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Adobe Flash Socket Policy
Port 843 is primarily associated with the Adobe Flash socket policy server, which was used historically to deliver cross-domain policy files to Flash applications. This enabled Flash clients embedded in browsers or standalone applications to determine whether they were allowed to establish socket connections to specific servers. Its usage peaked during the prominence of Adobe Flash content across the web, but it has since declined considerably due to the deprecation of Flash technology..
Port 843 was designated by Adobe Flash Player for serving socket policy files, which are XML documents that define permissions for Flash clients to connect to services on a server using sockets. When a Flash client attempts a socket connection, it sends a request to port 843 for the policy file to verify permissions, typically formatted in XML, before establishing a socket communication session.
This port was a standardized method allowing web administrators to define cross-domain rules in a centralized policy file. The server would respond to a policy-file request on this port, streamlining permission control for all Flash-based socket communications regardless of which port the actual application data uses.
Despite its once widespread use, the relevance of port 843 has greatly declined following the deprecation and end-of-life of Adobe Flash in 2020. Modern web applications and browsers no longer rely on Flash or its security model, favoring more secure and standardized methods such as WebSockets, CORS, or other API-based authentication and authorization techniques.