The program could then allow the attacker to take control of the user's machine and add, change or erase data, communicate with Web sites or reformat the machine's hard drive, Microsoft said in a security bulletin.
The vulnerability is caused by HTML-based e-mail that contains an executable attachment whose MIME (Multipurpose Internet Mail Extensions) is incorrectly given as one of several unusual types. The flaw in IE will cause the attachment to be executed without displaying a warning, Microsoft said. MIME is an Internet standard for encoding binary files as e-mail attachments.
The versions of IE affected are 5.01 and 5.5. These versions of the browser are not capable of handling the unusual MIME types and automatically execute the attachment without alerting the user, Microsoft said. IE should only open files if the user explicitly asks for them to be opened. However, the flaw allows an attacker to circumvent this security measure, the company said. By default, IE downloads files automatically.
An attacker could exploit the vulnerability in one of two ways, according to Microsoft. The attacker could host the HTML e-mail on his or her Web site and try to persuade the user to visit it or the attacker could send the e-mail directly to the user, Microsoft said.
Microsoft already has created a patch for the IE vulnerability, which was brought to the company's attention by Juan Carlos Cuartango, a telecommunications engineer with Kriptopolis, a Spanish online magazine that focuses on Internet cryptography, security and privacy. The patch prevents e-mails from automatically launching executable attachments, Microsoft said.
Russ Cooper, an analyst at TruSecure was unaware of the vulnerability but said there have been similar problems with MIME before. One was discovered in 1997 in the MIME header that allowed for a buffer overrun, he said. This would allow a hacker to run embedded malicious code in an e-mail and have it be executed when read. This affected both Microsoft Outlook and Netscape Communications' Communicator. Another MIME vulnerability was also identified in late 2000, he said.