Corel on Friday rebuked Microsoft for releasing an update to the Office 2003 software suite that disables some Corel files that are saved in older formats.

Microsoft has said it designed Office 2003 Service Pack 3 to block CorelDraw .CDR files, as well as some other formats, because they're vulnerable to hackers. Corel isn't buying that claim.

"Corel has unsuccessfully tried to figure out the basis for categorizing .CDR files as less secure," said Gerard Metrallier, Corel's director of product management, in an e-mailed statement.

Metrallier added that security research organizations such as FrSIRT and US-CERT have not included .CDR files in their lists of known threats.

Metrallier said Corel officials have contacted Microsoft in hopes of receiving an explanation. "We want to make 100% certain our file formats pose no security concern for any of our users," he said.

Along with Corel's .CDR format, Office 2003 SP3 won't open files that are stored in a number of other formats. Among them are those used by older versions of Microsoft Word, Excel, and PowerPoint, IBM's Lotus Notes, and Corel's Quattro spreadsheet.

Office 2003 users report that the service pack fails to warn that its installation could prevent access to the older files, according to a number of blog posts complaining about the move.

"It is unreasonable, and stupid to boot," said S Yousef, writing on the Slashdot technology blog.

Microsoft on its Web site provides a detailed workaround for those who want to defeat SP3 for Word 2003's auto-blocking. But the workaround calls for users to modify their computers' registry settings -- a risky procedure that can render a PC unusable if not done correctly.