My MCP Security Scanner Missed 2026's Worst MCP RCE: Here Is the One-Rule Fix

The hook A few months back I shipped mcpscan , a static analyzer that scans MCP (Model Context Protocol) servers for the vulnerability classes that keep showing up in this ecosystem: command injection, SSRF, and path traversal. Rule MCP007 was supposed to be the path traversal catch-all. This week I sat down with my own research notes and ran a simple gut-check: would MCP007 have caught the four real path-traversal CVEs disclosed against MCP servers this year? It would have missed every single one. Including the worst one. Real-world context Here is what actually shipped as CVEs in 2026, all in MCP servers, all sharing the same root cause: CVE Server Sink Impact CVE-2026-40576 excel-mcp-server file write Path traversal CVE-2026-84201 appium-mcp-server write_file Path traversal CVE-2026-44336 PraisonAI MCP Python .pth write RCE via site-packages injection CVE-2026-27825 mcp-atlassian confluence_download_attachment CVSS 9.1 , unauthenticated RCE (chained with SSRF CVE-2026-27826 to overwrite ~/.ssh/authorized_keys or drop a cron entry) Four different maintainers, four different tools, the exact same blind spot: a file path built from caller-controlled input, written without a directory-boundary check. The bug in mcp-atlassian is the nastiest: no auth needed, no restart needed, straight to a shell. So I opened my own rule file and read the docstring out loud: MCP007: path traversal in file-reading tools. There it is. My rule was scoped to reads from day one, and every real-world exploit this year happened on the write side. A scanner whose entire job is catching this bug class was structurally blind to the half of it that is actually landing CVSS 9+ scores. Architecture: how MCP007 actually works The rules in mcpscan are simple on purpose: line-scan regex matching without an AST, so they run fast across any language mcpscan supports. Each rule has three regex layers: ┌─────────────────────────────────────────────┐ │ 1. SINK: does this line call a │ │ file-open/read fun