Double dots in quopri transported emails

[Julien00859]

BPO	43922
Nosy	@warsaw, @bitdancer, @Julien00859
PRs	bpo-43922: escape email line starting with a dot #25562

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-04-30.13:18:23.508>
created_at = <Date 2021-04-23.14:36:50.407>
labels = ['type-feature', 'expert-email', '3.11']
title = 'Double dots in quopri transported emails'
updated_at = <Date 2021-04-30.13:18:23.507>
user = 'https://github.com/Julien00859'

bugs.python.org fields:

activity = <Date 2021-04-30.13:18:23.507>
actor = 'Julien Castiaux'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-30.13:18:23.508>
closer = 'Julien Castiaux'
components = ['email']
creation = <Date 2021-04-23.14:36:50.407>
creator = 'Julien Castiaux'
dependencies = []
files = []
hgrepos = []
issue_num = 43922
keywords = ['patch']
message_count = 6.0
messages = ['391698', '391707', '391718', '391726', '392319', '392428']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'Julien Castiaux', 'jev2']
pr_nums = ['25562']
priority = 'normal'
resolution = 'third party'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43922'
versions = ['Python 3.11']

[Julien00859]

Hello,

We received multiple bug reports about broken links in rich html emails. Sometime, in some emails, a link like <a href="https://example.com"\> would become <a href="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.

[bitdancer]

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.

[Julien00859]

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 ?

[bitdancer]

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.

[Julien00859]

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,

[Julien00859]

Fix deployed in the third party