* U.S. government urged users to disable Java on Thursday
* Bugs in Java make PCs vulnerable to attack by hackers
* Update sets security settings at "high" by default
By Jim Finkle
BOSTON, Jan 13 (Reuters) - Oracle Corp released an emergency update to its widely used Java software for surfing the Web on Sunday, days after the U.S government urged PC users to disable the program because of a bug it said made computers vulnerable to attack by hackers.
Java security expert Adam Gowdiak, who has discovered several bugs in the software over the past year, said that the update from Oracle leaves unfixed several critical security flaws.
"We don't dare to tell users that it's safe to enable Java again," said Gowdiak, a researcher with Poland's Security Explorations.
An Oracle spokeswoman declined to comment on Gowdiak's analysis.
Oracle said on its security blog on Sunday that its update fixed two vulnerabilities in the version of Java 7 for Web browsers.
It said that it also switched Java's security settings to "high" by default, making it more difficult for suspicious programs to run on a personal computer without the knowledge of the user.
Java is a computer language that enables programmers to write software utilizing just one set of codes that will run on virtually any type of computer, including ones that use Microsoft Corp's Windows, Apple Inc's OS X and Linux, an operating system widely employed by corporations.
It is installed in Internet browsers to access web content and also directly on PCs, server computers and other devices that use it to run a wide variety of computer programs. Analysts estimate that it may be used on more than 1 billion machines around the globe.
The Department of Homeland Security and computer security experts said on Thursday that hackers figured out how to exploit the bug in a version of Java used with Internet browsers to install malicious software on PCs. That has enabled them to commit crimes from identity theft to making infected computers part of an ad-hoc networks that used to attack websites.
Oracle said that the flaws only affect Java 7, the program's most-recent version, and versions of Java software designed to run on browsers.
Java is so widely used that the software has become a prime target for hackers. Last year, Java surpassed Adobe Systems Inc's Reader software as the most frequently attacked piece of software, according to security software maker Kaspersky Lab.
Java was responsible for 50 percent of all cyber attacks last year in which hackers broke into computers by exploiting software bugs, according to Kaspersky. That was followed by Adobe Reader, which was involved in 28 percent of all incidents. Microsoft Windows and Internet Explorer were involved in about 3 percent of incidents, according to the survey.
The Department of Homeland Security said attackers could trick targets into visiting malicious websites that would infect their PCs with software capable of exploiting the bug in Java.
It said an attacker could also infect a legitimate website by uploading malicious software that would infect machines of computer users who trust that site because they have previously visited it without experiencing any problems.
Security experts have been scrutinizing the safety of Java since a similar security scare in August, which prompted some of them to advise using the software only on an as-needed basis.