Skype callto:// BoF technical details

["Berend-Jan Wever" <skylined () edup tudelft nl>]

Skype reported they've found a remotely exploitable BoF in the callto:// URI handler. New version has been released.
http://www.skype.com/products/skype/windows/changelog.html
http://secunia.com/advisories/13191/

Technical details:

The bufferoverflow happens when a skype user clicks on a "callto://username" link with a username longer then 4096 
characters that does not exist: An error message is created and put into a buffer without correct size checks. The 
errormessage and buffer are unicode but unicode characters are filtered out and replaced with '?'. Only printable ascii 
characters seem to get through. A return address can be overwritten as well as the SEH. Exploitation is complicated by 
the fact that return addresses have to be in range 0x00??00??.

Webbrowsers like MSIE do not support URI's long enough to trigger the BoF. To exploit it, one could send a skype user a 
callto:// link in a private message and trick him/her into clicking it.

If one would want to, one could write a skype worm with this. User interaction would be required: they'd have to click 
the link.

Cheers,
SkyLined


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