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Re: Skype callto:// BoF technical details
From: Fabian Becker <neonomicus () gmx de>
Date: Tue, 16 Nov 2004 20:50:07 +0100

Berend-Jan Wever wrote:

Skype reported they've found a remotely exploitable BoF in the callto:// URI handler. New version has been released.

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.


They fixed it without knowing of the callto:// thing I suppose cause I wrote them an email saying that the quick-call field is exploitable, too. This was fixed within the new version. Maybe your flaw is fixed, too, if not, I think it soon will be :)

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