<p>Scenario: Document library in SharePoint with column x of "Person or Group" type.  From within a VBA macro (or VSTO add-in) we're trying to access the MetaProperty on the document to set/get the user name.  Any attempt to access the value via the ContentTypeProperties collection throws a </p>

<blockquote>
  <p>Type MisMatch error (13).</p>
</blockquote>

<p>The Type property of the MetaProperty object says it's <code>msoMetaPropertyTypeUser</code>.  I cannot find any examples of how to work with MetaProperties of this type.  Anyone have any experience with this?</p>

<p>Thanks!</p>

Are you executing the programs directly or going through the shell?  If you always launch an external program by giving the full path name to the executable and leaving the shell out of the equation, then you aren't really susceptible to any kind of command line injection.
EDIT: DrFloyd, the shell is responsible for evaluating things like the backtick.  No shell, no shell evaluation.  Obviously, you've still got to be aware of any potential security gotchas in the programs that you're calling -- but I don't think this question is about that.