Adobe has released an emergency software patch for Flash.
The patch was issued after the firm found a serious vulnerability that was being exploited by hackers.
The company said it had evidence of "limited targeted attacks" and urged people to update their software immediately.
The vulnerability allows hackers to take control of a computer and affects Windows, Mac and Linux systems.
The Flash software by Adobe has seen many occasions where it needs a security fix and many security researchers regard it as a weak point in many websites.
Security specialist from ESET Mark James said: "Since Flash is such a widely used plug-in, it stands to reason that it will be one of the most targeted apps for vulnerability.
"If you want to affect as many people as possible, then you need an application that a lot of users use, and Flash is one of them."
Security blogger Brian Krebs recently disabled Flash on his machine, entirely as an experiment.
He said: "It might be worth considering whether you really need to keep Flash Player installed at all."
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