[issue43922] Double dots in quopri transported emails

[Julien Castiaux]

Julien Castiaux  added the comment:

Fix deployed in the third party

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resolution:  -> third party
stage: patch review -> resolved
status: open -> closed

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[issue43922] Double dots in quopri transported emails

[Julien Castiaux]

Julien Castiaux  added the comment:

Hello David,

The third party smtp software that causes troubles have been identified ! We 
are still investigating how to fix the problem at its root, ultimately this 
"fix" would not even be necessary. I'll keep you informed, just don't review or 
close the PR yet.

Regards,

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[issue43922] Double dots in quopri transported emails

[R. David Murray]

R. David Murray  added the comment:

As far as I know the only resources are the context manager docs and the source 
code.  The stdlib content manager can serve as a model.  I have to admit that 
it was long enough ago that I wrote that code that I'd have to re-read the docs 
and code myself to figure it out :)

I'm afraid I don't really have time to do a complete review, but at a quick 
glance your patch doesn't look too complicated to me.  Quick observation:  the 
comment should explain why the dot check is done, and that it isn't needed for 
rfc compliance.

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[issue43922] Double dots in quopri transported emails

[Julien Castiaux]

Julien Castiaux  added the comment:

Hello David, thank you for your quick answer. I tried to keep it minimal with 
some unittests. Could you point me some resources to learn how to properly 
write a custom content manager ?

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[issue43922] Double dots in quopri transported emails

[Julien Castiaux]

Change by Julien Castiaux :


--
keywords: +patch
pull_requests: +24281
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/25562

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[issue43922] Double dots in quopri transported emails

[R. David Murray]

R. David Murray  added the comment:

Since python is doing the right thing here, I don't see a particularly good 
reason to put a hack into the stdlib to fix the failure of third party software 
to adhere to standards.  (On the output side.  We do follow Postel's rule on 
input and try hard to handle broken but recoverable input.)  I don't actually 
*object* to it, though, as long as it follows the standard on output, and is a 
*simple* change.

Please note that you can fix this locally by implementing and using a custom 
content manager.

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[issue43922] Double dots in quopri transported emails

[Jérôme Vanhaudenard]

Change by Jérôme Vanhaudenard :


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nosy: +jev2

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[issue43922] Double dots in quopri transported emails

[Julien Castiaux]

New submission from Julien Castiaux :

Hello,

We received multiple bug reports about broken links in rich html emails. 
Sometime, in some emails, a link like https://example.com";> would 
become https://example..com>, notice the double dot.

After multiple researches both in the Python email source code and in the RFC, 
it turns out that Python correctly implements the standard but that the distant 
(non-python) smtp server used by some of our customers doesn't.

The various email standard state the following:

1) As a single dot (".", chr(0x2e)) in a line ends the SMTP transmission, such 
single dots must be escaped when they are part of the message. RFC 5321, 
section 4.5.2 requires to escape all dots when they appear at the beginning of 
a line, using a dot as escape symbol. That is, when the user message contains: 
"\r\n.\r\n", it is escaped to "\r\n..\r\n". The other smtp side is responsible 
to remove the extra dot.

2) When we transport the email body using the quoted-printable encoding, RFC 
2045 requires each line to have maximum 78 characters and define a single equal 
("=", chr(0x3d)) as soft-warp sequence to fold lines too long. The RFC does 
only require to split the line outside of a quoted character (cannot split in 
the middle of "=2E"). Like any other character, it is allowed to split the line 
before a dot.

Take the following example:

from email.message import EmailMessage
from email.policy import SMTP

msg = EmailMessage(policy=SMTP)
msg.set_context("Hello there, just need some text to reach that seventy-six 
character, example.com")
#   
  ^
#   
  78th char

print(msg.as_string())
# Content-Type: text/plain; charset="utf-8"
# Content-Transfer-Encoding: quoted-printable
# MIME-Version: 1.0
#
# Hello there, just need some text to reach that seventy-six character, 
example=
# .com

When the message is sent over smtp, smtplib escapes the line ".com" to become 
"..com" as required by the RFC. So no problem in the python implementation, it 
is the other side that is buggy.

But! We have two solutions to "fix" the other side, the problem is that they do 
not correctly parse lines starting with a dot. A solution would be to ensure no 
line starts with the dot character. Two solutions : (1) quoted-printable encode 
dots when they are at the beginning of a line, (2) prevent the line folding 
code from splitting a line before a dot.

(1) is allowed by the RFC, any character can be quoted-printable encoded even 
those that have a safe ascii representation already. In our "example=\n.com" 
example above, we can qp the code: "example=\n=2Ecom". The line starts with a 
"2" instead of a dot and the content is the same.

(2) is allowed by the RFC, the RFC only states that a line must be at most 78 
chars long, it also states it is allowed to fold a line anywhere but in a 
quoted-printable sequence. It is safe to split a line earlier than the 78th 
character. In our "example=\n.com" example above, we could split the line at 
the 77th character: "exampl=\ne.com". The line starts with a "e" instead of a 
dot and the content is the same.

A pull request is coming shortly.

--
components: email
messages: 391698
nosy: Julien Castiaux, barry, r.david.murray
priority: normal
severity: normal
status: open
title: Double dots in quopri transported emails
type: enhancement
versions: Python 3.11

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