Jenkins Swarm Plugin allows clients to auto-discover Jenkins instances on the same network through a UDP discovery request. Responses to this request are XML documents.
Swarm Plugin does not configure the XML parser in a way that would prevent XML External Entity (XXE) processing. This allows unauthenticated attackers on the same network to have Swarm clients parse a maliciously crafted XML response that uses external entities to read arbitrary files from the Swarm client or denial-of-service attacks.
As of publication of this advisory, there is no fix.