IETF's job is not to recommend "practically .... what happens" but to recommend
what works well from a technical perspective. Masking the problem does not
work well. And in my experience, attempts to interpret malformed messages
and guess what they really meant often fail.
However, it is important to arrange things so that the "pressure" goes to the
right place - i.e. someone who can fix the actual problem.
In the case of a malformed message, that ends up being the sender. The sender
might then put pressure on his ISP ("why didn't my mail get there? why did
something complain about my mail reader generating a malformed message?") And
the ISP might well respond by having their mail submission servers try to
repair those messages. At least in that case, if the ISP's mail submission
servers do more harm than good, the feedback will still go back to the sender
and/or his ISP, and the ISP will be in a position to fix the problem with the
fix. When the "repair" is done by a party unrelated to the sender, that
opportunity is lost, and there is no convergence toward a working system.
In general, OPES rules would seem to apply here.
On Apr 15, 2011, at 11:54 AM, Murray S. Kucherawy wrote:
I think that’s true from the IETF side of things, but practically speaking
that’s rarely what happens. What arrives at an ingress MTA contains all
kinds of insane things, and in my experience the choice to reject them
outright puts huge pressure on customer service facilities that nearly always
results in demands for more relaxed software. And since that turns it into a
business case, the pressure usually wins.
So given that reality, this work seems to make sense to have out there,
rather than allowing widely varied choices about how to handle this case or
that one that result in weak ingress security all around.
From: Keith Moore [mailto:moore(_at_)network-heretics(_dot_)com]
Sent: Friday, April 15, 2011 5:51 AM
To: Murray S. Kucherawy
Subject: Re: Comments on Malformed Message BCP draft
I'm strongly opposed to MTAs "fixing" malformed messages (other than
submission servers fixing a small number of known problems caused by broken
If an MTA does anything at all when it thinks that a message is malformed, it
should be to bounce it _exactly as it received it originally_.
MTAs trying to fix malformed messages, at best, mask problems further
upstream that should be fixed. At worst, they exacerbate existing problems
and make such problems harder to diagnose.
On Apr 14, 2011, at 3:07 PM, Murray S. Kucherawy wrote:
This is some work starting up in the APPS area. Please comment on the
apps-discuss list if you’re interested in participating.
[mailto:apps-discuss-bounces(_at_)ietf(_dot_)org] On Behalf Of Simon Tyler
Sent: Thursday, April 14, 2011 2:59 AM
Subject: [apps-discuss] Comments on Malformed Message BCP draft
Having read the Malformed Message BCP draft I am interested in getting some
discussion going on its content and to find the best way forward.
For those who missed it, the draft is at:
I have a few comments on it.
One thing that concerns me is the proposal that processing should stop when
certain malformations are identified.
For example it is proposed that should a badly wrapped header field be found
(quite how we define this is left open, I presume a line that does not start
with a valid header field name followed by a colon) then the processing agent
should treat this as the end of the header. My feeling is that this opens up
a greater potential hole than the one closed and that can be exploited.
An example of the type of issue this could is cause is that should such a
malformation occur before the MIME header fields in the header then this
would cause the rest of the header and the message body to be treated as
plain text. This could cause content analysis system to fail as they would
not interpret the MIME content in the way that was presumably intended.
Given that these recommendations are unlikely to be followed by all clients
and servers, I feel that this suggestion will allow content through an agent
without suitable processing. My preference on this particular malformation
would be to continue processing the header fields, this is based on the
assumption that what follows the malformed header field is more likely to be
further header fields and not body content. What we do with the malformed
header field I am less certain about. We could just ignore it or we could
treat it as part of the previous header field - both will be right as often
as they wrong. I would welcome some additional thoughts on this.
I have similar feelings about some of the other suggestions including the
lack of a MIME-Version header. We cannot ignore intended meaning just because
a non-compliant application made a small error in the header fields. Everyone
will be best served if we subject such messages to more analysis, not less.
On the whole I think a set of guidelines in this area is good but it will be
hard to reach consensus without agreement on some basic underlying
principles. I would suggest that one such principle is to try to do what the
intended recipient would most likely prefer, which is generally to fix and
deliver non-spam messages.
I would also propose some additions to the draft. At Mimecast we see a lot of
messages generated by all sorts of systems and amongst these we see a lot of
different kinds of message malformations.
I'll suggest more as I think of them but for starters here are a few:
1. Excessively long lines in both headers and body. I commonly see lines that
are several hundred Kbs in length. These are often valid messages in the
sense that the content is desired by the receiver and in all respects other
than line length are well formed. Obviously a limit has to be enforced and I
would like to find a consensus on what sort of limit is reasonable. Initially
I felt 8K was a good limit - it is after all 8 times the limit in RFC 5321.
But it appears that this is too small a limit in real situations. When the
limit is exceeded, what is the best option – a rejection or forced line
wrap. I am open to both.
2. Invalid characters in headers. I often see headers with un-encoded 8bit
characters. These are often displayed correctly to the recipient where the
client happens upon the correct character set by virtue of chance.
3. 8bit characters in MIME sections with a content-transfer-encoding of 7bit.
If you have read this far then I think you will agree with me that Murray has
made a good start on a much needed set of guidelines. Now let's see if we can
support him to expand on the work he has done and reach a consensus on the