Microsoft plans to patch a newly discovered hole in Microsoft Word in its next monthly patch update, and may release a rare 'out of cycle' fix to address the hole, a company spokesman said.

Microsoft's Security Research Center is analysing the previously unknown vulnerability, which affects Microsoft Word XP and Word 2003 and is already being linked to targeted internet attacks on government agencies in the US and European Union, as well as US government defence contractors, according to security experts. The attacks, which use Word attachments to very targeted 'phishing' email messages, are being likened to a widespread attack on the US government last year that was code-named Titan Rain.

Antivirus firms warned of the unknown Word hole on Friday, after attacks were first detected. When a malicious Word document is opened, an exploit file installs a Trojan program on the affected Windows system with a rootkit component that makes the malicious program almost undetectable. According to research by F-Secure of Finland, the Trojan communicates with a host registered at, a free hosting service based in China.

Microsoft is completing development of a security update for Microsoft Word that addresses the vulnerability. The fix is being tested and could be released on 13 June, with the company's scheduled monthly patch release, or sooner, according to an email sent from a Microsoft spokesman to InfoWorld.

The attacks, or others like them, have been going on for months, and combine malicious Word attachments with email messages that are targeted at individual users within government agencies and corporations, said Johannes Ullrich, CTO at the Sans Internet Storm Center.

Sometimes referred to as 'spear phishing' attacks, the email-borne threats are sent from outside an organisation but are made to appear as if they come from within the company. Because the email attacks are sent to one or two specific users, they are very hard to spot using automated tools, and often use the names of actual employees within an organisation and other tricks to deceive recipients into opening the malicious attachments, Ullrich said.

Previous attacks have also buried attacks in Word documents, which are commonly sent between employees and businesses, and rarely blocked at the network gateway. Often those attacks used known vulnerabilities in Word, but were repackaged in ways that fooled antivirus scanners, Ullrich said.

The latest attacks are similar to an earlier, co-ordinated attack on US government computer systems that was known as Titan Rain. This was also believed to have originated in China, Ullrich said.

In a blog for the MSRC, security program manager Stephen Toulouse said the attacks are not widespread, and recommended reducing the privileges attached to Windows user accounts, a policy known as 'least user privilege', to stop the malicious code from being installed on Windows machines.

Microsoft customers who believe they have been affected by attackers using the Word vulnerability should visit the company's security website.