UPDATE: Microsoft late Wednesday confirmed a Windows kernel zero-day vulnerability is being exploited in "limited, targeted attacks" against Windows XP users.
Microsoft issued a security advisory to acknowledge the flaw after anti-malware vendor FireEye warned that the Windows bug is being used in conjunction with an Adobe Reader exploit to infect Windows machines with malware.
Microsoft described the issue as an elevation of privilege vulnerability that allows an attacker to run arbitrary code in kernel mode. An attacker could then install programs; view, change, or delete data; or create new accounts with full administrative rights.
The vulnerability cannot be used for remote code execution but could allow a standard user account to execute code in the kernel. Currently, the exploit appears to only work in Windows XP, FireEye said.
The exploit targets Adobe Reader 9.5.4, 10.1.6, 11.0.02 and prior on Windows XP SP3. Windows users running the latest versions of Adobe Reader are not be affected by this exploit.
Microsoft's advisory includes some "workarounds" to help Windows XP and Windows Server 2003 users mitigate the issue.
In addition, FireEye recommends that Windows users upgrade to the latest version of Adobe Reader to blunt the in-the-wild PDF exploit.
Anti-malware vendor Symantec also confirmed the attacks have been ongoing since the beginning of November this year. The company said the attack arrives as a malicious PDF file with file names such as syria15.10.pdf or Note_№107-41D.pdf either via attachments or via downloads from rigged websites.
"Upon successful exploitation of the vulnerability, another malicious file, observed since mid-October, is dropped onto the compromised computer. This Trojan collects system information and connects to a command-and-control (C&C) server," the company said.
Symantec has spotted infections in India, Australia, United States, Chile, Hungary, Germany, Norway, and Saudi Arabia.
*Updated with additional details from Symantec