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


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


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


#define _ATL_STREAM_MAX_SIZE  0x100000


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."));

          hr = E_ACCESSDENIED;



If you have valid scenario where you are streaming data as 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 amount of memory causing DOS attacks.

Inside the Active Template Library (ATL) Security Update

Active Template Library Security Update for Developers

Microsoft Internal Support Information

Steps to reproduce.

Product Bug Number:
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Article ID: 2831480 - Last Review: Apr 17, 2013 - Revision: 1