nextcloud · lubo92 · Apr 22, 2026
@@ -300,11 +300,13 @@
 	 */
 	private function getPart(Horde_Mime_Part $p, string $partNo, bool $isFetched): void {
 		// iMIP messages
-		// Handle text/calendar parts first because they might be attachments at the same time.
-		// Otherwise, some of the following if-conditions might break the handling and treat iMIP
+		// Handle text/calendar and application/ics parts first because they
+		// might be attachments at the same time. Otherwise, some of the
+		// following if-conditions might break the handling and treat iMIP
 		// data like regular attachments.
 		$allContentTypeParameters = $p->getAllContentTypeParameters();
-		if ($p->getType() === 'text/calendar') {
+		if ($p->getType() === 'text/calendar'
+				|| $p->getType() === 'application/ics') {
 			// Handle event data like a regular attachment
 			// Outlook doesn't set a content disposition
 			// We work around this by checking for the name only
@@ -320,18 +322,30 @@
 				];
 			}
 
-			// return if this is an event attachment only
-			// the method parameter determines if this is a iMIP message
-			if (!isset($allContentTypeParameters['method'])) {
+			// Try the Content-Type method= parameter first — that's the common
+			// case. If it is missing, fall back to the METHOD: line inside
+			// the ICS body: Proton Mail Bridge strips the parameter during
+			// E2E re-assembly, and requiring it here would mean every
+			// Proton-Bridge user silently loses inbound invitations.
+			$method = $allContentTypeParameters['method'] ?? null;
+			$contents = null;
+			if ($method === null) {
+				$contents = $this->loadBodyData($p, $partNo, $isFetched);
+				if (is_string($contents)
+						&& preg_match('/^METHOD:\s*(\S+)/mi', $contents, $m)) {
+					$method = $m[1];
+				}
+			}
+			if ($method === null) {
 				return;
 			}
 
-			if (in_array(strtoupper($allContentTypeParameters['method']), ['REQUEST', 'REPLY', 'CANCEL'])) {
+			if (in_array(strtoupper($method), ['REQUEST', 'REPLY', 'CANCEL'])) {
 				$this->scheduling[] = [
 					'id' => $p->getMimeId(),
 					'messageId' => $this->uid,
-					'method' => strtoupper($allContentTypeParameters['method']),
-					'contents' => $this->loadBodyData($p, $partNo, $isFetched),
+					'method' => strtoupper($method),
+					'contents' => $contents ?? $this->loadBodyData($p, $partNo, $isFetched),
 				];
 				return;
 			}

@@ -916,10 +916,17 @@ public function getBodyStructureData(Horde_Imap_Client_Socket $client,
 					$hasAttachments = true;
 				}
 
-				if ($part->getType() === 'text/calendar') {
-					if ($part->getContentTypeParameter('method') !== null) {
-						$isImipMessage = true;
-					}
+				// Flag the message as iMIP when we see a text/calendar or
+				// application/ics part. Proton Mail Bridge is known to strip the
+				// `method=` parameter from the Content-Type header while
+				// re-assembling a message after E2E-decryption, so requiring the
+				// parameter here would drop every Bridge-delivered invitation.
+				// The actual method (REQUEST/REPLY/CANCEL) is parsed out of the
+				// ICS body downstream in ImapMessageFetcher::getPart() — see the
+				// matching fallback there.
+				if ($part->getType() === 'text/calendar'
+						|| $part->getType() === 'application/ics') {
+					$isImipMessage = true;
 				}
 			}
 

@@ -749,6 +749,16 @@ public function isImipMessageProvider(): array {
 		return [
 			'google request' => ['request_google', true],
 			'outlook.com request' => ['request_outlook_com', true],
+			// Proton Mail Bridge strips the `method=` parameter from the
+			// text/calendar Content-Type header during E2E re-assembly. The
+			// message is still an iMIP REQUEST — the method lives in the ICS
+			// body. Must still be flagged as iMIP so downstream processing
+			// picks it up.
+			'proton bridge request (method= stripped)' => ['request_proton_bridge', true],
+			// Google occasionally attaches the same invitation twice: once as
+			// text/calendar and once as application/ics. We used to ignore the
+			// latter even though it is functionally equivalent.
+			'application/ics request' => ['request_application_ics', true],
 		];
 	}
 

