A PDF file that attacks Adobe's Reader and Acrobat software has been released, just hours after a patch became available, according to Symantec.

"This mass mailing of exploit files may be an attempt to leverage the exposure window between patch release and widespread adoption of the fix," Symantec warned customers of its DeepSight threat intelligence network.

The rogue PDF is attached to spam, and arrives with a file name such as YOUR_BILL.pdf or INVOICE.pdf, said Symantec. It exploits the "mailto:" protocol vulnerability disclosed more than a month ago by UK-based researcher Petko Petkov.

Adobe released patches in the 8.1.1. editions of Reader and Acrobat early this week. Users of older versions must either upgrade or apply one of the temporary workarounds Adobe has detailed. On Monday Adobe did say that it would update Adobe Reader 7.0.9 and Acrobat 7.0.9 "at a later date" but did not set a definitive time.

When recipients open the attacking PDF, it launches a Trojan horse dubbed "Pidief.a" that knocks out the Windows firewall and then downloads another piece of malware to the compromised computer. That second attack is a downloader that can retrieve files from a remote server and, at the attacker's command, pull them onto the hacked PC.

"The host [server] is live and still currently serving [the downloader] over FTP," said Symantec. The server is well known for hosting malicious software, the warning added.

Although Adobe patched the newest versions of Reader and Acrobat, the vulnerability is ultimately Microsoft's responsibility. Microsoft owned up to that two weeks ago, saying that it would patch common protocol handlers such as mailto: in Windows XP and Windows Server 2003.

Only users running the Internet Explorer 7 browser on XP or Windows Server 2003 are vulnerable.

"[Users] are advised to apply the patches outlined in Adobe Advisory APSB07-18 as soon as possible," Symantec recommended.