Re: [SOGo] Bug: non-root inline text/html part in multipart/related is rendered as message body

[Christian Mack]

Hello

You should open a bug report at https://sogo.nu/bugs


Kind regards,
Christian Mack

Am 18.08.26 um 14:34 schrieb Felix Oberst (f...@thorsis.com):

Hello,



Environment:

mailcow 2026-07a
SOGo version bundled with mailcow 2026-07a
Browsers tested: Firefox / Chromium
Control client: Mozilla Thunderbird

Problem:
SOGo renders a non-root text/html MIME part with

Content-Disposition: inline



The same message is displayed correctly in Thunderbird.

Minimal MIME structure:

multipart/related; type="multipart/alternative"
|
+-- multipart/alternative <-- root object
| |
| +-- text/plain
| |
| +-- text/html <-- actual message body
|
+-- text/html <-- related non-root part
Content-ID: <test-html-resource>
Content-Disposition: inline



Actual behavior in SOGo:
SOGo displays both

the normal HTML message body, and
the additional related text/html part below the message.



Expected behavior:




Thunderbird behavior:


A/B tests:

Test A:

additional related text/html part
Content-Disposition: inline
no CID reference from the root HTML

Result:

SOGo: incorrect, related HTML is still rendered below the message
Thunderbird: correct

This shows that a CID reference is not required to trigger the issue.

Test B:
Same structure, but:

Content-Disposition: attachment

Result:

SOGo: correct, the HTML part is shown as an attachment
Thunderbird: correct





I have attached a fully synthetic .eml reproducer.

The attached reproducer is:

sogo_ab_A_no_cid_reference.eml





Best regards
Felix

we found a reproducible rendering issue in SOGo with multipart/related messages. We encountered the problem on a mail coming from a customer using some microsoft mail. Whole mail was not readable in SOGo. inside a multipart/related message as additional visible message body content. There is no "start" parameter on the outer multipart/related, therefore the first body part (multipart/alternative) is the root object. If the related HTML part contains CSS such as position, z-index or large backgrounds, this can severely break the message display. Only the root object of multipart/related should be displayed as the message body. Non-root related MIME parts should not automatically be rendered as additional message body content. Thunderbird displays only the actual root HTML message. The additional inline text/html related part is not appended to the visible message body. This indicates that Content-Disposition: inline is an important part of the trigger. We initially found this with a real customer email. Removing only the additional related text/html MIME part caused the original message to render correctly in SOGo. It contains the minimal case where the related HTML part is not referenced by the root HTML at all. Please let me know if you need any additional information or further test cases.

--
Christian Mack
Universität Konstanz
Kommunikations-, Informations-, Medienzentrum (KIM)
Abteilung IT-Dienste Forschung, Lehre, Infrastruktur
78457 Konstanz
+49 7531 88-4416

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