@@ -0,0 +1,74 @@
+MIME-Version: 1.0
+Message-ID: <imip-testing-1234@example.org>
+Date: Thu, 06 Feb 2025 16:21:41 +0000
+From: alice@example.org
+To: bob@example.org, john@example.org
+Content-Type: multipart/mixed; boundary="f2d8330e8efc4039bd8073c70ec6cf24"
+Subject: Invitation: Imip Testing @ Thu Feb 20, 2025 7pm - 8pm
+	(CET) (alice@example.org)
+
+--f2d8330e8efc4039bd8073c70ec6cf24
+Content-Type: multipart/alternative; boundary="f55a70234308486098b936b62a6d5e33"
+
+--f55a70234308486098b936b62a6d5e33
+Content-Type: text/plain; charset="UTF-8"; format=flowed; delsp=yes
+Content-Transfer-Encoding: base64
+
+SW1pcCBUZXN0aW5nICh0ZXh0L3BsYWluKQo=
+--f55a70234308486098b936b62a6d5e33
+Content-Type: application/ics; name="invite.ics"
+Content-Transfer-Encoding: 7bit
+
+BEGIN:VCALENDAR
+PRODID:-//Google Inc//Google Calendar 70.9054//EN
+VERSION:2.0
+CALSCALE:GREGORIAN
+METHOD:REQUEST
+BEGIN:VTIMEZONE
+TZID:Europe/Berlin
+X-LIC-LOCATION:Europe/Berlin
+BEGIN:DAYLIGHT
+TZOFFSETFROM:+0100
+TZOFFSETTO:+0200
+TZNAME:GMT+2
+DTSTART:19700329T020000
+RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
+END:DAYLIGHT
+BEGIN:STANDARD
+TZOFFSETFROM:+0200
+TZOFFSETTO:+0100
+TZNAME:GMT+1
+DTSTART:19701025T030000
+RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
+END:STANDARD
+END:VTIMEZONE
+BEGIN:VEVENT
+DTSTART;TZID=Europe/Berlin:20250220T190000
+DTEND;TZID=Europe/Berlin:20250220T200000
+DTSTAMP:20250206T162141Z
+ORGANIZER;CN=alice@example.org:mailto:alice@example.org
+UID:69d4c40b4a274636bf23517938df9673@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=
+ TRUE;CN=john@example.org;X-NUM-GUESTS=0:mailto:john@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=ACCEPTED;RSVP=TRUE
+ ;CN=alice@example.org;X-NUM-GUESTS=0:mailto:alice@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=
+ TRUE;CN=bob@example.org;X-NUM-GUESTS=0:mailto:bob@example.org
+CREATED:20250206T162140Z
+DESCRIPTION:
+LAST-MODIFIED:20250206T162140Z
+LOCATION:
+SEQUENCE:0
+STATUS:CONFIRMED
+SUMMARY:Imip Testing
+TRANSP:OPAQUE
+BEGIN:VALARM
+ACTION:DISPLAY
+DESCRIPTION:This is an event reminder
+TRIGGER:-P0DT0H30M0S
+END:VALARM
+END:VEVENT
+END:VCALENDAR
+
+--f55a70234308486098b936b62a6d5e33--
+--f2d8330e8efc4039bd8073c70ec6cf24--
@@ -0,0 +1,74 @@
+MIME-Version: 1.0
+Message-ID: <imip-testing-1234@example.org>
+Date: Thu, 06 Feb 2025 16:21:41 +0000
+From: alice@example.org
+To: bob@example.org, john@example.org
+Content-Type: multipart/mixed; boundary="f2d8330e8efc4039bd8073c70ec6cf24"
+Subject: Invitation: Imip Testing @ Thu Feb 20, 2025 7pm - 8pm
+	(CET) (alice@example.org)
+
+--f2d8330e8efc4039bd8073c70ec6cf24
+Content-Type: multipart/alternative; boundary="f55a70234308486098b936b62a6d5e33"
+
+--f55a70234308486098b936b62a6d5e33
+Content-Type: text/plain; charset="UTF-8"; format=flowed; delsp=yes
+Content-Transfer-Encoding: base64
+
+SW1pcCBUZXN0aW5nICh0ZXh0L3BsYWluKQo=
+--f55a70234308486098b936b62a6d5e33
+Content-Type: text/calendar; charset="UTF-8"
+Content-Transfer-Encoding: 7bit
+
+BEGIN:VCALENDAR
+PRODID:-//Google Inc//Google Calendar 70.9054//EN
+VERSION:2.0
+CALSCALE:GREGORIAN
+METHOD:REQUEST
+BEGIN:VTIMEZONE
+TZID:Europe/Berlin
+X-LIC-LOCATION:Europe/Berlin
+BEGIN:DAYLIGHT
+TZOFFSETFROM:+0100
+TZOFFSETTO:+0200
+TZNAME:GMT+2
+DTSTART:19700329T020000
+RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
+END:DAYLIGHT
+BEGIN:STANDARD
+TZOFFSETFROM:+0200
+TZOFFSETTO:+0100
+TZNAME:GMT+1
+DTSTART:19701025T030000
+RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
+END:STANDARD
+END:VTIMEZONE
+BEGIN:VEVENT
+DTSTART;TZID=Europe/Berlin:20250220T190000
+DTEND;TZID=Europe/Berlin:20250220T200000
+DTSTAMP:20250206T162141Z
+ORGANIZER;CN=alice@example.org:mailto:alice@example.org
+UID:69d4c40b4a274636bf23517938df9673@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=
+ TRUE;CN=john@example.org;X-NUM-GUESTS=0:mailto:john@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=ACCEPTED;RSVP=TRUE
+ ;CN=alice@example.org;X-NUM-GUESTS=0:mailto:alice@example.org
+ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=
+ TRUE;CN=bob@example.org;X-NUM-GUESTS=0:mailto:bob@example.org
+CREATED:20250206T162140Z
+DESCRIPTION:
+LAST-MODIFIED:20250206T162140Z
+LOCATION:
+SEQUENCE:0
+STATUS:CONFIRMED
+SUMMARY:Imip Testing
+TRANSP:OPAQUE
+BEGIN:VALARM
+ACTION:DISPLAY
+DESCRIPTION:This is an event reminder
+TRIGGER:-P0DT0H30M0S
+END:VALARM
+END:VEVENT
+END:VCALENDAR
+
+--f55a70234308486098b936b62a6d5e33--
+--f2d8330e8efc4039bd8073c70ec6cf24--