CVE-2026-77604: Code Graph RAG: MCP paginated read_file path traversal / arbitrary file read

The read_file MCP tool in codebase_rag/mcp/tools.py contains a path traversal vulnerability in its paginated branch (when offset or limit is supplied). The handler constructs a file path using Path(self.project_root) / file_path without subsequently validating that the resolved path remains within the configured project root. Python’s pathlib silently discards the left-hand component when the right-hand argument is an absolute path, allowing an attacker to read any UTF-8-readable file on the server by passing an absolute path such as /etc/passwd.