Virus

HTML/Exploit.MIME

Analysis

MIME Exploits
There is a flaw in certain versions of Internet Explorer prior to (5.5 and earlier except for v5.01 with SP2) that would allow an attacker to send a specially crafted "wrapper" email that, when opened,could automatically launch the file attachment. This flaw affects all programs that uses Internet Explorer HTML rendering engine (IE, Outlook, etc.). Several viruses are known to use this exploit, including W32/Klez.H-mm, W32/Frethem.F-mm and W32/Swen.A-mm.

FortiGate systems with FortiOS version 2.36 and 2.50 will detect known variants of this "wrapper" email generically under the name MIME.Exploit.gen. FortiGate systems with FortiOS 2.5x and above will detect this threat type generically under the names MIME.Suspicious.exe or MIME.Exploit.gen.

This flaw, and instructions on how to correct it, is covered in great detail by Microsoft Corporation in Technical Security Bulletin MS01-020: Incorrect MIME Header Can Cause IE to Execute E-mail Attachment .

The following section of this document contains several important excerpts from MS01-020. For the complete bulletin, see the following Microsoft bulletins:
http://www.microsoft.com/technet/security/bulletin/MS01-020.asp
http://support.microsoft.com/default.aspx?scid=kb;EN-US;290108

An expanation, to start...
Because HTML e-mails are simply web pages, IE can render them and open binary attachments in a way that is appropriate to their MIME types. However, a flaw exists in the type of processing that is specified for certain unusual MIME types. If an attacker created an HTML e-mail containing an executable attachment, then modified the MIME header information to specify that the attachment was one of the unusual MIME types that IE handles incorrectly, IE would launch the attachment automatically when it rendered the e-mail.

An attacker could use this vulnerability in either of two scenarios. She could host an affected HTML e-mail on a web site and try to persuade another user to visit it, at which point script on a web page could open the mail and initiate the executable. Alternatively, she could send the HTML mail directly to the user. In either case, the executable attachment, if it ran, would be limited only by user’s permissions on the system.

This vulnerability can be eliminated either by installing the patch or upgrading to an unaffected version. However, as discussed in the FAQ and in Knowledge Base article Q308411, customers who upgrade to IE 6 on Windows 95, 98, 98SE or ME must select either Typical Install (this is the default) or Full Install in order to eliminate the vulnerability.

Digging deeper. What’s a MIME type?
Let’s start with what MIME is. MIME is an acronym for Multipurpose Internet Mail Extensions. It’s a widely used Internet standard for encoding binary files as e-mail attachments. When an e-mail contains a binary attachment, it must specify what type of file the attachment is, so the mail program can interpret it correctly.

In the case of this vulnerability, IE doesn’t correctly handle certain types of fairly unusual MIME types. If an attacker created an e-mail message containing an executable attachment, and specified that it was one of these MIME types, IE would execute the attachment rather than prompting the user.

For those who want to dig deeper still, a longer and more complicated description of MIME can be found in the following RFC's:

Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, see: http://www.faqs.org/rfcs/rfc2045.html
Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types, see: http://www.faqs.org/rfcs/rfc2046.html
MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text, see: http://www.faqs.org/rfcs/rfc2047.html
Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures, see: http://www.faqs.org/rfcs/rfc2048.html
Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples, see: http://www.faqs.org/rfcs/rfc2049.html

The MIME Multipart/Related Content-type, see: http://www.faqs.org/rfcs/rfc2387.html

Would IE always execute the attachment?
IE will only execute the attachment if File Downloads were enabled in the Security Zone that the e-mail was opened in. However, File Downloads are enabled in all zones by default.

For more information on how the Security Zone in Internet Explorer works, see: http://support.microsoft.com/default.aspx?scid=KB;EN-US;Q174360

What would this vulnerability enable an attacker to do?
If an attacker created an e-mail that exploits this vulnerability, she could use it in an attempt to run the executable attachment on another user’s computer. She could try to do this through either of two scenarios. First, she could host the HTML mail on her web site, and try to persuade the user to visit it. Second, she could send the email directly to the user.

What kind of actions could the attachment take if it ran?
The attachment would be able to take any action that the user himself could take on his system. If he were an unprivileged user, it might be able to do very little. However, if the user were an administrator on his system, the attachment would be able to do virtually anything, including reformatting the hard drive.

Could an e-mail accidentally be created that would exploit this vulnerability?
No. To create such an e-mail, an attacker would need to create an e-mail containing an executable attachment, then deliberately edit the MIME headers in the mail to be one of the affected types.

Does this vulnerability affect IE 6?
No. You can eliminate the vulnerability by upgrading to IE 6. However, if you are running Windows 95, 98, 98SE or ME, you should be aware that you will need to install IE 6 in a certain way. Specifically, you will need to choose either the Full Install or Typical Install option. (The default installation type is Typical Install). If you choose Minimal Install or Custom Install, the files containing the vulnerability might not be upgraded, and your system could remain vulnerable.

Customers running Windows NT 4.0, Windows 2000, or Windows XP do not need to concern themselves with this contingency, as IE 6 does not provide either a Minimal or Custom Install option when installing on these systems. Any upgrade to IE 6 on one of these systems would fully eliminate the vulnerability. More information on this is available in Knowledge Base article Q308411.

Recommended Action

Check the main screen using the web interface for your FortiGate unit to ensure that the latest AV/NIDS database has been downloaded and installed on your system - if required, enable the "Allow Push Update" option