Oracle released an emergency software update today to fix a security vulnerability in its Java software that could allow attackers to break into computers.

The update, which is available on Oracle's Web site, fixes a critical vulnerability in Oracle's Java 7 that could allow a remote, unauthenticated attacker to execute arbitrary code. The attack can be induced if someone visits a Web site that's been set up with malicious code to take advantage of the hole.

Oracle said the update modifies the way Java interacts with Web applications.

"The default security level for Java applets and web start applications has been increased from 'medium' to 'high," Oracle said in an advisory today. "This affects the conditions under which unsigned (sandboxed) Java web applications can run. Previously, as long as you had the latest secure Java release installed applets and web start applications would continue to run as always. With the 'high' setting the user is always warned before any unsigned application is run to prevent silent exploitation."

The vulnerability was being exploited by a zero-day Trojan horse called Mal/JavaJar-B, which was already identified as attacking Windows, Linux and Unix systems and being distributed in exploit kits "Blackhole" and "NuclearPack," making it far more convenient to attackers.