The flaws disclosed last week in Adobe System's Reader and Acrobat programs have been used to exploit computers since at least January via malicious banner advertisements, security analysts are reporting.

Adobe issued patches last Wednesday for Reader and Acrobat, but the company did not detail the flaws.

Problems with Adobe's software can potentially affect millions of PC users, since the company's software is widely used to read PDF (Portable Document Format) files. Most people regard PDFs as harmless.

"From our standpoint, it appears that this PDF-based attack has been quite successful, affecting many thousands of users throughout the world," wrote Hon Lau on Symantec's Security Response Weblog.

Greg McManus of iDefense Labs, the security arm of VeriSign, reported one of the vulnerabilities to Adobe in October, according to a post by the SANS Institute, a computer security organization.

Since hackers have been apparently using the Adobe flaws since January, it raises the question how they discovered the flaw.

Lau wrote that the "swiftness of the exploit appearing in the wild suggests that leaks had occurred."

However, it appears that the vulnerabilities in Reader and Acrobat were disclosed in a responsible way, Lau wrote.

The flaws in the programs allow a hacker to create a malicious PDF document. If opened by a victim, that document downloads a malicious Trojan that Symantec calls "Zonebac."

Zonebac was first detected in 2006. It shuts off a user's security software as well as downloads other bad software. The latest version also appears to taint search engine results, Lau wrote.

In January, iDefense noticed that the malicious PDF document was being delivered through malicious banner advertisements. Symantec's Lau wrote that it's not immediately clear how the PDF file is delivered, but that the banner ads could be redirecting people to other harmful Web sites with the file. Also, spam messages may be carrying the bad file as an attachment.

Malicious banner ads can be particularly dangerous since the ads can show up on legitimate Web sites. Online advertising companies have struggled to keep these ads off their networks. Sometimes, hackers will approach the networks with what is a legitimate ad and then substitute it for a malicious one. Many of those bad ads have exploited vulnerabilities in Adobe's Flash multimedia technology.

Adobe's Reader and Acrobat are designed to regularly look for updates, but users are advised to upgrade to the patched version, 8.1.2.