CVE-2026-55603: http-proxy-middleware: multipart/form-data field injection via unescaped CRLF in `fixRequestBody`

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.