CComVariant::ReadFromStream() returns an error (Access Denied) for stream > 1 MB

This article helps you resolve the problem that an error (Access Denied) occurs when your application uses CComVariant::ReadFromStream() to read data from a stream.

Original product version:   Visual C++
Original KB number:   2831480


You have developed an application that uses CComVariant::ReadFromStream() to read data from a stream and it returns an error message (Access Denied) for stream size > 1 MB.


Reviewing atlcomcli.h we find that the size has been set here:

#define _ATL_STREAM_MAX_SIZE0x100000

and if the size of the stream increases the MAX length it should throw an Access Denied error.

else if (cbStrLen > _ATL_STREAM_MAX_SIZE)
    ATLTRACE(atlTraceCOM, 0, _T('String exceeded the maximum allowed size see _ATL_STREAM_MAX_SIZE.'));


If you have valid scenario where you are streaming data as Batch Stirred Tank Reactor (BSTR) that is larger than the predefined size you can change it. However, if you are using any untrusted code this workaround should not be employed.

One approach would be to override CCOmVariant::ReadFromStream(). Another way is to change _ATL_STREAM_MAX_SIZE itself.

More information

We are reading from a stream and the stream can be from untrusted source. The MAX value is there to catch any issues with streams that have been manipulated to try the code to allocate huge number of memory causing DOS attacks.