The vulnerability, found by Finnish security researcher Jouko Pynnonen in April, was patched last month by Sun, but its details were not made public until Tuesday. Security information provider Secunia posted information about the flaw in an advisory that rated it a "highly critical" threat.
The Java plug-in enables small Web programs, known as applets, to run safely on a user's computer. But the security flaw allows a malicious Web site accessed through a victim's browser to bypass those protections.
"It allows execution of attacker-supplied code without user interaction (apart from viewing a Web page) which usually means a 'critical' classification," Pynonnen stated in an e-mail interview with CNET News.com.
"The same exploit could also be used against various operating systems and browsers, which makes it more serious," he added. The vulnerability can be used to attack systems running on Windows or Linux, for example, and using major browser software such as Microsoft's Internet Explorer and Firefox--meaning a large number of systems are vulnerable to attack.
An attacker could use the flaw to do anything the victim normally could, including browse, modify or run files, upload more programs to the victim's system, or send out data from the system, Pynnonen wrote in an advisory dated Tuesday.
While the major browsers have had to deal with a significant number of security issues, the flaw is a rare black eye for the security of Sun's Java technology. Java is designed to be able to run programs downloaded from the Internet on various operating systems safely, without danger to a PC. The "sandbox" that cordons off Java applets from the rest of the system has typically worked well.
However, the flaw allows small snippets of Web code, known as Javascript, to execute functions of Java that were never meant to be run by external programs.
Last week, while announcing details of Sun's forthcoming Solaris 10 operating system, President Jonathan Schwartz noted that Java hasn't been afflicted by a single Java virus.
However, the new security hole could allow a virus to use the Java plug-in to invade PC systems. In October, a flaw in the Java plug-in for cell phones raised the specter that a malicious program disguised as a helpful application could attack a phone's software, if run by a user.
Like the recent iFrame vulnerability in Microsoft's Internet Explorer, the Java flaw could allow a malicious Web site to download and execute a program that would compromise a visitor's PC.
"It could be easily used for spreading viruses or other malware," Pynnonen said in the e-mail. "The exploit itself can't be easily embedded in e-mail, because Java applets contained in e-mail aren't normally started automatically. However an e-mail message could contain a link to a Web page which has the exploit."
While Sun would not speculate on how the flaw could be used by attackers, the company did say that it worked hard to distribute the patch for it to all users.
"We took this very seriously, and we have gone the extra mile to post these patches," a Sun representative said on Tuesday.
The advisories from Sun, Secunia and Pynnonen do not address whether the problem could affect Apple Computer's Mac OS X operating system, which is based on a Unix-like core of code, similar to Linux. The Sun representative said that the Mac issue is being investigated.
Apple Computer was not immediately available for comment.
CNET News.com's Stephen Shankland contributed to this report.
