Adobe's popular PDF file format has generally been considered immune to viruses. But a new virus carried by programs embedded in PDF files raises concerns that the format itself could become susceptible. On Tuesday morning, Network Associates' McAfee antivirus division became aware of the first virus -- known as "Peachy" -- that uses PDF to spread, said Vincent Gullotto, senior director of McAfee's Avert group. Fortunately, those who are simply viewing a PDF, or Portable Document Format, file aren't vulnerable. The virus spreads only by way of Adobe's Acrobat software -- the program used to create PDF documents -- not through Acrobat Reader, the free program that is used to view the files. "There is no way for this to affect Acrobat Reader," said Adobe's Sarah Rosenbaum, director of Acrobat product management. "The code in Acrobat that recognises attachments does not exist in Reader." Peachy exploits an Acrobat feature that allows people to embed other files within a PDF -- attachments that can be opened only by people using Acrobat. "Right now it's considered to be a low risk because we haven't seen it reported to us from a customer," Network Associates' Gullotto said. But the Peachy virus raises the issue that PDF files -- widely used to display documents within Web browsers and email -- could become a new channel for spreading viruses. "What I'm concerned about here is that this could be a new frontier," said Richard Smith, chief technology officer of the Privacy Foundation. "It's considered to be a safe file format." Smith posted news of the virus to the Bugtraq security mailing list on Tuesday. It's clear that if Adobe modified future versions of Reader so that it could read attachments embedded in PDF files, the program could fall victim to Peachy's descendents. Rosenbaum said that while it's possible Adobe might add attachment-handling capability in future editions of Acrobat Reader, the company has no immediate plans to do so. Smith said he believes Acrobat Reader software ultimately could prove susceptible in any case. Indeed, the Computer Emergency Response Team posted news of a vulnerability in the Windows version of Acrobat last year that could let an outside attacker gain control over the computer of a person who simply viewed a PDF file. Adobe patched that hole. Adobe said any popular software becomes a target for security attacks and Acrobat has crossed that threshold. "I think the attraction...has reached a critical level recently," Rosenbaum said. "It's only been in the last 18 to 24 months that PDF...use has really exploded." Acrobat lets people embed different file types within a PDF, including everything from the VBScript programs -- used in the LoveLetter virus -- to an actual executable program, Gullotto said. Peachy is named after a small game in a PDF file that involves finding peaches, Gullotto said. According to a person called Zulu, who said he wrote Peachy, showing the solution to the game runs a VBScript file. The virus then spreads to others using email addresses collected from Microsoft Outlook, Gullotto said. Using PDF bypasses the filters in newer versions of Outlook that ordinarily screen out VBScript files. Through an agreement with Adobe announced in June, McAfee's software is able to scan PDF files, Gullotto said. However, as with other virus types, the software isn't always able to catch new viruses until its definitions are updated. Updated virus descriptions released by McAfee next week will be able to detect Peachy, Gullotto said. But Adobe doesn't currently plan to prevent VBScript or other files from running. To prevent Peachy from being able to run, "the change we would have to make is not to allow VBScript attachments. That is a problem for a lot of our customers," she said. "If they change their opinion, we will do what they want." Users with the full version of Acrobat will have to exercise caution when opening attachments to PDF files. However, opening attachments isn't automatic: a cautionary dialogue box asks if the user wants to proceed. See the Viruses and Hacking News Section for the latest headlines. Have your say instantly, and see what others have said. Click on the TalkBack button and go to the Security forum. Let the editors know what you think in the Mailroom. And read other letters.