nextcloud
Multipart / Attachment handling overview
The purpose of this issue is to document known issues with our multipart and attachment handling.
Hidden attachments
Original report https://github.com/nextcloud/mail/issues/5282 and partly fixed by https://github.com/nextcloud/mail/pull/5339.
Sample eml
Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: from fcf416df7902
by fcf416df7902 with LMTP
id oKwwBSNDuWSRAQAAuSYRyA
(envelope-from <[email protected]>)
for <[email protected]>; Thu, 20 Jul 2023 14:22:27 +0000
Received: from localhost (unknown [172.19.0.1])
by fcf416df7902 (Postfix) with ESMTP id 063732260B32
for <[email protected]>; Thu, 20 Jul 2023 14:22:27 +0000 (UTC)
From: [email protected]
To: [email protected]
Subject: Patches
Message-ID: <20230720162227.Horde.tQbO75MfhHKIerc-38l3V5j@pc>
User-Agent: Horde Application Framework 5
Date: Thu, 20 Jul 2023 16:22:27 +0200
Content-Type: multipart/mixed; boundary="=_ltOS0Zz8DtIKjXr3jbdMGDN"
MIME-Version: 1.0
This message is in MIME format.
--=_ltOS0Zz8DtIKjXr3jbdMGDN
Content-Type: multipart/alternative; boundary="=_sVhlJCYnkclzOHfh6XZ-w5L"
This message is in MIME format.
--=_sVhlJCYnkclzOHfh6XZ-w5L
Content-Type: text/html; charset=utf-8
Content-Description: HTML Version of Message
<html><meta http-equiv="content-type" content="text/html; charset=UTF-8"><body>Hello Hello</body><html>
--=_sVhlJCYnkclzOHfh6XZ-w5L
Content-Type: text/plain; charset=utf-8
Content-Description: Plaintext Version of Message
Hello Hello
--=_sVhlJCYnkclzOHfh6XZ-w5L--
--=_ltOS0Zz8DtIKjXr3jbdMGDN
Content-Type: text/x-patch; name=some.patch
Content-Disposition: inline; filename=some.patch
hello world
--=_ltOS0Zz8DtIKjXr3jbdMGDN--
Generated via: https://github.com/kesselb/weird-emails/blob/main/attachment_content_disposition_inline.php
Message structure
graph TD;
multipart/mixed-->multipart/alternative;
multipart/alternative-->text/html;
multipart/alternative-->text/plain;
multipart/mixed-->text/x-patch;
Screenshots
Thunderbird
HTML message Content-Disposition for text/x-patch is inline
Issue: We assume that mime parts with content-disposition = inline are referenced in via content-id / cid in the html document. Is the mime part not referenced, it's hidden.
Possible solutions:
- Merge inlineAttachments and attachments like for text/plain emails.
- If content-disposition = inline && content-id = null, then treat mime part as regular attachment.
Hidden message
Sample eml
Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: from fcf416df7902
by fcf416df7902 with LMTP
id lko6KsJTuWQUBAAAuSYRyA
(envelope-from <[email protected]>)
for <[email protected]>; Thu, 20 Jul 2023 15:33:22 +0000
Received: from localhost (unknown [172.19.0.1])
by fcf416df7902 (Postfix) with ESMTP id 9BEA52260B32
for <[email protected]>; Thu, 20 Jul 2023 15:33:22 +0000 (UTC)
From: [email protected]
To: [email protected]
Subject: Multipart with multiple parts
Message-ID: <20230720173322.Horde.Lbu6mga1p0l6Yh4ZFm-Exh4@pc>
User-Agent: Horde Application Framework 5
Date: Thu, 20 Jul 2023 17:33:22 +0200
Content-Type: multipart/mixed; boundary="=_Xs2dih_sEN07PRgH-bx7MWq"
MIME-Version: 1.0
This message is in MIME format.
--=_Xs2dih_sEN07PRgH-bx7MWq
Content-Type: text/html; charset=utf-8
Content-Description: HTML Version of Message
<html><meta http-equiv="content-type" content="text/html; charset=UTF-8"><body>Hello Hello</body><html>
--=_Xs2dih_sEN07PRgH-bx7MWq
Content-Type: text/html; charset=utf-8
Content-Description: HTML Version of Message
<html><meta http-equiv="content-type" content="text/html; charset=UTF-8"><body>Hope you are dooing fine</body><html>
--=_Xs2dih_sEN07PRgH-bx7MWq--
Generated via: https://github.com/kesselb/weird-emails/blob/main/multipart_with_multiple_parts.php
Message structure
graph TD;
multipart/mixed-->text/html;
multipart/mixed-->text/html;
Screenshots
Thunderbird
Multipart message with multiple text/html parts.
Issue: We show the first text/html part. Without looking at the source, you don't know there's something else.
Possible solutions:
- Render both text/html parts
- Add the mime parts as attachment
Some possible mime structures...
multipart/mixed
text/plain, content-disposition=inline - A
multipart/mixed
multipart/alternative
multipart/mixed
text/plain, content-disposition=inline - B
image/jpeg, content-disposition=inline - C
text/plain, content-disposition=inline - D
multipart/related
text/html - E
image/jpeg - F
image/jpeg, content-disposition=attachment - G
application/x-excel - H
message/rfc822 - J
text/plain, content-disposition=inline - K
In this case, the above algorithm would decompose this to:
textBody => [ A, B, C, D, K ]
htmlBody => [ A, E, K ]
attachments => [ C, F, G, H, J ]
Hi, here is a simplified sample of one more mime structure, that fails to display correctly in Nextcloud Mail. This is is a Domain-based Message Authentication, Reporting, and Conformance (DMARC) aggregate report sent by Google (some information removed from message). Basically, it is a MIME message whose Content-Type is application/zip (attachment only, no other parts/content):
p.s. Just found - this is also discussed in #4423 !
Return-Path: <[email protected]>
Delivered-To: [email protected]
Received: from mail.example.com
by mail.example.com with LMTP
id tZvOJQASw2ZcsA4ARJn5MQ
(envelope-from <[email protected]>)
for <[email protected]>; Mon, 19 Aug 2024 12:36:00 +0300
MIME-Version: 1.0
X-Received: by 2002:a05:6820:1acc:b0:5d5:d7fc:955c with SMTP id
006d021491bc7-5da9801a875mr10422414eaf.5.1724060153323; Mon, 19 Aug 2024
02:35:53 -0700 (PDT)
Date: Sun, 18 Aug 2024 16:59:59 -0700
Message-ID: <[email protected]>
Subject: Report domain: example.com Submitter: google.com Report-ID: 6356864336886002680
From: [email protected]
To: [email protected]
Content-Type: application/zip;
name="google.com!example.com!1723939200!1724025599.zip"
Content-Disposition: attachment;
filename="google.com!example.com!1723939200!1724025599.zip"
Content-Transfer-Encoding: base64
UEsDB... <actual-base64-content-as-well-as-unrelated-headers-removed> ...AAAA=
