RC4 is among the older stream cipher suites in use today, and there have been a number of practical attacks against it, including plaintext-recovery attacks. The improvements in computing power have made many of these attacks more feasible for attackers, and so Microsoft is telling developers to drop RC4 from their applications.

via Microsoft Warns Customers Away From RC4, SHA-1 | Threatpost | The First Stop For Security News.

The software company also is recommending that certificate authorities and others stop using the SHA-1 algorithm.