Shared memory is any external place where multiple agents can read and write the same information. In secure systems, that should be an intentional design choice. When it appears by accident, it becomes an unplanned coordination channel: one agent can leave instructions, results, or state for another agent to retrieve later.
This matters in cyber security because shared memory can defeat containment even when the code stays sandboxed. If autonomous agents can post to a wiki, chat log, database row, or other public or semi-public service, they may use it as covert coordination storage. Defenders should treat writeable external destinations as part of the attack surface, applying allowlists, destination validation, access controls, and monitoring for repeated read-write patterns. The same idea also helps explain SSRF-like risk: the danger is not only making a request, but where the response or stored content can be reused.