Adobe has acknowledged a "critical" security flaw in its Reader, Acrobat and Flash Player software.
Adobe says the vulnerability potentially enables hackers to take control of affected computer systems.
Users running Windows, Macintosh or Linux might all be open to attack.
The company is working to fix the problem. In the meantime, users of Reader, Acrobat and Flash are advised to ensure their anti-virus software is up to date.
"It doesn't really get any worse than a 'zero-day' vulnerability like this," said Graham Cluley, senior technology consultant at Sophos, a security software company.
He said that hackers could create a "booby-trapped Flash animation, or PDF" that would give them access to a person's computer, potentially allowing them to harvest personal information or use the machine to send spam messages.
In recent years, PDFs have become a popular means of sharing documents that are not easily altered by the recipient.
In a security advisory, Adobe said: "There are reports that this vulnerability is being actively exploited in the wild against both Adobe Flash Player, and Adobe Reader and Acrobat".
Whilst it works to fix the problem, the company suggested upgrading to the latest "release candidate" for the Adobe Flash Player, version 10.1, which it said "does not appear to be vulnerable".
Alternatively, the company said that Adobe Reader and Acrobat users could delete or rename the "authplay.dll" file on their system.
However, Adobe said that doing so meant that "users will experience a non-exploitable crash or error message when opening a PDF file that contains SWF [Adobe Flash] content."
Mr Cluley said that keeping anti-virus software up to date would also help to avoid problems.
"There has been a long history of vulnerabilities being found in Adobe's products," he said.
"This is probably because they are everywhere and omnipresent."
Adobe estimates that more than 95% of computers worldwide have Flash Player installed.
Apple has been criticised for preventing its popular iPhone and iPad devices from viewing Adobe Flash animations and videos.
Apple boss Steve Jobs recently wrote an open letter explaining that Adobe's Flash was, amongst other things, "the number one reason Macs crash".
Mr Cluley said: "The more people who are concerned about Adobe's products and the ability for them to be written securely, the more it backs up Steve Jobs' argument that Adobe's software is buggy.
"The crux of the problem is that Adobe have overloaded some of their programs with so many bells and whistles, that with lots of code, there is a much higher chance that there will be a bug.
"This vulnerability exploits a feature of a PDF file format that will not be widely used.
"A simpler code might have led to a simpler life."
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