String expansion and securely running programs on Unix

July 22, 2011

One of the corollaries of how to securely run programs on Unix is that a general purpose, generic string expansion system is a bad fit with securely running programs. The problem is that there is a fundamental clash of goals between the two systems: a generic string expansion system wants to treat everything as a generic string to be expanded (regardless of what it actually is), and a secure system for running programs wants to tokenize everything using simple rules.

At this point I am going to pick on Exim for illustrative examples. Unfortunately, Exim tries to have it both ways at once and thus is a great source for showing the problems that this causes, no matter how much I like it otherwise. Please note that the problems here are generic; any program that takes either approach (or both at once as Exim does) will have the same issues.

First up is Exim's av_scanner setting. This is not expanded at all unless it starts with a '$', at which point the entire string must be expanded before Exim knows how to tokenize it:

av_scanner = ${if bool{true} {cmdline:/opt/avscanner $recipients %s}}

If you are concerned about arbitrary characters appearing in $recipients, there is no way to make this secure (as discussed before).

Second, the command setting for running things in pipes. This tokenizes things before string expansion, but it does the tokenization purely on a textual basis. As the documentation notes, this causes serious problems:

command = /some/path ${if eq{$local_part}{postmaster} {xx} {yy}}

Since tokenization is expansion-blind, this fails because all the string expansion evaluator winds up seeing is '${if' (which is a clear syntax error). To get this to work you have to force the tokenizer to treat the entire string expansion as a single token by 'quoting' it.

(The documentation does not quite put it the way that I have here.)

A side effect of tokenization before expansion is that a single string expansion can only ever expand to a single argument. (You may or may not be able to expand to nothing instead of a '' empty argument, depending on the implementation.)

What this points out is that command line tokenization and string expansion need to be aware of each other. Once the dust settles, either string expansion needs to be able to mark hard token boundaries (so that $recipients can be marked as a single token regardless of contents) or tokenization needs to know about the string expansion language (so that ${if ...} can be parsed into a single token despite the presence of internal spaces or other special characters).

(I have opinions on the answer here, but this entry is already long enough as it is.)

PS: if you want to be secure with minimal effort, it's clear that you need to do tokenization before expansion and provide some sort of 'quoting' mechanism to glue a string expansion expression into a single token. This is secure while being merely inconvenient and annoying to people writing configuration files. Simple expansion before tokenization cannot be made secure at all, as previously discussed.

Written on 22 July 2011.
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Last modified: Fri Jul 22 01:07:08 2011
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