Fixing SSL RC4 Cipher Suites Supported (Bar Mitzvah)

Nessus Summary

Nessus ID: 65821

CVSS v3.0 Base Score: 2.6

Nessus Description:

The remote host supports the use of RC4 in one or more cipher suites. The RC4 cipher is flawed in its generation of a pseudo-random stream of bytes so that a wide variety of small biases are introduced into the stream, decreasing its randomness. If plaintext is repeatedly encrypted (e.g., HTTP cookies), and an attacker is able to obtain many (i.e., tens of millions) ciphertexts, the attacker may be able to derive the plaintext.

How to Fix

This vulnerability is cased by a RC4 cipher suite present in the SSL cipher suite. Fixing this is simple.

Apache Fix

The follow configuration should be added to the security.conf file to apply globally or to virtual host:

SSLCipherSuite EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH
SSLProtocol All -SSLv2 -SSLv3 -TLSv1 -TLSv1.1
SSLHonorCipherOrder On
SSLCompression off

IIS Fix

The Microsoft Knowledge Base article "How to Restrict the Use of Certain Cryptographic Algorithms and Protocols in Schannel.dll" describes how to enable just the FIPS 140 algorithms. Here's a summary:

Disable weak ciphers

Open the registry editor and locate HKLMSYSTEMCurrentControlSetControlSecurityProviders

Set "Enabled" dword to "0x0" for the following registry keys:

SCHANNELCiphersRC4 128/128
SCHANNELCiphersRC2 128/128
SCHANNELCiphersRC4 64/128
SCHANNELCiphersRC4 56/128
SCHANNELCiphersRC2 56/128
SCHANNELCiphersRC4 40/128
SCHANNELCiphersRC2 40/128
SCHANNELCiphersNULL
SCHANNELHashesMD5

Enable strong ciphers

Open the registry editor and locate HKLMSYSTEMCurrentControlSetControlSecurityProviders

Set "Enabled" dword to "0xffffffff" for the following registry keys

SCHANNELCiphersTriple DES 168/168
SCHANNELHashesSHA
SCHANNELKeyExchangeAlgorithmsPKCS

If the Enabled dword doesn't exist yet, please create the dword and set the value to "0x0" or "0xffffffff" as required.