ChangeWindowMessageFilter function (winuser.h)

Article
10/13/2021

[Using the ChangeWindowMessageFilter function is not recommended, as it has process-wide scope. Instead, use the ChangeWindowMessageFilterEx function to control access to specific windows as needed. ChangeWindowMessageFilter may not be supported in future versions of Windows.]

Adds or removes a message from the User Interface Privilege Isolation (UIPI) message filter.

Syntax

BOOL ChangeWindowMessageFilter(
  [in] UINT  message,
  [in] DWORD dwFlag
);

Parameters

[in] message

Type: UINT

The message to add to or remove from the filter.

[in] dwFlag

Type: DWORD

The action to be performed. One of the following values.

Value	Meaning
MSGFLT_ADD 1	Adds the message to the filter. This has the effect of allowing the message to be received.
MSGFLT_REMOVE 2	Removes the message from the filter. This has the effect of blocking the message.

Return value

Type: BOOL

TRUE if successful; otherwise, FALSE. To get extended error information, call GetLastError.

Note A message can be successfully removed from the filter, but that is not a guarantee that the message will be blocked. See the Remarks section for more details.

Remarks

UIPI is a security feature that prevents messages from being received from a lower integrity level sender. All such messages with a value above WM_USER are blocked by default. The filter, somewhat contrary to intuition, is a list of messages that are allowed through. Therefore, adding a message to the filter allows that message to be received from a lower integrity sender, while removing a message blocks that message from being received.

Certain messages with a value less than WM_USER are required to pass through the filter regardless of the filter setting. You can call this function to remove one of those messages from the filter and it will return TRUE. However, the message will still be received by the calling process.

Processes at or below SECURITY_MANDATORY_LOW_RID are not allowed to change the filter. If those processes call this function, it will fail.

For more information on integrity levels, see Understanding and Working in Protected Mode Internet Explorer.

Requirements

Requirement	Value
Minimum supported client	Windows Vista [desktop apps only]
Minimum supported server	Windows Server 2008 [desktop apps only]
Target Platform	Windows
Header	winuser.h (include Windows.h)
Library	User32.lib
DLL	User32.dll

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