Anthropic Filesystem MCP Server: Directory Access Bypass via Improper Path Validation

Anthropic's filesystem MCP server (a tool that lets AI assistants like Claude access your computer's files) had a path validation vulnerability where it only checked if a file path started with an allowed directory name, rather than confirming it was actually in that directory. This meant if you allowed access to /mnt/finance/data, the AI could also access sibling files like /mnt/finance/data-archived because the path string starts the same way.