Windows malware slipped past Apple's eye and has been found tucked into software available on the company's iOS App Store.
Although the malware, labeled a worm by Microsoft and tagged as "Win32/VB.CB" by the company, is ineffective against Apple's iOS and OS X operating systems, it may pose a threat to iTunes customers who download iPhone and iPad apps to their Windows PCs before syncing to their mobile devices.
CNET first reported on the worm earlier Tuesday.
A user reported Win32/VB.CB to Apple's support forum around 10:30 a.m. ET Tuesday. The user, identified only as "deesto," said that his or her OS X antivirus warned that "Instaquotes-Quotes Cards for Instagram" was infected.
"I just downloaded two apps from iTunes, and one of them has been flagged by ClamXav as being a virus," wrote deesto.
While others commenting on the same support thread initially suspected the warning may have been a false positive by the antivirus software, CNET and Computerworld separately confirmed that the app was, in fact, harboring the worm.
Computerworld extracted the app's .ipa archive -- the packaging format Apple uses to deliver its apps -- on a Windows 7 PC, then scanned the system using Microsoft's free Security Essentials antivirus program.
Security Essentials flagged the file and warned that it contained Win32/VB.CB.
According to Microsoft's website, the worm harks back to 2008 and is known by several other names, including W32.Imaut.AS (Symantec), W32/Autorun.worm.h (McAfee) and W32/VB-DGA (Sophos).
Microsoft said that the worm "attempts to spread via Yahoo! Messenger ... [and] may also connect to a remote server to download arbitrary files."
Another commenter on the support thread downplayed the threat, even to Windows users, and posed a possible explanation for the infected app.
"It is nothing to worry about," said the commenter, tagged as "etresoft," several hours after deesto opened the discussion. "Considering where this virus is located, it wouldn't even hurt a Windows machine. It looks like an accident from the developer's infected Windows machine."
As of 3 p.m. ET, Instaquotes-Quotes Cards for Instagram was still available on Apple's iOS App Store. According to iTunes, the app launched July 19. On Tuesday, the developer's Facebook page included a comment from a user warning that the app was infected.
The developer, identified as Ilyas Hassani of Morocco, could not be reached for comment, and the website he referenced on his Facebook page appeared to be offline.
Hassani joined Facebook on June 1.
Microsoft's Security Essentials antivirus flagged the installation package for an iOS app as infected with a four-year-old worm.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is email@example.com.
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