fix(mime-viewer): sanitize charset parameter containing full MIME type

[jcdelepine]

Some buggy mailers set the charset parameter to the full MIME type (e.g. charset="text/html") instead of a valid character set name, causing a fatal error when attempting to convert the character set. Fall back to UTF-8 when the charset value contains a slash, which is a reliable indicator of a misplaced MIME type.

[ralflang]

Thank you. Do you have a sample of such a buggy mailer software or created mail so I can add a unit test?

[jcdelepine]

mail.eml

As you can see, this mailer uses

Content-Type: text/html; charset="text/html"

And horde said :

2026-08-03T11:44:38+02:00 DEBUG: HORDE [imp] Unable to convert character set from "text/html" to "utf-8". Attempted methods: iconv, mbstring, UConverter. Failures: iconv failed or does not support charset; mbstring: mb_convert_encoding(): Argument #3 ($from_encoding) contains invalid encoding "text/html"; UConverter returned empty/false result. [pid 941789 on line 232 of "/var/www/horde-git/horde/Util/src/HordeString.php"]
2026-08-03T11:44:38+02:00 DEBUG: HORDE  1. require_once() /var/www/horde/web/horde/services/ajax.php:3
 2. Horde\Core\Ajax\Application->doAction() /var/www/horde-git/horde/base/services/ajax.php:65
 3. call_user_func() /var/www/horde-git/horde/Core/src/Ajax/Application.php:210
 4. IMP_Ajax_Application_Handler_Common->showMessage()
 5. IMP_Ajax_Queue->message() /var/www/horde-git/horde/imp/lib/Ajax/Application/Handler/Common.php:719
 6. IMP_Contents_Message->showMessage() /var/www/horde-git/horde/imp/lib/Ajax/Queue.php:462
 7. IMP_Contents_Message->getInlineOutput() /var/www/horde-git/horde/imp/lib/Contents/Message.php:176
 8. IMP_Contents_Message->_getInlineOutput() /var/www/horde-git/horde/imp/lib/Contents/Message.php:337
 9. IMP_Contents->renderMIMEPart() /var/www/horde-git/horde/imp/lib/Contents/Message.php:595
10. Horde_Mime_Viewer_Base->render() /var/www/horde-git/horde/imp/lib/Contents.php:665
11. IMP_Mime_Viewer_Html->_renderInline() /var/www/horde-git/horde/Mime_Viewer/lib/Horde/Mime/Viewer/Base.php:159
12. IMP_Mime_Viewer_Html->_IMPrender() /var/www/horde-git/horde/imp/lib/Mime/Viewer/Html.php:76
13. Horde_Mime_Viewer_Html->_cleanHTML() /var/www/horde-git/horde/imp/lib/Mime/Viewer/Html.php:160
14. Horde_Mime_Viewer_Base->_textFilter() /var/www/horde-git/horde/Mime_Viewer/lib/Horde/Mime/Viewer/Html.php:152
15. Horde_Core_Factory_TextFilter->filter() /var/www/horde-git/horde/Mime_Viewer/lib/Horde/Mime/Viewer/Base.php:439
16. Horde_Text_Filter::filter() /var/www/horde-git/horde/Core/lib/Horde/Core/Factory/TextFilter.php:71
17. Horde_Text_Filter_Xss->postProcess() /var/www/horde-git/horde/Text_Filter/lib/Horde/Text/Filter.php:111
18. Horde\Util\Domhtml->__construct() /var/www/horde-git/horde/Text_Filter/lib/Horde/Text/Filter/Xss.php:76
19. Horde\Util\HordeString::convertCharset() /var/www/horde-git/horde/Util/src/Domhtml.php:99
20. Horde\Util\HordeString::_convertCharset() /var/www/horde-git/horde/Util/src/HordeString.php:116