ActivityPub is a federation protocol that lets decentralized social platforms exchange posts, follows, reactions, and other activity between independent servers. Instead of one central service, each server can host its own users while still participating in the same network. That design improves resilience and user control, but it also creates a trust boundary between servers that security teams must protect.
In cyber security, ActivityPub matters because each federated request may carry identity, authorization, and content that must be validated before it is accepted. Bugs in request handling can let a server process actions it should reject, expose private data, or be abused for server-side request forgery if outbound fetches are not tightly controlled. Defenders secure ActivityPub deployments by verifying signatures, checking permissions carefully, limiting which remote servers can interact, and treating federation endpoints as internet-facing attack surfaces.