Spring Framework, versions 5.0 prior to 5.0.5 and versions 4.3 prior to 4.3.15 and older unsupported versions, provide client-side support for multipart requests. When Spring MVC or Spring WebFlux server application receives input from a remote client, and then uses that input to make a multipart request to another server , it can be exposed to an attack, where an extra multipart is inserted in the content of the request from server A, causing server B to use the wrong value for a part it expects. This could to lead privilege escalation, for example, if the part content represents a username or user roles.