Re: [Extra] Ben Campbell's Discuss on draft-ietf-extra-sieve-fcc-08: (with DISCUSS and COMMENT)

Ben Campbell <ben@nostrum.com> Thu, 10 January 2019 22:16 UTC

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From: Ben Campbell <ben@nostrum.com>
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Date: Thu, 10 Jan 2019 16:16:32 -0600
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Cc: draft-ietf-extra-sieve-fcc@ietf.org, Alexey Melnikov <aamelnikov@fastmail.fm>, extra@ietf.org, yaojk@cnnic.cn, The IESG <iesg@ietf.org>, extra-chairs@ietf.org
To: Ned Freed <ned.freed@mrochek.com>
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Subject: Re: [Extra] Ben Campbell's Discuss on draft-ietf-extra-sieve-fcc-08: (with DISCUSS and COMMENT)
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> On Jan 10, 2019, at 11:53 AM, Ned Freed <ned.freed@mrochek.com>; wrote:
> 
> 
> 
>>> On Jan 10, 2019, at 2:42 AM, Alexey Melnikov <aamelnikov@fastmail.fm>; wrote:
>>> 
>>> Hi Ben,
>>> 
>>>> On 9 Jan 2019, at 21:51, Ben Campbell <ben@nostrum.com>; wrote:
>>>> 
>>>> ----------------------------------------------------------------------
>>>> DISCUSS:
>>>> ----------------------------------------------------------------------
>>>> 
>>>> Thanks for the work on this. I plan to ballot "yes", but have one item I think
>>>> needs to be discussed first:
>>>> 
>>>> The security considerations say that this extension adds no new considerations
>>>> not already present in [RFC5228], [RFC5230], [RFC5435], and [RFC6131]. I'm not
>>>> sure that that is true.
>>>> 
>>>> It seems like the ability to insert a copy of message into a mailbox might have
>>>> security and/or privacy considerations.
>>> 
>>> Can you give me an idea of what you have in mind here, other than putting the user (Sieve script owner) over quota?
> 
>> I can’t say that I know what the security considerations might be; I’m
>> just skeptical that the answer is “no new considerations."
> 
> I think this is demonstrably true.
> 
> I'll first note that in terms of the sorts of messages it is capable of
> generating, the enotify extension with mailto: is effectively a superset of
> vacation. (Vacation is much more convenient to use and includes some additional
> builtin checks, but these aren't relevant here.)
> 
> So it suffices to consider the notify :fcc case.
> 
> Now compare the following sieves, each belonging to bob@example.com:
> 
> [1]
> 
> require ["enotify", "fcc"];
> notify :fcc "INBOX.Sent" :message "Bob got mail!" "mailto:ken@example.com";
> 
> [2]
> 
> require ["enotify", "fileinto"];
> if header :is "subject" "Bob got mail!" {
>  fileinto "INBOX.Sent";
> }
> else {
>  notify :message "Bob got mail!" "mailto:ken@example.com?to=bob@example.com";
> }
> 
> In case this isn't clear, the first script uses :fcc to make a copy of the
> notification and file it while the second uses a second notification recipient,
> test, and a fileinto to make the copy and file it. The results will likely
> differ in minor ways, e.g., Received: fields will probably be different, but
> since :fcc is constrained to have essentially the same semantics as fileinto,
> the security considerations aren't going to differ much if at all.
> 
> The use of :fcc with actions defined in the future may result in additional
> security considerations, but we won't know what those are until it happens.
> 

I absolutely agree the security considerations for fcc are similar, perhaps identical, to those for fileinto.


> Of course this also sidesteps the issue of whether or not the security
> considerations for fileinto are properly documented.

That is my concern.

