Whether that's through RAG, Web Search, MCP, user input, or apis...etc doesn't matter. MCP just scales this greatly. Any sort of "agent" will have this same limitation.
Prompting is just natural language. There are a million different ways to express the same thing in natural language. Combine that with a non-deterministic model "interpreting" said language and this becomes a very difficult and unpredictable attack vector to protect against - other than simply not using untrusted content in agents.
Also, given prompting is natural language, it is incredibly easy to do these attacks. For example, it's trivial to gain access to confidential emails of a user using Claude Desktop connected to a Gmail MCP server [2].
[1] https://joedivita.substack.com/p/ugc-in-agentic-systems-feel...
[2] https://joedivita.substack.com/p/mcp-its-the-wild-west-out-t...