Microsoft is recommending IT administrators block users opening Office documents as a way to prevent attacks, tacitly acknowledging that Office cannot be completely secured.

Called "File Block," the feature allows administrators - or technically astute end users - to declare the specific Office file types that can or cannot be opened by Word 2003/2007, Excel 2003/2007 and PowerPoint 2003/2007. File type restrictions are spelled out by editing the Windows registry or through Group Policy settings.

Microsoft pitched File Block as a way to keep individual users from doing the enterprise harm. "Blocking specific Office file types allows administrators to temporarily deny users the ability to open certain files, such as when a threat of attack from a given Office file type exists," the company said in a security advisory.

Even if Office is religiously patched, Microsoft recognises that attacks may be successful and sees File Block as a necessary stop-gap. "Threat[s] may occur even if your Office 2003 or 2007 Office software has been kept up-to-date, for example if a previously unknown new 'zero-day' vulnerability in Office is discovered and then used to attack users of Microsoft Office," read the advisory.

Attacks carried out via malicious Office documents climbed during 2006, and are continuing this year. Many of the attacks have been very tightly targeted, say security professionals, often one-on-one, with a single email carrying an Office attachment sent to just one recipient. Last month, MessageLabs noted that micro-managed attacks like these had increased ten times over last year. In April, 95 percent of such attacks used malformed Office documents as an exploit vector.

File Block functionality is built into Word, Excel, and PowerPoint 2007 - although disabled by default - and was added to the 2003 editions by updates unveiled two weeks ago as part of the regularly monthly security patch cycle. (Users can also manually download the Word 2003, Excel 2003, and PowerPoint 2003 updates from the Microsoft site.)

Security researchers urged users to consider activating the new feature. "File Block ability is a natural addition for most Office environments," said Symantec in a posting to its DeepSight threat management system customers today. "These new applications provide additional remediation and control over what is considered a significant threat to today's Office environments."

Also on Monday, Microsoft released MOICE (Microsoft Office Isolated Conversion Environment), the conversation tool announced earlier this month that converts Office 2003 format documents into the more secure Office 2007 formats to strip out possible exploit code.

MOICE converts files in an isolated sandbox so that if the worst happens and an attacker tries to exploit and/or infiltrate the converter itself, the utility simply crashes in its protected cocoon without damaging the system or allowing rogue code to install. MOICE requires Office 2003 and the Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats; the latter is a free add-on download.

Microsoft labelled MOICE, like the new File Block, a defence against imminent attacks or those already in play. "MOICE provides a mitigation that administrators and users can use to protect themselves when the threat of an attack from potentially unsafe Office 2003 binary documents exists," said Microsoft.

Symantec applauded MOICE - with one caveat. "It has been reported that MOICE may add noticeable overhead when opening and converting Office files," the security company told customers. "However, for those who strive to ensure that received Office documents are safe to handle and do not contain malicious code these free tools are a highly suggested addition for your MS Office security arsenal."

MOICE's performance overhead stems from its two-stage file conversion: the first from old 2003 format to the new 2007 format, then another from new back to old.

MOICE can be downloaded immediately by visiting Microsoft Update and installing the recommended update. It will also be pushed out to users in the next scheduled security update, 12 June.