Vulnerabilities have been identified in two widely-used Microsoft Corp. products, Microsoft Word and Outlook Express.
In Microsoft Word's case, an attacker could steal data from a victim's hard disk, according to alerts posted on the Bugtraq Web site weeks ago and acknowledged by Microsoft on Friday.
It would work like this: The attacker creates a Word 97 document and embeds hidden fields, such as the "IncludeText" field, in it. The attacker then e-mails the malicious document to the intended victim. When the victim opens the document, the fields retrieve data from the hard disk. The attacker would then receive the stolen data in the document when the victim e-mails it back to him.
In order to exploit this vulnerability, an attacker would have to know the names and the locations of the victim's files containing the information he wanted to steal. The hidden fields would look for data in specific files, and not do a general scan of the hard disk. Because of this complexity, Microsoft and most security experts qualified their warnings concerning the vulnerability by noting that the potential threat of being attacked using fields is limited.
"We believe there are some important mitigating factors," said Lynn Terwoerds, Security Program Manager at Microsoft, referring to the difficulty of exploiting the vulnerability. "A successful attack, in which several best practices and mitigating factors are not applied, could potentially allow a malicious user to view the contents of a targeted file."
A long-standing feature of Microsoft Word, fields allow users to create documents with information that can be automatically updated or changed without requiring action by the author, such as dates and page numbers.
Fields can easily be hidden in documents -- buried in the header or footer region and formatted with invisible text, for example. Once hidden in the body of a document, security experts warn, fields can easily and silently access text and graphics on a user's hard drive. The vulnerability was shown to exist in Microsoft Word 97, but could not be duplicated in Word 2000 or Word 2002, according to statements posted on the Bugtraq Web page.
And even with the seeming difficulty of exploiting the vulnerability, some security experts warn, the widespread use of Microsoft Word increases the likelihood of successful attacks.
"The fact that you have all these organizations using Microsoft Office products as their main communications tools, and the fact that you have (files) in standard predictable places with standard predictable names puts those things at risk," said Sean Smith, Assistant Professor of Computer Science at Dartmouth College, who has studied the use of fields in Word documents to construct attacks.
Microsoft's popular Outlook Express e-mail program was named in another advisory issued Thursday by Beyond Security Ltd., a computer security consulting company based in Israel.
By taking advantage of a standard Outlook Express feature that enables users to automatically break up large outgoing e-mails into two or more smaller files, technicians at Beyond Security were able to sneak viruses past many common Simple Mail Transport Protocol (SMTP) content filtering engines, which organizations use to weed out viruses and other attacks from incoming e-mail.
The problem lies not with Microsoft's e-mail program, experts say, so much as with the filtering engines from third parties that failed to detect the ruse.
"Outlook Express is the only program that's doing stuff right. This is a documented feature," said Aviram Jenik, CEO of Beyond Security, citing the Internet Engineering Task Force (IETF) standards documentation that calls for such fragmenting of e-mail messages. "(SMTP filtering) products are supposed to be able to filter these messages."
Many popular filtering programs did not, however. Among the products that released patches to eliminate the vulnerability was Trend Micro Inc.'s InterScan VirusWall. The vulnerability of many other e-mail scanning programs to SMTP fragmenting was not clear.
Trend Micro could not be reached for comment.
Some products, including Symantec Corp.'s Norton AntiVirus for Gateways were not affected because their software already blocked or contained options that allow administrators to block multipart e-mail messages.
"This is a default option (of Norton AntiVirus) and has been (in the product) for a significant amount of time," said Vincent Weafer, Senior Director of Symantec Security Response at Cupertino, California-based Symantec.
But that approach may have its own limitations when it is used as the sole means of preventing attacks, experts warn.
"That's a (solution) that's just crying out for another vulnerability to surface," said Jenik. "These programs should assemble the message and check its content before rejecting it."
Besides, Jenik added, e-mail fragmenting has its place, even in a world where more and more people enjoy high bandwidth Internet connections.
"We've all probably had e-mails rejected by a server because they were too large. I've personally had to manually fragment messages into smaller chunks, so if Outlook does it for me, great."