CVE-2026-30827: Express-Rate-Limit Project Express-Rate-Limit

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

express-rate-limit is a basic rate-limiting middleware for Express. In versions starting from 8.0.0 and prior to versions 8.0.2, 8.1.1, 8.2.2, and 8.3.0, 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. This issue has been patched in versions 8.0.2, 8.1.1, 8.2.2, and 8.3.0.

Affected products

• Express-Rate-Limit Project Express-Rate-Limit: from 8.0.0, before 8.0.2 (fixed in 8.0.2); from 8.2.0, before 8.2.2 (fixed in 8.2.2); version 8.1.0 only

Published 2026-03-07. Last modified 2026-06-17.