Mac computers have stopped running programs written using the Java programming language in their browsers, as Apple blocked it because of security problems.
Apple has previously blocked, then unblocked, the latest version of Java on the most recent versions of its Mac operating system.
On Friday (AEDT) Apple also started blocking an older version of the Mac system, called Snow Leopard, from running Java 6, also an older version.
The US Department of Homeland Security recommends disabling Java in web browsers because it has provided pathways for hackers to take control of computers that visit a website rigged with malicious software.
Oracle Corp, which owns Java, has issued updates that fix known vulnerabilities, but the DHS expects that there are more.