Embodiments gather historical information about data propagation by monitoring requests to and replies from a server. When a request is received from a client system to upload code onto a web site, a user identity associated with the client system is determined and a tag that uniquely identifies the uploaded data is created and mapped with the user identity into a propagation graph. The propagation graph includes nodes and edges associated with a number of client systems that made similar requests such that each node of the propagation graph corresponds to both a tag and user identity of a client system and edges within the propagation graph represent causality links between the nodes. The propagation graph can then be used for finding long propagation chains, which can be useful for detecting worm-like propagation activity.