A malicious PDF document that exploits bugs in Adobe's Reader and Acrobat software is on the loose, Symantec said Tuesday, just hours after Adobe patched the programs.

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

The rogue PDF document is attached to spammed e-mail, and arrives with a filename such as YOUR_BILL.pdf or INVOICE.pdf, said Symantec. It exploits the "mailto:" protocol vulnerability disclosed more than a month ago by U.K.-based researcher Petko Petkov.

Adobe fixed the flaw Monday and released updated 8.1.1. editions of both Reader and Acrobat that plug the hole. Users of older versions of the popular programs must either upgrade to 8.1.1 or apply one of the temporary work-arounds Adobe provided to stifle attacks. 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 timeline.

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 piece of attack code is a dedicated 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 this morning. 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 Corp.'s responsibility. The Redmond, Wash.-based software vendor 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 to the PDF exploit.

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