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Title: Incorrect VBScript Handling in IE can Allow Web Pages to
Read Local Files
Date: 21 February 2002
Software: Internet Explorer
Impact: Information Disclosure
Max Risk: Critical
Bulletin: MS02-009

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-009.asp.

---

Issue:

Frames are used in Internet Explorer to provide for a fuller
browsing experience. By design, scripts in the frame of one site or
domain should be prohibited from accessing the content of frames
in another site or domain. However, a flaw exists in how VBScript
is handled in IE relating to validating cross-domain access. This
flaw can allow scripts of one domain to access the contents of
another domain in a frame.

A malicious user could exploit this vulnerability by using
scripting to extract the contents of frames in other domains,
then sending that content back to their web site. This would
enable the attacker to view files on the user's local machine
or capture the contents of third-party web sites the user visited
after leaving the attacker's site. The latter scenario could,
in the worst case, enable the attacker to learn personal
information like user names, passwords, or credit card information.

In both cases, the user would either have to go to a site under
the attacker's control or view an HTML email sent by the attacker.
In addition, the attacker would have to know the exact name and
location of any files on the user's system. Further, the attacker
could only gain access to files that can be displayed in a browser
window, such as text files, HTML files, or image files.

Mitigating Factors:

• The vulnerability could only be used to view files. It could not
  be used to create, delete, modify or execute them.

• The vulnerability would only allow an attacker to read files that
  can be opened in a browser window, such as image files, HTML
  files and text files. Other file types, such as binary files,
  executable files, Word documents, and so forth, could not be read.

• The attacker would need to specify the exact name and location of
  the file in order to read it.

• The email-borne attack scenario would be blocked if the user were
  using any of the following: Outlook 98 or 2000 with the Outlook
  Email Security Update installed; Outlook 2002; or Outlook
  Express 6.

Risk Rating:

• Internet systems: Moderate
• Intranet systems: Moderate
• Client systems: Critical

Patch Availability:

• A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-009.asp
  for information on obtaining this patch.

Acknowledgment:

• Zentai Peter Aron, Ivy Hungary Ltd (http://w3.ivy.hu/)

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