Date: Thu Oct 27 02:56:23 2005
From: sk at groundzero-security.com (sk)
Subject: Skype security advisory

i didnt test it myself, but since UDP is a connectionless protocol i suspect
it works without any user interaction.

- sk
http://www.groundzero-security.com

----- Original Message ----- 
From: "Brown, Bobby (US - Hermitage)" <bobbrown@...oitte.com>
To: <full-disclosure@...ts.grok.org.uk>
Sent: Wednesday, October 26, 2005 9:53 PM
Subject: RE: [Full-disclosure] Skype security advisory


> I have the question, can the exploit be perform with no interaction of the
user other than having the program running waiting for a connection or is it
only valid after a user accepted a connection and then the flaw is
exploited?
>
> BB
>
>
> -----Original Message-----
> From: full-disclosure-bounces@...ts.grok.org.uk
[mailto:full-disclosure-bounces@...ts.grok.org.uk] On Behalf Of . EADS CCR
DCR/STI/C
> Sent: Tuesday, October 25, 2005 12:17 PM
> To: full-disclosure@...ts.grok.org.uk; bugtraq@...urityfocus.com;
vulndev@...urityfocus.com
> Subject: [Full-disclosure] Skype security advisory
>
> Synopsis
> ========
>
> The EADS/CRC security team discovered a flaw  in  Skype  client.
>
> Skype is a P2P VoIP software that can bypass firewalls  and  NAT
> to connect to the Skype network. Skype is very  popular  because
> of its sound quality and ease of use.
>
> Skype client is available for Windows, Linux,  Mac  OS  X,   and
> PocketPC.
>
> A remotely exploitable flaw exists in  the  parser  of  packets.
> Exploitation  is  possible  through  a  single    UDP    packet.
>
>
> Impact
> ======
>
> An attacker can  send  a  specially  crafted  packet  that  will
> trigger a heap overflow condition and execute arbitrary code  on
> the target. Hence, an attacker can  gain  full  control  of  the
> target. Conversely to  what  is  written  in  Skype's  advisory,
> remote code execution *is* possible.
>
>
> Affected Versions
> =================
>
> Skype for Windows (including XP SP2 hosts):
> All releases prior to and including 1.4.*.83
>
> Skype for Mac OS X:
> All releases prior to and including 1.3.*.16
>
> Skype for Linux:
> All releases prior to and including 1.2.*.17
>
> Skype for Pocket PC:
> All releases prior to and including 1.1.*.6
>
>
> Description
> ===========
>
> Skype uses several  data  formats.   Each  format  has  its  own
> specific parser. Note that data format  will  not  be  described
> here, for the sake of clarity. A specific encoding  is  used  to
> store numbers, that will be referred  as  VLD  (Variable  Length
> Data) in this advisory.
>
> The data causing the overflow has the following format:
> ------------------------------------ 
> | Object Counter*  | M objects     |
> | M (VLD)          | (VLD)         |
> ------------------------------------ 
> * The first number in the packet is the amount of forthcoming
> objects.
>
> The amount of memory allocated by the  parser  is  prone  to  an
> integer wrap-around. The allocated  size  is  4*M.   Thus,   the
> overflow occurs when M is greater than 0x40000000:  e. g.   when
> M=0x40000010, HeapAlloc(0x40) is called, but  up  to  0x40000010
> objects are effectively read in  the  packet  and  written  into
> memory.
>
> Since the attacker controls both M and all other objects in  the
> packet, he can overwrite an  arbitrary  amount  of  memory  with
> chosen values, thus easily  gaining  control  of  the  execution
> flow.
>
> The corresponding parsing code roughly translates in C as
> following:
>
> --------------------------------------------------------- 
> // read a VLD from input stream
> // return 0 on error
> int get_vld(unsigned int*);
>
> unsigned int object_counter;
> unsigned int i;
> unsigned int * tab_objects;
>
> // read object count (M)
> if (get_vld(&object_counter)==0)
>         fault();
>
> // allocate memory to store sub-objects
> tab_objects = HeapAlloc( sizeof(unsigned int) * object_counter );
> if (tab_objects ==NULL)
>         fault();
>
> // read and store M sub-objects
> for (i=0;i<object_counter;i++)
> {
>         if (get_vld(&tab_objects[i])==0)
>                 fault();
> }
>
> return;
> --------------------------------------------------------- 
>
>
> Exploitation
> ============
> We were able to  design  a  proof-of-concept  exploitation  code
> targeting Windows XP SP2 and Linux clients using  a  single  UDP
> packet.  Remote  exploitation  is  also  possible  through  TCP.
>
> Due to favorable environmental conditions, this particular  heap
> overflow *is* also exploitable on  heap-protected  systems  such
> as Windows  XP  SP2  and  some  Linux  distributions.   This  is
> possible because Skype stores function  pointers  in  the  heap,
> and  those  pointers  can  be  overwritten  by  the    overflow.
>
>
> Detection
> =========
> As Skype uses encryption mechanisms, it seems difficult for  any
> IDS/IPS  to  be  able  to  detect   the    offensive    payload.
>
>
> Solution
> ========
> Skype has issued fixes. Details are available in their advisory:
> http://www.skype.net/security/skype-sb-2005-03.html
>
>
> Vendor response
> ===============
> Skype advisory:
> http://www.skype.com/security/skype-sb-2005-03.html
>
> Disclosure timeline
> ===================
> Oct 17 2005: EADS CRC contacted Skype Security Team
> Oct 17 2005: Skype responded to EADS CRC
> Oct 25 2005: new patched version available
>
>
> Legal notices
> =============
> Copyright (c) 2005 EADS/CRC All rights reserved.
>
> This  EADS  CRC  Security  Bulletin  may  be   reproduced    and
> distributed, provided that the Bulletin is not modified  in  any
> way, is attributed to EADS/CRC, and provided  that  reproduction
> and  distribution  is  performed  for  non-commercial  purposes.
>
> This EADS CRC Security Bulletin is provided to  you  on  an  "AS
> IS"  basis  and  may  contain  information  provided  by   third
> parties. EADS CRC makes no guarantees or warranties  as  to  the
> information contained herein.
>
> ANY AND ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING  WITHOUT
> LIMITATION  WARRANTIES  OF  MERCHANTABILITY,   FITNESS  FOR    A
> PARTICULAR PURPOSE, OR NON-INFRINGEMENT, ARE HEREBY DISCLAIMED.
>
> Contact
> =======
> dcrstic.ccr <.a.t.> eads.net
>
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