no-ssrf

Keywords: no-ssrf, SSRF, server-side request forgery, CWE-918, OWASP A10:2021, fetch, axios, http.request, internal services, ESLint rule CWE: CWE-918: Server-Side Request Forgery (SSRF) OWASP: A10:2021 — Server-Side Request Forgery

Detect outbound HTTP requests whose URL is derived from user-controlled input. An attacker can pivot the request at internal services (metadata endpoints, admin panels, databases) to bypass perimeter controls.

This rule is part of eslint-plugin-node-security.

Quick Summary

Why SSRF matters

A server inside a cloud VPC can typically reach:

• The cloud metadata service (169.254.169.254) — AWS IMDS, GCP, Azure — where credentials live.
• Other internal services (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
• The loopback (127.0.0.1) and Unix sockets.

If your server accepts a URL from a user request and fetches it, the user can borrow your server's identity to reach those targets. The 2019 Capital One breach was a textbook SSRF chain that exfiltrated 100M records via IMDS.

Examples

❌ Incorrect

app.get('/preview', async (req, res) => {
  const r = await fetch(req.query.url);            // attacker controls target
  res.send(await r.text());
});

app.post('/import', async (req, res) => {
  const r = await axios.get(req.body.feedUrl);     // same exposure via axios
  res.json(r.data);
});

const r = await fetch(`${API_BASE}${userPath}`);   // path traversal lets attacker swap host

✅ Correct

import { URL } from 'node:url';

const ALLOWED_HOSTS = new Set(['api.partner.example', 'cdn.partner.example']);
const BLOCKED_CIDRS = ['10.0.0.0/8', '172.16.0.0/12', '192.168.0.0/16', '127.0.0.0/8', '169.254.0.0/16'];

function safeFetch(rawUrl) {
  const url = new URL(rawUrl);
  if (url.protocol !== 'https:') throw new Error('https only');
  if (!ALLOWED_HOSTS.has(url.hostname)) throw new Error('host not allowed');
  // Resolve DNS before request; reject if the resolved IP sits in BLOCKED_CIDRS.
  return fetch(url, { redirect: 'error' });   // refuse redirects to avoid TOCTOU
}

Error Message Format

🔒 NODE-SECURITY CWE-918 OWASP:A10 | SSRF: user-controlled URL passed to outbound request | CRITICAL
   Fix: Validate the URL against an allowlist of trusted hosts, refuse redirects, and resolve DNS to block internal IPs (RFC 1918, link-local, loopback).

Known False Negatives

• URL fragments derived from a chain of variables (more than one assignment hop) are not always traced.
• Constructed URL objects passed to request.get(urlObject) are detected when the constructor receives user input directly, not when input enters via setter methods.
• Allowlists declared in environment variables (consumed at runtime) cannot be validated statically — the rule still fires; suppress with an eslint-disable-next-line comment that names the validating function.