulnerability in the CBC cipher suite used in the SSL and TLS protocols was discovered that could allow an attacker to gain access to encrypted information. While the attack is not easily implemented, it will show up on compliance audits and auditors don’t like that. Fortunately, there is a server-side fix for Server 2008 and above that can be easily implemented without breaking compatibility with clients.

More information about the attack and workarounds can be found here: http://blogs.msdn.com/b/kaushal/archive/2011/10/03/taming-the-beast-browser-exploit-against-ssl-tls.aspx.

The workaround is to enable TLS 1.1 and/or 1.2 on servers that support it, and prioritize cipher suites so RC4 takes precedence over CBC. Server 2008 R2 and above supports TLS 1.1 and 1.2 – you can enable those protocols by following the instructions in KB 2588513. You’ll also want to change the priority of cipher suites on all Server 2008 and above systems using group policy (either a local group policy object for a single server, or by modifying domain policy in an AD environment).

1. Open Group Policy Editor (locally, Start>Run>gpedit.msc).
2. Browse to Computer Configuration>Administrative Templates>Network>SSL Configuration Settings.
3. Modify SSL Cipher Suite Order: set it as enabled, and enter a comma delimited list of cipher suites. I recommend the following:

TLS_RSA_WITH_RC4_128_SHA,TLS_RSA_WITH_RC4_128_MD5,SSL_CK_RC4_128_WITH_MD5,TLS_RSA_WITH_AES_128_CBC_SHA,TLS_RSA_WITH_AES_256_CBC_SHA,TLS_RSA_WITH_3DES_EDE_CBC_SHA,TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P256,TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P384,TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P521,TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P256,TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P384,TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P521,TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P256,TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P384,TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P521,TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P256,TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P384,TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P521,TLS_DHE_DSS_WITH_AES_128_CBC_SHA,TLS_DHE_DSS_WITH_AES_256_CBC_SHA,TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA,SSL_CK_DES_192_EDE3_CBC_WITH_MD5,SSL_CK_DES_64_CBC_WITH_MD5

4. Reboot the server for the setting to take effect.

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