US-CERT (the United States Computer Emergency Readiness Team) issued an advisory yesterday that a security vulnerability in Oracle Java 7 has been found, and is currently being exploited by malware in the wild. This exploit may allow attackers to excute arbitrary code on vulnerable systems.
No update has been issued yet, the only recommendation at this time is to disable Java on web browsers. This is considered to be a high risk threat.
CERT notice: http://www.kb.cert.org/vuls/id/625617
How to turn off Java applets in Firefox: https://support.mozilla.org/en-US/kb/How%20to%20turn%20off%20Java%20applets
How to disable Java in Safari: https://support.apple.com/kb/HT5241