It isn't hard to think of a simple example in which Claude.md can be written to by the LLM to allow accessing endpoints not whitelisted by the user by smuggling a base64 encoded payload that then gets decoded by a subroutine it wrote to a file without you noticing. Or realizing it can't use the WebFetchTool but it can write a script to do manual DNS resolution and then use bash TCP sockets instead of curl in case it is hardened to not be able to use curl.