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EEYE: Yahoo Webcam ActiveX Controls Multiple Buffer Overflows
From: "eEye Advisories" <Advisories () eeye com>
Date: Fri, 8 Jun 2007 10:58:08 -0700

Yahoo Webcam ActiveX Controls Multiple Buffer Overflows

Release Date:
June 8, 2007

Date Reported:
June 5, 2007

High (Remote Code Execution)

Yahoo!, Inc.

Systems Affected:
Yahoo Messenger 8 for Windows

eEye Digital Security has discovered two critical vulnerabilities in
ywcupl.dll (version and ywcvwr.dll (version included
by default in all releases of Yahoo! Messenger 8.x.  Ywcupl.dll is
Yahoo's Webcam Upload ActiveX Control used by Yahoo! Messenger to stream
content from a user's webcam to other users.  Ywcvwr.dll is Yahoo!
Messenger's Webcam Viewer ActiveX Control used to view any streamed
content.  These files are normally used only when viewing or streaming
webcam content to and from Yahoo Messenger, but they are incorrectly
marked safe for scripting and can be instantiated by any website.
Furthermore they both fail to perform bounds checking on variables
resulting in 2 stack-based buffer overflow conditions that could allow
arbitrary code to execute in the context of the logged-in user. 

The vulnerability is a simple strcpy-based stack buffer overflow within
the ActiveX controls, and can be reliably exploited on all versions of
Windows in order to execute arbitrary code.

Technical Details:
The code for both vulnerabilities is similar, in which they both call
mbscpy on the vulnerable string after allocating only 1023 bytes of

        03ada506        or      dword ptr [ebp-4],0FFFFFFFFh
        03ada50a        cmp     eax,ebx
        03ada50c        mov     dword ptr [esi+210h],eax  
        03ada512        je      ywcupl+0xa5f4 
        03ada518        lea     ecx,[esi+174h]
; Loads the payload into ECX
        03ada51e        call    dword ptr
        7c3a44b3        cmp     dword ptr [ecx+18h],10h
        7c3a44b7        jb      MSVCP71!std::basic_string
        7c3a44b9        mov     eax,dword ptr [ecx+4]
; Payload is in EAX
        7c3a44bc        ret
        03ada524        push    eax
; push payload to the stack
        03ada525        push  3FFh
; Max size of 1023              
        03ada52a        lea   eax,[ebp-414h]
; EAX = 159dfb98 
; (Destination buffer Location)
        03ada530        push    eax

        03ada531        push  offset ywcupl!DllUnregisterServer+0x6498;
"WebcamServer" gets 
; pushed on the stack
        03ada536        lea     ecx,[esi+0F8h]
; ECX loads payload
        03ada53c        call  ywcupl+0x4a72
        03ad4ac1        push    dword ptr [ebp+14h]
; pushes our payload to the stack
        03ad4ac4        push  dword ptr [ebp+0Ch]
; pushes 0x159dfb98 
; (Destination Buffer to copy payload)
        03ad4ac7        call    ywcupl!DllUnregisterServer+0x28f2
        03aebfa6        jmp   dword ptr
[ywcupl!DllUnregisterServer+0x4b9c]     ; Call MSVCR71!mbscpy 

At this point either of the vulnerable ActiveX controls call mbscpy
without bounds checking the 'server' string variable. If this variable
is excessively large it causes a page boundary access violation upon
reaching the bottom of the stack.

After hitting a page boundary the exception handler gets called and
attempts to clean up the access violation.  Since the stack has been
overflowed with a payload, the exception handler loads the payload and
calls it in the following code:  

        7c9037ae        push    dword ptr [ebp+14h]
        7c9037b1        push  dword ptr [ebp+10h]
        7c9037b4        push    dword ptr [ebp+0Ch]
        7c9037b7        push  dword ptr [ebp+8]
        7c9037ba        mov   ecx,dword ptr [ebp+18h]
; ECX loads payload
        7c9037bd        call  ecx
; Calls payload

Retina - Network Security Scanner has been updated to identify this
Blink - Unified Client Security has proactively protected from this
vulnerability since its discovery.

Vendor Status:
Yahoo! has released a patch for this vulnerability which is available

Greg Linares

Related Links:
Retina - Network Security Scanner - Free Trial:
Blink - Unified Client Security Personal - Free For Home Use:
Blink - Unified Client Security Professional - Free Trial:

Terrell Karlsten, Dre, the Super Soeder Bros, Matt, Laurentiu, Richard
D. James, Pikey, Cash, Expanders, Str0ke, William Kurnik, Big Perm,
Dooritto, CSam, Shadow Penguin, and Reverse.  Best of luck to where ever
the winds may take you.

Copyright (c) 1998-2007 eEye Digital Security
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