Title: MS Office Excel (all versions) Arbitrary Code Execution Vulnerability
Date: September 30th, 2017.
Author: Eduardo Braun Prado
Vendor Homepage: http://www.microsoft.com/
Software Link: https://products.office.com/
Version: 2007,2010,2013,2016 32/64 bits (x86 and x64)
Tested on: Windows 10/8.1/8.0/7/Server 2012/Server 2008/Vista (X86 and x64)
MS Excel contains a remote code execution vulnerability upon processing OLE objects. Although this is a different issue from the
MS Word HTA execution vulnerability, it has been patched together, 'silently'. By performing some tests from the Word HTA PoC posted
on exploit-db[dot]com, it´s possible to exploit it through Excel too, however the target would need to either accept a security warning
regarding external links or double click inside the Excel window, same applies for Powerpoint, so I guess this is the reason, Word caught
the attention and no exploit PoC was made available to other Office apps.
This vulnerability exists in the way Excel handles parameters passed to the "DDEService" attribute of links, leading to the search for a
program to display it. As it does not impose restrictions on what program is going to be executed, for instance, only programs located in the
Office install directory, it is possible to invoke arbitrary local programs with parameters, leading to system compromise.
Since Excel blocks automatic update of linked files, the target must be tricked into double clicking anywhere inside the document.
(The linked object occupies basicly the whole document window). Without the patch applied no warning/prompt is shown;
With the patch a prompt is shown asking if it´s ok to run 'xxxx.exe', where 'xxxx.exe' can have arbitrary names as long as it´s at most 8
chars long, so we could still fake/spoof it as another Office app (the app name cannot be the same of the legitimate, eg. 'Excel').
Proof of Concept: