November 13, 2013, 9:59 AM — Microsoft patched serious vulnerabilities Tuesday in Windows, Internet Explorer and Office, but also urged customers to stop using the aging RC4 cipher and SHA-1 hashing function in their systems and services.
These algorithms have known weaknesses and should be replaced with more secure alternatives in SSL deployments and digital certificates.
Microsoft released an update for Windows 7, Windows 8, Windows RT, Windows Server 2008 R2 and Windows Server 2012 that allows system administrators to disable RC4 using registry settings. The update also adds a SCH_USE_STRONG_CRYPTO flag that allows developers to remove RC4 support in their Internet applications that use the Windows Secure Channel (Schannel) library.
The settings added by the update are not enabled by default, but their use is recommended because of known weaknesses in the RC4 stream cipher. For SSL/TLS implementations, Microsoft recommends the AES-GCM cipher as an alternative, but this requires customers to enable support for TLS 1.2 in their services.
TLS, the successor of SSL, offers a choice of ciphers, but versions 1.0 and 1.1 of the protocol support only block ciphers that operate in cipher-block chaining (CBC) mode and the RC4 stream cipher. In recent years, several attacks have been demonstrated by security researchers against both CBC mode ciphers and RC4, leaving TLS 1.2, which adds support for block ciphers operating in Galois/Counter Mode (GCM), as the secure alternative.
Unfortunately, real-world support for TLS 1.2 is not yet widespread. According to statistics from the SSL Pulse project, as of Nov. 2, only around 20 percent of the world's 162,480 most popular HTTPS-enabled websites support TLS 1.2. Furthermore, default TLS 1.2 support has been added in Chrome 30, Mozilla Firefox 28, Internet Explorer 11, Opera 17 and Safari 7 on Mac OS, which have only been recently released or have yet to be released as stable versions.
As far as RC4 use on the Internet goes, Microsoft found that 43 percent of HTTPS websites prioritize this cipher in their configurations and about 4 percent of those actually require it.
"RC4 is a very widely used cipher suite," the SSL Pulse Project says. "Before 2013, we knew of some RC4 weaknesses, but it was thought that they did not affect SSL. With new research published in early 2013, we now know that RC4 is weak and should not be used."