GHSA-4mfg-r38w-w8fg - mcp-sanitizer: URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6

## URL sanitizer allows localhost/IPv6 loopback SSRF bypass via explicit port or bracketed IPv6 ### Summary `mcp-sanitizer` v1.3.3 fails to block Server-Side Request Forgery (SSRF) targets when a `localhost` URL includes an explicit port (e.g., `http://localhost:8080/`) or when an IPv6 loopback/link-local address is expressed in bracketed notation (e.g., `http://[::1]:8080/`, `http://[fe80::1]:8080/`). Because the library is explicitly designed to gate MCP tool parameters such as `web_fetch` and `web_scraper` before outbound HTTP requests are made, a bypass here directly enables an attacker-controlled URL to reach internal services, leaking their responses. ### Details The vulnerability exists in `src/utils/validation-utils.js` inside the `validateURLLocation()` function that is called by `_secureSanitizeURL()` in `src/sanitizer/mcp-sanitizer.js:698`. **Bug 1 — Port-guarded localhost check (line 274–276):** ```js // src/utils/validation-utils.js:274-276 if ((hostname === 'localhost' || hostname === '127.0.0.1' || hostname === '::1') && !parsedUrl.port) { throw new Error('URL points to localhost without explicit port'); } ``` The `&& !parsedUrl.port` guard means that any localhost URL that includes a port number is *not* thrown. `http://localhost:8080/` passes the check entirely, returning `blocked: false` with the original URL preserved. **Bug 2 — Bracketed IPv6 hostname mismatch (lines 272, 293–295):** ```js // src/utils/validation-utils.js:293-295 if (hostname.startsWith('169.254.') || hostname.startsWith('fe80:')) { throw new Error(`URL points to link-local address: ${hostname}`); } ``` The Node.js `URL` API returns `parsedUrl.hostname` for IPv6 addresses *with* surrounding brackets: `[::1]`, `[fe80::1]`. The string comparison `hostname === '::1'` (line 274) and `hostname.startsWith('fe80:')` (line 293) therefore never match, because the actual string starts with `[`. **Data flow through the middleware:** 1. `src/middleware/express.js:499` — tool