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Win32: Postmessage API security flaw
From: Palan <palan () myrealbox com>
Date: 13 Mar 2003 21:07:08 -0000
I would like to bring to your notice a certain vulnerability that has
existed in Win 9x platforms for many years and now in Win2k/XP. Most of
us our familiar with password revealers and password stealing trojans.
Though flaws in Windows Messaging API have been show before this one
relates to Windows 2000/XP WM_GETTEXT and EM_SETPASSWORDCHAR message.
The idea behind password capture is a concept called windows sub
classing. You send a SendMessage API with the WM_GETTEXT to the password
edit box it will give you the password stored in the edit box control.
SendMessage API has been secured in Win2k, it will not let you send a
WM_GETTEXT message to another processes window that doesnt below to the
current process thread.
The traditional exploit would look like this:
Visual Basic Code Snippet
'The WM_GETTEXT technique has been blocked in Windows 2k/XP
nchrs = SendMessage(hwnd, WM_GETTEXTLENGTH, 0, 0) + 1
pStr = Space$(nchrs)
nchrs = SendMessage(hwnd, WM_GETTEXT, nchrs, ByVal pStr)
But the same password revealing functionality is possible with the same
relative easiness as before, due to a vulnerability in PostMessage API.
PostMessage API does not seem to check the handle and the type of the
message before sending a message to any message queue.
What Ive shown below demonstrates that we can copy message from a
password edit box control by acting like a regular application message.
'SendMessage posts the message and activates the message
'Postmessage waits for the app to service the message queue
hwnd is the target Edit box handle
'Get the password character from the Edit box
current_passchar = SendMessage(hwnd, EM_GETPASSWORDCHAR, 0, 0)
'Disable the Password Character
'Only Post Message will work for SETPASS in win2k
ret = PostMessage(hwnd, EM_SETPASSWORDCHAR, 0, 0)
This will unmask the password in the field after a few milliseconds;
using a combination of Clipboard copying techniques this flaw could be
As application programmers what you can do:
1.In your Win32 programs make sure you handle the Message Queues for the
Password text boxes, kill all EM_SETPASSWORD and WM_GETTEXT messages.
2.Harden your registry key access, eg. Run/RunOnce keys especially in the
most neglected hive: HKEY_CURRENT_USER.
Finally some day PostMessage API has to be corrected so that it checks
the handle and the type of the message before sending a message to any
Researcher, Networking Lab,
palan () myrealbox com
- Win32: Postmessage API security flaw Palan (Mar 14)