Nothing in life is completely secure; Java is no exception. Several specific security problems have been discovered and fixed since Java was first released. If you're using an up-to-date Web browser, you are usually safe against the known attacks. However, nobody is safe against attacks that haven't been discovered yet.
If somebody says Java is safe because "hackers aren't smart enough to exploit the problems,'' don't believe them. We're disappointed that some people who should know better are still spouting this nonsense. We've discovered several security problems, and we're pretty sure we're not the smartest people in the world. If one group of hackers creates a Java-based attack and shares it with their friends, we're all in trouble.