High [CVE-2026-55603] Data integrity compromise via CR/LF injection

http-proxy-middleware is node.js http-proxy middleware. From 3.0.4 until 3.0.7 and 4.1.1, 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. 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. This vulnerability is fixed in 3.0.7 and 4.1.1.

A remote attacker could exploit a vulnerability in the fixRequestBody() function, which is used to re-emit a request body. By injecting carriage return and line feed characters (\r\n) into a request body key or value, an attacker can bypass security policies and validation performed by the proxy.

This desynchronization between the proxy and the backend server can lead to a compromise of data integrity.