Some ways to test if a program securely runs other programs
Suppose that you have a program that can run other programs, and you want to find out if it securely runs the other programs. In an ideal world, the program's documentation would tell you, and you could trust it. Sadly we do not live in an ideal world.
First you need a test environment where you can control what external program your program runs and force it to actually run the program. In many cases (such as testing daemons, web servers, MTAs, and so on), the easiest test environment is a virtual machine. Next you need a program that logs its arguments in some appropriate place.
Let's say that we're testing a daemon with a configuration option
av_scanner' and that our program to report arguments
Our first test is whether straight shell metacharacters have any effect:
av_scanner = /opt/argreporter 'test' >/tmp/canary
After you get the daemon to run your configured 'AV scanner', check
argreporter's logs; if it is run securely, it should have seen exactly
these two arguments. If it was run through the shell, it will have seen
one argument that won't have quotes (and
/tmp/canary will exist).
Variants of this are possible; for example, some programs will helpfully
av_scanner command line through the shell if they see shell
If your program fails this check, you can stop now. Otherwise, though,
we still need to test the substitutions that your program does. For
this, we need to find some substitution that introduces a space; the
best one is a variable substitution, because those are usually the
simplest. Suppose that we have a
$recipients variable that has a
space separated list of destinations; then:
av_scanner = /opt/argreporter $recipients
The logs should show that argreporter was called with one argument and that the argument had spaces in it. If it was called with several arguments and each argument was a single destination, your program makes substitutions before breaking the command line up into arguments and you've just seen why this is somewhere between annoying and dangerous.
(Another important test is what happens with empty or blank
substitutions. You want these to result in an argument of
instead of the argument just disappearing.)