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Re: base64
From: MightyE <trash () mightye org>
Date: Thu, 25 Sep 2003 09:06:58 -0400

Lawrence MacIntyre wrote:

Whatever happened to the strategy:

Be conservative in what you send and liberal in what you accept.

This strategy falls on its face when you're looking at scanning emails for viruses. If you're liberal in what you accept, then someone has the ability to encode their virus, targeted toward a specific mail program, which behaves in a certain way when it encounters improperly encoded base64 data. There are two methods which you can use in the writing of your email virus scanner; you can either decode it every known way that any client does so, in which case you are only ever able to perform damage control, and must update your software each time a program exhibits new behavior. This method also introduces a high load on the mail server since you have to decode the same attachment many times, and scan it for viruses many times. If there are only 5 conceivable means to decode an error in the base64 encoding, then you've introduced 5 times the load on your mail server compared to traditional approaches. This thus also opens you wider to DoS attacks.

Alternatively you can accept it only if it is properly encoded, relying on the 99.9% of clients out there that know how to read an RFC. If all mail servers had always only accepted proper base64 encoding, then the authors of all email clients would have their clients encoding correctly from the very start, and you'd never need to worry about this now (or else they'd never actually get any users since they'd have a hard time sending attachments).

The real problem isn't badly formed MIME attachments or blown BASE64
encoding, it is instead mail clients that execute email as if it were
code, when we all know that email is simply data.  You don't execute

No, the problem at hand is how best to scan for viruses in incoming email, that's what the discussion is centered around. This latest virus going around uses social engineering to get the user to run the executable; the mail program never runs it. In your paragraph above, you thus describe this as not a problem, or failing that, this certainly doesn't fall within your description of what a problem is. I disagree, and insist that in an increasingly hostile Internet environment, stricter acceptance policies are needed more and more. Non compliance with RFC's should not be tolerated, this sort of non compliance is specifically the sort of loop hole that crackers can exploit to bypass the likes of virus scanners.

p.s.  If your theory on broken clients were true, then Outlook would be
gone by now:-)

No, because my theory depends on being conservative in what you accept; being liberal in what you accept does not give users of broken clients any incentive to find a functioning client. When users are hit by an Outlook worm, everything blows over in a week or two, it's a big inconvenience, but they're again sending emails again, and the problem is "fixed" as far as they're concerned, so no reason to change now, they after all have their address book set up in this client. If a worm came around that was hitting them, and there never *was* a fix for their client, the user is faced with either dropping off the face of the 'net for eternity, or finding a new client. I think we're both aware of what the obvious choice is for the user here.

Eric Stevens
mightye a () t  mightye d.o.t org

On Wed, 2003-09-24 at 15:13, MightyE wrote:
I agree, I don't think it's unreasonable to reject improperly formatted messages. Chances are much higher that they're spam or virii, and the minority with broken clients will find their way to non-broken clients. If you are parsing the message, particularly looking for malware, and encounter an improper encoding, bounce the message with a meaningful error, this way you don't have to worry about a targeted exploit that depends on the way one email client (mis)interprets a message in a different way from your virus scanner.

The RFC does declare an = to indicate that the end of the data stream has been reached, further data should be truncated, though it seems each email client actually handles this differently. Take the low road catchall, and simply reject them as a matter of course.

-Eric Stevens
mightye a () t mightye d.o.t org

Christian Vogel wrote:


On Tue, Sep 23, 2003 at 07:50:56PM +0300, Alexander Ogol wrote:

decision in all situations. Some mailing lists (debian-russian, for example)
add some 7bit information after letter body while re-forwarding, regardless
of was the letter base64/QP encoded or not, resulting of such malformed
Then this software is severly broken (MIME-wise), imho, and needs to be

So I think that the right solution (before antivirus software would be
rewritten) is to write filters by yourself - decode base64 as that do
popular mail clients and give them to antivirus.
With this approach, you are always on the "one step behind" side of
the problem. It's only a matter of time that someone finds out that
(made up example:) you can use a UTF8-mis-encoded "=" in Microsoft's
base64-decoder... The only sane way is to check if it's in the
standard-form ("abcABC=") and reject or convert if it's not.

99.99% of all software should create the standard form, so please
let the tiny fraction of users with broken software suffer
when their mails get rejected.

(Note: this of course applies not only to Base64 but to all aspects
of header-parsing, file-format guessing etc...)


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