A message handling computing system that provides security across even transport-independent communication mechanisms, and which allows for convenient extension of security to different security token types, and may provide end-to-end security across different transport protocols. The message handling computing system includes a message handling component configured to send and receive network messages having security tokens. The message handling component interfaces with an expandable and contractible set of security token managers through a standardized application program interface. Each security manager is capable of providing security services for messages that correspond to security tokens of a particular type. A security token plug-in component registers new security token managers with the message handling component.