express-rate-limit: IPv4-mapped IPv6 addresses bypass per-client rate limiting on servers with dual-stack network

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-30827: 0.52% estimated exploitation activity in the next 30 days; percentile 44.01 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The default keyGenerator in express-rate-limit applies IPv6 subnet masking (/56 by default) to all addresses that net.isIPv6() returns true for. This includes IPv4-mapped IPv6 addresses (::ffff:x.x.x.x), which Node.js returns as request.ip on dual-stack servers. Because the first 80 bits of all IPv4-mapped addresses are zero, a /56 (or any /32 to /80) subnet mask produces the same network key (::/56) for every IPv4 client. This collapses all IPv4 traffic into a single rate-limit bucket: one client exhausting the limit causes HTTP 429 for all other IPv4 clients. Details Root Cause In source/ip-key-generator.ts: net.isIPv6('::ffff:192.168.1.1') returns true, so IPv4-mapped addresses enter the subnet masking path. With a /56 prefix, the start address for any ::ffff:x.x.x.x is ::, producing the key ::/56. Proof of Concept End-to-End Validation On a dual-stack Express server (app.listen(port, '::')), tested with Express 5.2.1: - request.ip for IPv4 clients is ::ffff:127.0.0.1 -…

Published by the source

6 Mar 2026, 18:36 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 12:43 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-46wh-pxpv-q5gq

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-46wh-pxpv-q5gq

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# Patch Next advisory brief: CVE-2026-30827

Advisory: express-rate-limit: IPv4-mapped IPv6 addresses bypass per-client rate limiting on servers with dual-stack network
IDs: CVE-2026-30827, GHSA-46wh-pxpv-q5gq
Record: https://patchnext.com/advisories/cve-2026-30827 (revision 2, source checked 26 Oct 2026, 12:43 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: express-rate-limit 8.2.2, 8.1.1, 8.0.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm express-rate-limit: affected >= 8.2.0, < 8.2.2; >= 8.1.0, <= 8.1.0; >= 8.0.0, < 8.0.2; fixed in 8.0.2, 8.1.1, 8.2.2

## Suggested inspection (read-only)
- `npm ls express-rate-limit --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 express-rate-limit.
- 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-46wh-pxpv-q5gq (OSV record GHSA-46wh-pxpv-q5gq lists affected ranges: npm express-rate-limit >=8.2.0 =8.1.0 =8.0.0 <8.0.2 (fixed))
- Verified: https://github.com/advisories/GHSA-46wh-pxpv-q5gq (GitHub-reviewed advisory GHSA-46wh-pxpv-q5gq lists affected ranges: npm express-rate-limit >=8.2.0 =8.1.0 =8.0.0 <8.0.2)

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