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Yahoo Webcam ActiveX Controls Multiple Buffer Overflows
EEYE: Yahoo Webcam ActiveX Controls Multiple Buffer Overflows
EEYE: Yahoo Webcam ActiveX Controls Multiple Buffer Overflows

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	=09
	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.

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