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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-22 · updated 2026-10-25
Weakness: CWE-93

Description

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.

Affected products

chimurai http-proxy-middleware: from 3.0.4 before 3.0.7; from 4.0.0 before 4.1.1 — fixed in 3.0.7, 4.1.1

How to fix

Upgrade to a fixed version: chimurai http-proxy-middleware 3.0.7, 4.1.1. Check the vendor advisory for mitigations.

References

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