> 
>> The authors of
>> 5228 thought “fileinto” could be dangerous. Do we know why?
> 
> Yes, but I doubt you'll care for the answer much. (I certainly don't.) First,
> it's pretty obvious that there are risks with using fileinto. Off the top of my
> head:
> 
> (1) Lost messages. An incorrectly set up fileinto, or a correct one that's
>    set up and then forgotten can result in important messages being put in a
>    place where nobody bothers to look.
> 
> (2) Excessive disk usage. Lots of sites have automatic rules for cleaning out
>    unread messages from commonly used mailboxes like INBOX or Junk. However,
>    the assumption tends to be that since other folders have to be set up
>    manually that they should have longer expiration times or not be subject
>    to expiration policies at all.
> 
> (3) Breaking non-idempotent workflows. Fileinto can be used to create
>    additional copies of messages, and this can have bad effects on
>    processors (including people) that fail to check for duplicates.
> 
> (4) Creating large numbers of mailboxes. Combine fileinto and Sieve
>    variables with a bit of carelessness, and you can achieve some
>    spectacular results. Example:
> 
>    require ["fileinto", "variables"];
>    # File tagged list mail in an appropriate folder.
>    if header :matches "subject" "*[*]*" { fileinto "List-${2}"; }
> 
>    (I note that this issue is described in more general terms in RFC 5229.)
> 
> And so on. Similar lists can also be constructed for discard, redirect,
> ereject, etc.
> 
> But here's the thing: Are these really the sorts of things that need to be
> listed in a security considerations section?

I don’t think they all are. I’m primarily concerned about things that could unintentionally expose information to third parties. I guess data loss could be a secondary concern, but I’m not as concerned about that.

In email discussion so far, the only things that have come up that seem to fit that is filing into a shared mailbox, or into a mailbox that is otherwise not well protected.

> Sure, they all have potential
> security ramifications, but the common factor in all of these is that somebody
> did something dumb. And the list of dumb things is effectively endless. Do we
> need to caution against having
> 
>    discard;
> 
> or
> 
>    require "fileinto"; fileinto "Junk";
> 
> as your entire Sieve script? And if not, where do we draw the line?
> 
> And perhaps more to the point, there's little if anything a Sieve implementer
> can do to prevent people from doing dumb things. In every case I've listed
> above - including the last two examples - there's a legitimate use-case  that
> isn't dumb.
> 
> Note: Speaking as someone who supports a Sieve implementation, users definitely
> do dumb things with sieves on a regular basis. In fact if I could add one
> recommendation to the base document, it would be that Sieve implementations
> running at or just before final delivery SHOULD log all actions the user' Sieve
> takes. Including explicit keep. Logging makes it possible to tell users what
> happened, and in most cases why. Of course the log then has privacy issues ...
> 
> Even worse, we've now opened the door to a bunch of meta-issues that are well
> known to be ratholes: Exactly who is the target audience for security
> considerations? Where do we draw the line between "Obvious problem but
> sufficiently dangerous to be worth belaboring" and "Duh!!!"? Etc.
> 
> And this brings us back to what's in RFC 5228. What's in there and what's not
> is a result of the fact that the process that produced that text in both RFC
> 3228 and 5228 was - and I'm being generous here - distinctly suboptimal.
> 
> I don't think it's productive to get into the specifics of what went wrong, so
> let me just say that ratholes were involved, and by the time we extricated
> ourselves from those there was neither sufficient energy nor sufficient
> patience to try and flesh out what dangerous aspects of various actions needed
> to be discussed and which ones didn't. So we stuck in a dangerous bend sign and
> left it at that.
> 

My point was not to do a post-mortum on 5228, or to try to fix it. I’m only concerned about any issue there to the extent that _this_ draft relies on it.

> In any case, while I acknowledge that the security considerations in RFC 5228
> could and should be improved, I think doing so in a document that provides -
> let's face it - a power user feature and which is therefopre unlikely to be
> consulted by base specification implementors doesn't meet a cost-benefit
> analysis. I therefore support the text Alexey has suggested which I think goes
> just far enough.

I think it’s likely that I agree; which text that Alexey suggested do you refer to? If it’s down to mentioning shared mailboxes and moving on, I’m fine with it at this point.

Thanks!

Ben.