Security firm, Trend Micro, has identified an unpatched vulnerability in Adobe Flash player for Windows.
In a recent series of blog posts the company outlined how the flaw can be used by attackers to run code or programs on Windows computers as if the user themselves had run it.
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The vulnerability is said to effect Windows systems running Internet Explorer and Firefox web browsers.
According to Trend Micro, once infecting a system, the attacker's program can do anything a user can do on their system. In the worst case scenario, this includes installing malware onto the PC.
Vulnerabilities are constantly being found, usually these are normally fixed with a patch before hackers are able to exploit them. In this case, hackers found the vulnerability before anyone else and have been exploiting the gap in protection.
Trend Micro said this is called a "zero-day" situation, because defenders have "zero days" to protect against attacks. This means even if a system is kept up-to-date, users are still at risk of attack until Adobe releases a patch.
The situation has become more serious due to the fact that the attacks have been using banner ads on web pages, known as "malvertisements", to spread malware.
Users going to trusted sites they expect to be safe and still have their system infected with malware. They work by attackers targeting and compromising the third-party ad servers that offer the ads on legitimate and popular sites. Trend Micro said this is a particularly nasty form of attack, one that puts average users at great risk.
In one of its blog posts, the firm said "The situation is even more serious because this vulnerability is being used by what we call an "exploit kit": a tool that cyber-criminals make and sell to other cyber-criminals so they can carry out attacks. An exploit kit spreads attacks much more widely. This particular vulnerability is being used in the "Angler" exploit kit."
Trend Micro claims this is a vulnerability that can be widely attacked.
"It's a potentially very serious situation that everyone running Microsoft Windows should be aware of."
The firm said the two most important things users should do to guard against potential attacks are keeping systems and programs up-to-date and running a mature, full-featured security program.
The latest update for Adobe Flash was released on January 13, 2015. The update fixed similar vulnerabilities but not the one outlined by Trend Micro.