Original URL: http://www.theregister.co.uk/1999/05/18/ms_office_virus_could_infect/

MS Office virus could infect without you opening attachment

If your browser automatically executes a CALL function, then all sorts of horrid things may be possible

By John Lettice

Posted in Business, 18th May 1999 10:21 GMT

Israeli security outfit Finjan Software has warned of how a potentially serious Excel-related virus could spread and inflict damage without the recipient opening an email attachment. Finjan says that "All 3.x and 4.x versions of the Microsoft Internet Explorer Browsers and Netscape Navigator browsers 3.x and 4.x (except Navigator 4.5) are vulnerable, as well as all HTML-aware email applications such as Outlook 98." The virus in question is Russian New Year, which uses the Excel CALL function in Office 95 and Office 97. This allows external executables to be started from within a spreadsheet cell, without the user knowing it's happening. Finjan explains how this would work via a browser: "On a Web page, Web developers include services to various file content types from a server to a browser. Suppose the files end with .XLS extensions. Then it is likely that these files will be associated with the Excel program. In this case, the .XLS files transferred to a browser will be passed immediately and processed by the referenced application - in this case, Excel. When Excel is opened, it executes functions in the cells of the spreadsheet. If one of the functions has a maliciously coded CALL function then it is possible that the Excel spreadsheet can be used to copy an executable program to the hard disk and execute it." But that doesn't mean you have to physically open the link yourself. Vulnerable browsers and email programs can execute the CALL function automatically without the email actually being opened, therefore it seems conceivable that the infection could spread without users even noticing it was happening. Freelance writer Deborah Radcliff reported on this a few days ago in Computerworld, and she comes up with some possible consequences. A mass mail could be used to distribute the virus, which could be used for espionage purposes (suck data from your corporate rivals) or for sheer destruction, creating and writing data to the recipients' hard disks. She also suggests the possibility that the Melissa approach, where the virus apparently comes from a colleague or friend, could be used in conjunction with Russian New Year. According to Finjan, the solutions are convoluted, and not particularly attractive for people who use the CALL function frequently. You need to run Office 97 (there's no fix for 95) with service packs 1 and 2 installed and the Microsoft patch to disable the CALL function. If you're using IE 3.x, upgrade to 4.x and set the security level to highest. Navigator users should switch to 4.5. Our thanks to Windows 98 Central, a useful site for monitoring all things Windows-related, for drawing this one to our attention. ®