diff --git a/apps/dav/lib/CalDAV/Plugin.php b/apps/dav/lib/CalDAV/Plugin.php
index 24448ae71ab3c..1b257ae21a702 100644
--- a/apps/dav/lib/CalDAV/Plugin.php
+++ b/apps/dav/lib/CalDAV/Plugin.php
@@ -7,9 +7,31 @@
  */
 namespace OCA\DAV\CalDAV;
 
+use Sabre\DAV\Server;
+use Sabre\HTTP\RequestInterface;
+use Sabre\HTTP\ResponseInterface;
+use Sabre\VObject\Component\VCalendar;
+use Sabre\VObject\Component\VEvent;
+use Sabre\VObject\Property;
+use Sabre\VObject\Reader;
+
 class Plugin extends \Sabre\CalDAV\Plugin {
 	public const SYSTEM_CALENDAR_ROOT = 'system-calendars';
 
+	/**
+	 * Initializes the plugin
+	 *
+	 * @param Server $server
+	 * @return void
+	 */
+	public function initialize(Server $server) {
+
+		parent::initialize($server);
+
+		$server->on('calendarObjectChange', [$this, 'calendarObjectChange'], 90);
+		
+	}
+
 	/**
 	 * Returns the path to a principal's calendar home.
 	 *
@@ -34,4 +56,150 @@ public function getCalendarHomeForPrincipal($principalUrl) {
 			return self::SYSTEM_CALENDAR_ROOT . '/calendar-rooms/' . $principalId;
 		}
 	}
+
+	/**
+	 * @param RequestInterface $request
+	 * @param ResponseInterface $response
+	 * @param VCalendar $vCal
+	 * @param mixed $calendarPath
+	 * @param mixed $modified
+	 * @param mixed $isNew
+	 */
+	public function calendarObjectChange(RequestInterface $request, ResponseInterface $response, VCalendar $alteredObject, $calendarPath, &$modified, $isNew) {
+		
+		// determine if the calendar has an event
+		// if there is no event there is nothing to do
+		if (!$alteredObject->VEVENT) {
+			return;
+		}
+		// determine if altered calendar event is a new event
+		// if calendar event is new sanitize and return
+		if ($isNew) {
+			$this->sanitizeCreatedInstance($alteredObject->VEVENT, $modified);
+			return;
+		}
+		// retrieve current calendar event node
+		/** @var \OCA\DAV\CalDAV\CalendarObject $currentNode */
+		$currentNode = $this->server->tree->getNodeForPath($request->getPath());
+		// convert calendar event string data to VCalendar object
+		/** @var \Sabre\VObject\Component\VCalendar $currentObject */
+		$currentObject = Reader::read($currentNode->get());
+		// find what has changed (base, recurrence, both) between altered and current calendar event
+		$delta = $this->findEventInstanceDelta($alteredObject->VEVENT, $currentObject->VEVENT);
+		//
+		foreach ($delta as $entry) {
+			// determine if this instance was created or updated
+			if ($entry['current'] !== null) {
+				$this->sanitizeUpdatedInstance($entry['altered'], $entry['current'], $modified);
+			} else {
+				$this->sanitizeCreatedInstance($entry['altered'], $modified);
+			}
+		}
+
+	}
+
+	public function sanitizeCreatedInstance(VEvent $altered, $modified): void {
+		
+		// sanitize attendees
+		if (isset($altered->ATTENDEE)) {
+			$this->sanitizeEventAttendees($altered, $modified);
+		}
+
+	}
+
+	public function sanitizeUpdatedInstance(VEvent $altered, VEvent $current, $modified): void {
+		
+		// find differences in properties
+		$delta = $this->findEventPropertyDelta($altered, $current, $modified);
+		// determine if any important properties have changed sanitize attendees
+		if (isset($delta['DTSTART']) || isset($delta['DTEND']) || isset($delta['LOCATION']) || isset($delta['RRULE'])) {
+			$this->sanitizeEventAttendees($altered, $modified);
+		}
+
+	}
+
+	public function sanitizeEventAttendees(VEvent $event, $modified): void {
+		
+		// iterate thought attendees
+		foreach ($event->ATTENDEE as $id => $entry) {
+			// determine attendee participation status
+			// if status is missing or NOT set correctly change the status
+			if (!isset($entry['PARTSTAT']) || $entry['PARTSTAT']->getValue() !== 'NEEDS-ACTION') {
+				$event->ATTENDEE[$id]['PARTSTAT']->setValue('NEEDS-ACTION');
+				$modified = true;
+			}
+		}
+
+	}
+
+	protected function findEventInstanceDelta(VEvent $altered, VEvent $current): array {
+		
+		$list = [];
+		// iterate through altered event instances
+		foreach ($altered as $event) {
+			// create instance id
+			if (!isset($event->{'RECURRENCE-ID'})) {
+				$id = $event->UID->getValue() . ':Base';
+			} else {
+				$id = $event->UID->getValue() . ':' . $event->{'RECURRENCE-ID'}->getValue();
+			}
+			// add instance to list
+			$list[$id] = ['altered' => $event, 'current' => null];
+		}
+		// iterate through current event instances
+		foreach ($current as $event) {
+			// create instance id
+			if (!isset($event->{'RECURRENCE-ID'})) {
+				$id = $event->UID->getValue() . ':Base';
+			} else {
+				$id = $event->UID->getValue() . ':' . $event->{'RECURRENCE-ID'}->getValue();
+			}
+			// determine if id exists in list
+			if (isset($list[$id])) {
+				// compare altered instance to current instance
+				if ($list[$id]['altered']->{'LAST-MODIFIED'}->getValue() == $event->{'LAST-MODIFIED'}->getValue() &&
+					$list[$id]['altered']->SEQUENCE->getValue() == $event->SEQUENCE->getValue()) {
+					// remove entry from list if instance has not changed
+					unset($list[$id]);
+				} else {
+					// update entry in list with current instance
+					$list[$id]['current'] = $event;
+				}
+			} else {
+				// add entry to list
+				$list[$id] = ['altered' => null, 'current' => $event];
+			}
+		}
+
+		return $list;
+
+	}
+
+	protected function findEventPropertyDelta(VEvent $altered, VEvent $current): array {
+		
+		$list = [];
+		// iterate through altered event properties
+		foreach ($altered->children() as $property) {
+			// add property to list
+			$list[$property->name] = ['altered' => $property->getValue(), 'current' => null];
+		}
+		// iterate through altered event properties
+		foreach ($current->children() as $property) {
+			if (isset($list[$property->name])) {
+				if ($list[$property->name]['altered'] == $property->getValue()) {
+					// remove entry from list if instance has not changed
+					unset($list[$property->name]);
+				} else {
+					// update entry in list with current instance
+					$list[$property->name]['current'] = $property->getValue();
+				}
+			} else {
+				// add entry to list
+				$list[$property->name] = ['altered' => null, 'current' => $property->getValue()];
+			}
+		}
+		
+		return $list;
+	}
+
 }