If you use Mozilla Firefox, stop what you're doing and check to make sure your version of the popular web browser is 72.0.1, or 68.4.1 if you're using the extended support release (ESR) build.
That's because just a couple of days after releasing Firefox 72 and Firefox ESR 68.4, Mozilla learned that Qihoo 360 researchers found a serious security flaw that, according to the U.S. Department of Homeland Security, could let an attacker "exploit this vulnerability to take control of an affected system."
That is indeed what is already happening, Mozilla said in its advisory posted yesterday (Jan. 8): "We are aware of targeted attacks in the wild abusing this flaw."
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It's also telling what Mozilla left out of this security advisory: any mention that this might be specific to one operating system. (Compare that to the two previous Mozilla security advisories, which both specified Windows.) Until we learn otherwise, we have to assume that this flaw affects Windows, macOS and Linux alike.
To check your version of Firefox, go to Help --> About Firefox on Windows, or Firefox --> About Firefox on a Mac. Many instances of Firefox update automatically when you launch them, so if you did so this morning, you may have versions 72.0.1 or ESR 68.4.1 already.
If not, checking the version number gives you the opportunity to check for updates, or often just starts the update process on its own.