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High, CVSS 7.5amended

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

CVE-2026-55603GHSA-gcq2-9pq2-cxqm

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (http-proxy-middleware 4.1.1, 3.0.7), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
7.5 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N
EPSS
  • CVE-2026-55603: 0.29% estimated exploitation activity in the next 30 days; percentile 21.15 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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: 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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-gcq2-9pq2-cxqm

  2. Revision 1 · created ·

    Published from osv record GHSA-gcq2-9pq2-cxqm

Agent brief

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# Patch Next advisory brief: CVE-2026-55603

Advisory: http-proxy-middleware: multipart/form-data field injection via unescaped CRLF in `fixRequestBody`
IDs: CVE-2026-55603, GHSA-gcq2-9pq2-cxqm
Record: https://patchnext.com/advisories/cve-2026-55603 (revision 2, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: http-proxy-middleware 4.1.1, 3.0.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm http-proxy-middleware: affected >= 3.0.4, < 3.0.7; >= 4.0.0, < 4.1.1; fixed in 4.1.1, 3.0.7

## Suggested inspection (read-only)
- `npm ls http-proxy-middleware --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on http-proxy-middleware.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-gcq2-9pq2-cxqm (OSV record GHSA-gcq2-9pq2-cxqm lists affected ranges: npm http-proxy-middleware >=3.0.4 =4.0.0 <4.1.1 (fixed))
- Verified: https://github.com/advisories/GHSA-gcq2-9pq2-cxqm (GitHub-reviewed advisory GHSA-gcq2-9pq2-cxqm lists affected ranges: npm http-proxy-middleware >=3.0.4 =4.0.0 <4.1.1)

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