Via Ivan - "...RC4 has long been considered problematic, but until very recently there was no known way to exploit the weaknesses. After the BEAST attack was disclosed in 2011, we—grudgingly—started using RC4 in order to avoid the vulnerable CBC suites in TLS 1.0 and earlier. This caused the usage of RC4 to increase, and some say that it now accounts for about 50% of all TLS traffic.
Last week, a group of researchers (Nadhem AlFardan, Dan Bernstein, Kenny Paterson, Bertram Poettering and Jacob Schuldt) announced significant advancements in the attacks against RC4, unveiling new weaknesses as well as new methods to exploit them. Matthew Green has a great overview on his blog, and here are the slides from the talk where the new issues were announced.
At the moment, the attack is not yet practical because it requires access to millions and possibly billions of copies of the same data encrypted using different keys. A browser would have to make that many connections to a server to give the attacker enough data. A possible exploitation path is to somehow instrument the browser to make a large number of connections, while a man in the middle is observing and recording the traffic.
We are still safe at the moment, but there is a tremendous incentive for researchers to improve the attacks on RC4, which means that we need to act swiftly...."