A flaw in the Outlook Web Access module in Microsoft's Exchange 5.5 email system could allow unauthorised access to users' mailboxes, the company warned late yesterday.

The problem lies in the way Outlook Web Access handles inline script in HTML email messages, Microsoft said in a security bulletin.

An attacker can get full control over a mailbox when an email with embedded malicious code is opened using Microsoft's Internet Explorer browser and Outlook Web Access, Microsoft said.

Although the attacker can delete mailbox contents, move and send messages as if they were the user, it isn't possible to send email to addresses in the user's address book, preventing a mass-mailing attack, Microsoft said.

Outlook Web Access allows users to access their email via the web, rather than using the Outlook client software on their PCs.

Microsoft is having a tough time securing Outlook Web Access. In June it took the company three patches to plug a similar hole. The first and second patches for the hole, which affected both Exchange 2000 Server and Exchange 5.5, left administrators with dysfunctional email systems.

Microsoft, which gives the vulnerability a 'moderate' severity rating, urges administrators who have deployed Outlook Web Access to immediately install a patch to fix the flaw. The patch is available from Microsoft's TechNet website.