diff --git a/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php b/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
index 6ac8bf28877c2..611bd4953a101 100644
--- a/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
+++ b/apps/dav/lib/CalDAV/Reminder/NotificationProvider/EmailProvider.php
@@ -144,19 +144,31 @@ private function addBulletList(IEMailTemplate $template,
 		IL10N $l10n,
 		string $calendarDisplayName,
 		VEvent $vevent):void {
-		$template->addBodyListItem($calendarDisplayName, $l10n->t('Calendar:'),
-			$this->getAbsoluteImagePath('actions/info.png'));
+		$template->addBodyListItem(
+			htmlspecialchars($calendarDisplayName),
+			$l10n->t('Calendar:'),
+			$this->getAbsoluteImagePath('actions/info.png'),
+			htmlspecialchars($calendarDisplayName),
+		);
 
 		$template->addBodyListItem($this->generateDateString($l10n, $vevent), $l10n->t('Date:'),
 			$this->getAbsoluteImagePath('places/calendar.png'));
 
 		if (isset($vevent->LOCATION)) {
-			$template->addBodyListItem((string)$vevent->LOCATION, $l10n->t('Where:'),
-				$this->getAbsoluteImagePath('actions/address.png'));
+			$template->addBodyListItem(
+				htmlspecialchars((string)$vevent->LOCATION),
+				$l10n->t('Where:'),
+				$this->getAbsoluteImagePath('actions/address.png'),
+				htmlspecialchars((string)$vevent->LOCATION),
+			);
 		}
 		if (isset($vevent->DESCRIPTION)) {
-			$template->addBodyListItem((string)$vevent->DESCRIPTION, $l10n->t('Description:'),
-				$this->getAbsoluteImagePath('actions/more.png'));
+			$template->addBodyListItem(
+				htmlspecialchars((string)$vevent->DESCRIPTION),
+				$l10n->t('Description:'),
+				$this->getAbsoluteImagePath('actions/more.png'),
+				htmlspecialchars((string)$vevent->DESCRIPTION),
+			);
 		}
 	}
 
diff --git a/apps/dav/lib/CalDAV/Schedule/IMipService.php b/apps/dav/lib/CalDAV/Schedule/IMipService.php
index 9fe0e0b9a80ea..7b75cda699752 100644
--- a/apps/dav/lib/CalDAV/Schedule/IMipService.php
+++ b/apps/dav/lib/CalDAV/Schedule/IMipService.php
@@ -25,6 +25,7 @@
 use Sabre\VObject\Parameter;
 use Sabre\VObject\Property;
 use Sabre\VObject\Recur\EventIterator;
+use function htmlspecialchars;
 
 class IMipService {
 
@@ -80,10 +81,11 @@ private function generateDiffString(VEvent $vevent, VEvent $oldVEvent, string $p
 		if (!isset($vevent->$property)) {
 			return $default;
 		}
-		$newstring = $vevent->$property->getValue();
+		$value = $vevent->$property->getValue();
+		$newstring = $value === null ? null : htmlspecialchars($value);
 		if (isset($oldVEvent->$property) && $oldVEvent->$property->getValue() !== $newstring) {
 			$oldstring = $oldVEvent->$property->getValue();
-			return sprintf($strikethrough, $oldstring, $newstring);
+			return sprintf($strikethrough, htmlspecialchars($oldstring), $newstring);
 		}
 		return $newstring;
 	}
@@ -96,8 +98,8 @@ private function generateLinkifiedDiffString(VEvent $vevent, VEvent $oldVEvent,
 			return $default;
 		}
 		/** @var string|null $newString */
-		$newString = $vevent->$property->getValue();
-		$oldString = isset($oldVEvent->$property) ? $oldVEvent->$property->getValue() : null;
+		$newString = htmlspecialchars($vevent->$property->getValue());
+		$oldString = isset($oldVEvent->$property) ? htmlspecialchars($oldVEvent->$property->getValue()) : null;
 		if ($oldString !== $newString) {
 			return sprintf(
 				"<span style='text-decoration: line-through'>%s</span><br />%s",
@@ -797,10 +799,10 @@ public function buildCancelledBodyData(VEvent $vEvent): array {
 		$strikethrough = "<span style='text-decoration: line-through'>%s</span>";
 
