excel-mcp-server 0.1.8 fails to enforce path confinement in stdio mode when EXCEL_FILES_PATH is unset, allowing attackers to read and write arbitrary files. Attackers can supply unchecked file paths to read and write tools to access any file accessible to the process.

excel-mcp-server version 0.1.8 contains a path confinement enforcement failure in stdio mode when the EXCEL_FILES_PATH environment variable is unset. This vulnerability allows attackers to supply unchecked file paths to read and write tools, enabling access to any file accessible to the process. The flaw effectively bypasses intended file access restrictions, resulting in arbitrary file read and write capabilities. The issue is tracked as CVE-2026-85661 and was reported via GitHub issue #149. A fix or mitigation requires proper path validation and confinement enforcement regardless of environment variable configuration. The vulnerability affects the server.py and validation.py components of the package. It poses a significant risk in environments where the MCP server is exposed to untrusted input.