Update: Microsoft launches last-ditch defenses against Office exploits

IT can block users from opening Office docs as last resort during zero-day attacks

Tacitly acknowledging that it cannot completely secure its Office suite against attackers, Microsoft Corp. yesterday promoted a last-ditch defense that lets administrators block users from opening documents.

Prosaically dubbed "File Block," the feature allows enterprise IT staffers -- or a technically astute end user -- 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 posted yesterday.

Even if Office is religiously patched, Microsoft recognizes that attacks may be successful and sees File Block necessary as a stopgap. "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 e-mail carrying an Office document attachment sent to just one recipient in a company or organization. Last month, MessageLabs Ltd. noted that micromanaged attacks like these had increased 10 times over last year. In April, 95% of such attacks used malformed Office documents as an exploit vector.

Microsoft designed File Block for just those victims, and in fact targeted the technique at enterprises by declining to build a tool sporting a graphical interface for turning file type blocking on and off. 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 regular monthly security patch cycle. (Users can also manually download the Word 2003, Excel 2003 and PowerPoint 2003 updates from the Microsoft site.)

"The update for file format management is geared towards being deployed and managed through tools like Group Policy," said Josh Edwards, a Microsoft technical product manager. "The types of attacks that we are seeing, using file formats, are extremely targeted at specific users in specific organizations. These new tools are designed to assist those organizations concerned about these issues to better address the evolving security landscape."p>

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), a 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-style environment 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 labeled MOICE, like the new File Block, a defense 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 Cupertino, Calif.-based 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.

Both File Block and MOICE, Microsoft said, will be integrated into its security advisories and bulletins, where it often spells out defensive steps users can take until a patch is deployed. "We will include the ability for customers to implement the group policy changes, in combination with MOICE, in our evaluation of mitigations and workarounds for customers," said Edwards.

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, June 12.

Copyright © 2007 IDG Communications, Inc.

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