 		$newMeetingWhen = $this->generateWhenString($eventReaderCurrent);
-		$newSummary = isset($vEvent->SUMMARY) && (string)$vEvent->SUMMARY !== '' ? (string)$vEvent->SUMMARY : $this->l10n->t('Untitled event');
-		$newDescription = isset($vEvent->DESCRIPTION) && (string)$vEvent->DESCRIPTION !== '' ? (string)$vEvent->DESCRIPTION : $defaultVal;
+		$newSummary = htmlspecialchars(isset($vEvent->SUMMARY) && (string)$vEvent->SUMMARY !== '' ? (string)$vEvent->SUMMARY : $this->l10n->t('Untitled event'));
+		$newDescription = htmlspecialchars(isset($vEvent->DESCRIPTION) && (string)$vEvent->DESCRIPTION !== '' ? (string)$vEvent->DESCRIPTION : $defaultVal);
 		$newUrl = isset($vEvent->URL) && (string)$vEvent->URL !== '' ? sprintf('<a href="%1$s">%1$s</a>', $vEvent->URL) : $defaultVal;
-		$newLocation = isset($vEvent->LOCATION) && (string)$vEvent->LOCATION !== '' ? (string)$vEvent->LOCATION : $defaultVal;
+		$newLocation = htmlspecialchars(isset($vEvent->LOCATION) && (string)$vEvent->LOCATION !== '' ? (string)$vEvent->LOCATION : $defaultVal);
 		$newLocationHtml = $this->linkify($newLocation) ?? $newLocation;
 
 		$data = [];
@@ -1067,22 +1069,22 @@ public function addAttendees(IEMailTemplate $template, VEvent $vevent) {
 	 */
 	public function addBulletList(IEMailTemplate $template, VEvent $vevent, $data) {
 		$template->addBodyListItem(
-			$data['meeting_title_html'] ?? $data['meeting_title'], $this->l10n->t('Title:'),
+			$data['meeting_title_html'] ?? htmlspecialchars($data['meeting_title']), $this->l10n->t('Title:'),
 			$this->getAbsoluteImagePath('caldav/title.png'), $data['meeting_title'], '', IMipPlugin::IMIP_INDENT);
 		if ($data['meeting_when'] !== '') {
-			$template->addBodyListItem($data['meeting_when_html'] ?? $data['meeting_when'], $this->l10n->t('When:'),
+			$template->addBodyListItem($data['meeting_when_html'] ?? htmlspecialchars($data['meeting_when']), $this->l10n->t('When:'),
 				$this->getAbsoluteImagePath('caldav/time.png'), $data['meeting_when'], '', IMipPlugin::IMIP_INDENT);
 		}
 		if ($data['meeting_location'] !== '') {
-			$template->addBodyListItem($data['meeting_location_html'] ?? $data['meeting_location'], $this->l10n->t('Location:'),
+			$template->addBodyListItem($data['meeting_location_html'] ?? htmlspecialchars($data['meeting_location']), $this->l10n->t('Location:'),
 				$this->getAbsoluteImagePath('caldav/location.png'), $data['meeting_location'], '', IMipPlugin::IMIP_INDENT);
 		}
 		if ($data['meeting_url'] !== '') {
-			$template->addBodyListItem($data['meeting_url_html'] ?? $data['meeting_url'], $this->l10n->t('Link:'),
+			$template->addBodyListItem($data['meeting_url_html'] ?? htmlspecialchars($data['meeting_url']), $this->l10n->t('Link:'),
 				$this->getAbsoluteImagePath('caldav/link.png'), $data['meeting_url'], '', IMipPlugin::IMIP_INDENT);
 		}
 		if (isset($data['meeting_occurring'])) {
-			$template->addBodyListItem($data['meeting_occurring_html'] ?? $data['meeting_occurring'], $this->l10n->t('Occurring:'),
+			$template->addBodyListItem($data['meeting_occurring_html'] ?? htmlspecialchars($data['meeting_occurring']), $this->l10n->t('Occurring:'),
 				$this->getAbsoluteImagePath('caldav/time.png'), $data['meeting_occurring'], '', IMipPlugin::IMIP_INDENT);
 		}
 
@@ -1090,7 +1092,7 @@ public function addBulletList(IEMailTemplate $template, VEvent $vevent, $data) {
 
 		/* Put description last, like an email body, since it can be arbitrarily long */
 		if ($data['meeting_description']) {
-			$template->addBodyListItem($data['meeting_description_html'] ?? $data['meeting_description'], $this->l10n->t('Description:'),
+			$template->addBodyListItem($data['meeting_description_html'] ?? htmlspecialchars($data['meeting_description']), $this->l10n->t('Description:'),
 				$this->getAbsoluteImagePath('caldav/description.png'), $data['meeting_description'], '', IMipPlugin::IMIP_INDENT);
 		}
 	}