GHSA-g5cg-6c7v-mmpw HackMD MCP Server has Server-Side Request Forgery (SSRF) vulnerability

• npm hackmd-mcp: >= 1.4.0, < 1.5.0 → fixed in 1.5.0

Impact A Server-Side Request Forgery (SSRF) vulnerability that affects all users running the HackMD MCP server in HTTP mode. Attackers could exploit this vulnerability by passing arbitrary `hackmdApiUrl` values through HTTP headers (`Hackmd-Api-Url`) or base64-encoded JSON query parameters. This allows malicious users to: • Redirect API calls to internal network services • Potentially access sensitive internal endpoints • Perform network reconnaissance through the server • Bypass network access controls The vulnerability affects the HTTP transport mode specifically - stdio mode is not impacted as it only accepts requests from stdio. Patches The vulnerability has been patched in version `1.5.0`. Users should: 1. Update to the latest version of the HackMD MCP server 2. Set the `ALLOWED_HACKMD_API_URLS` environment variable to restrict allowed HackMD API endpoints 3. If not set, the server will default to only allowing the official HackMD API URL (`https://api.hackmd.io/v1`) Example configuration: ``` ALLOWED_HACKMD_API_URLS=https://api.hackmd.io/v1,https://your-hackmd-instance.com/api/v1 ``` Workarounds Users can mitigate this vulnerability without upgrading by: 1. Use stdio mode instead of HTTP mode: Set `TRANSPORT=stdio` or remove the `TRANSPORT` environment variable to disable HTTP mode entirely 2. Network-level restrictions: Use firewall rules or network policies to restrict outbound connections from the server 3. Reverse proxy filtering: Place the MCP server behind a reverse proxy that validates and filters both the `Hackmd-Api-Url` header and the base64-encoded JSON `config` query parameter to prevent malicious `hackmdApiUrl` values References • OWASP Server-Side Request Forgery Prevention Cheat Sheet • HackMD MCP Server Documentation