A previously undocumented flaw in the latest version of Adobe Systems' ubiquitous Reader application is being exploited in online hacks that allow attackers to surreptitiously install malware on end-user computers, a security firm said.

The attacks, according to researchers from security firm FireEye, work against Reader 11.0.1 and earlier versions and are actively being exploited in the wild. If true, the attacks are notable because they pierce security defenses Adobe engineers designed to make malware attacks harder to carry out. Adobe officials said they're investigating the report.

"Upon successful exploitation, it will drop two DLLs," FireEye researchers Yichong Lin, Thoufique Haq, and James Bennett wrote of the online attacks they witnessed. "The first DLL shows a fake error message and opens a decoy PDF document, which is usually common in targeted attacks. The second DLL in turn drops the callback component, which talks to a remote domain." DLL is the researchers' shorthand for a file that works with the Microsoft Windows dynamic link library.

Update: Researchers with antivirus provider Kaspersky Lab have confirmed the exploit can successfully escape the Adobe sandbox, making it the first known in-the-wild attack to do so, Threatpost reporter Michael Mimoso reported. He cited a Kaspersky researcher reporting he observed an attack working against Reader 11.0.1 running on a 64-bit version of Windows 7.

So far, there have been no documented in-the-wild exploits that have successfully bypassed the Reader sandbox. The protection is designed to minimize the damage of attacks that exploit buffer overflows and other types of software bugs by isolating Web content from sensitive parts of the underlying operating system. As a result, the application will typically crash when flaws are exploited, but attackers remain unable to remotely execute malicious code on vulnerable computers.

In November, hackers claimed to possess an exploit that also pierced the security mechanism. At the Kaspersky Security Analyst Summit last week, Adobe officials said their researchers conducted a three-month investigation but never received confirmation that the attack existed.

FireEye's post was the latest to remind Reader users "not open any unknown PDF files." This advice is well-intended but largely ineffective, since many booby-trapped documents are contained in e-mails from people the victim knows or are hosted on websites the victim regularly visits. Better recommendations are to avoid PDF files whenever possible or to use an alternative PDF reader such as the Foxit Reader until Adobe has had time to diagnose the bugs and if necessary close the security hole.

Story updated to include details from Kaspersky.