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Re: Questions and remarks on draft-ietf-sieve-include-01.txt

2009-04-11 04:19:43

Hi Aaron,

Aaron Stone wrote:
Thanks for your comments. I'd been preparing to send an email to the list
announcing the changes and requesting feedback. You've touched on all the
good points. So let's run with this thread!

On Sat, 11 Apr 2009 01:39:58 +0200, Stephan Bosch 
First of all, I am very glad that the work on the specification is being continued now. This week I quickly updated my implementation to match the new specification. Merging import and export into a single global command is a good choice. The use of a global variable namespace is also a good idea (but I did not implement that yet).

I actually wonder if we should drop the global keyword and rely on
namespaces entirely.
The global namespace has one disadvantage: it makes all global variable names longer ('global.*'). Especially when a variable is used often in a script, this may become tedious. Using the global command, it is possible to pull global variables into the script namespace, which is convenient in such situations. I guess it would be best to support both methods of accessing global variables (and to even allow mixing these freely).

- Where the ManageSieve protocol specifies what characters are allowed for a script name, the include extension for the Sieve language does not. Would it be useful to adopt the same limitations? Especially things like '/' can cause problems.

Good suggestion. I think this makes sense to give a consistent opinion on
what script names should look like, but on the other hand, perhaps it's
possible that someone isn't using ManageSieve but IS using include and
might need to get at weird names? Do we care in that case?
You can also make it a recommendation. As long as implementers know that there is some consensus about what a script name is supposed to be and that script names should not be mapped directly to files for security reasons.

- The global command is required to follow 'require' or another global command. I am worried what happens when other extensions have commands with similar requirements. Shouldn't we account for this eventuality?
I don't like this restriction anyways. Any objection to lifting it?
Hmm, interesting suggestion. The main implication of this is that the inclusion of global variables into the script namespace can become conditional, i.e. when it is placed into an if statement. I don't know whether this is desirable.

You could also state that the global command needs to be placed after require commands or any command that also needs to be placed directly below the require commands (choose better wording though). This implies no ordering between the commands that follow the require section, avoiding problems with future commands with similar needs.

- Are there any special security implications for using variables in the value argument in the include command, i.e. to include a script specified by a variable? Is that even (intended to be) allowed?

Oh, that's a good question. That'd be an easy vector for contamination of
email contents into a filesystem request. Can we reasonably place such a
restriction, though? I'd hate to force implementations to special-case
their string expansions.
Other extensions already have such limitations (e.g. notify) and the variables extension explicitly states that future commands may choose not to allow variables in some of their arguments (the so-called constant string). However, I agree that we should not limit the flexibility of the command more than necessary. Perhaps an explicit mention in the Security Considerations section would suffice to cover our bases.

On the other hand, one may argue how useful it is to use variables in the include argument in the first place: there are a (known) limited amount of scripts in the Sieve storage and making choices can be done using if commands surrounding multiple alternative include commands.

People using variables could even make their scripts highly unreliable, since include commands kill Sieve execution when the indicated script does not exist. Perhaps in combination with an ':optional' modifier, variables could be used to change the behavior of a script without actually modifying it. For instance, when a script is added to the Sieve store with a name that matches the sender address, it is included automatically when a message from that person comes in. That sounds a bit freaky though :)

- The scope of the :once modifier could be a bit confusing. I am assuming it holds for the whole Sieve execution and not only for the identical include commands within the current script.

Correct. Could you suggest how I might clarify that it's the whole
execution? I feel like I'm missing the right word for being inside one file
vs. being inside one delivery/execution instance.
I am still not sure what to do here. I guess it is also important to mention that the scope of the :once modifier does not extend into the other scripts of a multiscript sequence (and any scripts included by those).. or does it? For this distinction it matters whether you call it the 'whole Sieve execution', as this would include truly everything.


Stephan Bosch

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