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Internet Explorer URL Decoding Zone Spoofing Technical Details (MS05-014)
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SUMMARY
The method used for Windows security zone evaluation fails when characters
in the URL are encoded in a certain way. Internet Explorer can be tricked
to think that a document belongs in "My Computer" zone when it actually
resides on an Internet server. JavaScript in such document can be used to
execute arbitrary code because documents in "My Computer" zone are
normally trusted and given more privileges than documents on Internet.
A malicious user can use this vulnerability to do any action on the victim
system with the victim user's privileges - transfer files, run programs,
etc. No further user interaction is required apart from viewing a web page
created by the attacker. In the e-mail attack scenario the victim user is
usually required to click a link in the e-mail.
DETAILS
Somewhere in the process of evaluating the security zone for URLs,
hex-decoding (the %xy notation) is done more than once for a single URL,
ie. the decoded URL is decoded again. This causes some undesired effects
if the URL contains certain special characters multiply encoded.
Unlike some other operating systems, Windows allows the % sign in
hostnames, so a URL containing such encoding works in Internet Explorer -
given that the hostname resolves correctly to the attacker's IP address.
The attacker can then host e.g. an HTML document on the server, which
Internet Explorer misinterprets as belonging in "My Computer" zone.
A proof-of-concept exploit was tested with Internet Explorer 6 on Windows
2000 and Windows XP. The exploit successfully launches an
attacker-supplied EXE program when the victim user visits a web page
containing the exploit. A full list of vulnerable versions is included in
Microsoft's bulletin.
Vendor Status:
Microsoft was informed of the problem on February 16th, 2004. A
preliminary patch was first produced in September 2004 and Microsoft sent
it to me for testing. However it turned out that the fix didn't correctly
protect from a variation of the exploit, so the release was delayed.
The final patch and Microsoft's bulletin is available at:
<http://www.microsoft.com/technet/security/bulletin/ms05-014.mspx>
http://www.microsoft.com/technet/security/bulletin/ms05-014.mspx
ADDITIONAL INFORMATION
The information has been provided by <mailto:jouko@iki.fi> Jouko
Pynnonen.
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