Attackers are now exploiting a Java zero-day vulnerability

The new issue is a combination of two vulnerabilities, he said. One of them abuses the new Reflection API in order to bypass Oracle's October patch for a different issue that Security Explorations reported to the company on Aug. 31, Gowdiak said.

The exploit vector used in the new attack is also known to Oracle, as it was reported by Security Explorations in September along with additional proof-of-concept code for the August issue, he said.

Oracle has yet to confirm the vulnerability or comment on its patching plans. The next critical patch update (CPU) for Java is scheduled for Feb. 19. Oracle doesn't have a comment available at the moment, a representative from the company's outside PR agency said Thursday via email.

When faced with a similar situation in August of cybercriminals exploiting an unpatched Java vulnerability, Oracle decided to break out of its quarterly patch release cycle and release an emergency update.

"I think that Oracle will not issue an out-of-band patch again without thoroughly investigating the full extent of the damage and ensuring the quality of the patch," Botezatu said. "The last out-of-band patch for Java that was released in August actually opened the door for a similar exploitation technique on Java versions that were not vulnerable before the exploit. I believe this was an important lesson that might delay the release of a fix."

Users should disable the Java plug-in their browsers as soon as possible and keep it disabled until a patch is released, Botezatu said. Users who need Java support in the browser on certain websites should only allow the plug-in to run on those websites, he said.

The latest version of Java, Java 7 Update 10 (7u10), which was released on Dec. 11, enables users to have better control over Web-based Java content. The version provides an option in the Java control panel to disable all Java content in browsers.