Disclosure summary
### Impact `proxy-addr` determines which network hops are trusted proxies so that `X-Forwarded-For` can be believed. When an application configures a trust subnet as an IPv4-mapped IPv6 address with a short prefix, such as `::ffff:10.0.0.0/8` (the correct spelling is `::ffff:10.0.0.0/104`), the subnet compiles with all-zero leading bits and matches every IPv4 address instead of the block it names. The same happens for any IPv6 trust subnet with zero leading bits, such as `::/1`. Every unauthenticated client is then trusted as a proxy at hop 0, so `proxyaddr(req, trust)`, and therefore `req.ip` and `req.ips` in Express, returns whatever the client sends in `X-Forwarded-For`. This defeats IP-based access control, rate limiting, geolocation, and audit logging. The misconfiguration compiles without any error, and the same block written correctly behaves correctly, so the difference is not visible from the configuration. ### Patches Upgrade to `proxy-addr` 2.0.8. An IPv4 address now matches an IPv6 trust subnet only when that subnet is a genuine IPv4-mapped subnet whose prefix covers the mapped marker. ### Workarounds Write IPv4 trust subnets in plain IPv4 notation (for example `10.0.0.
Source-specific records & product guidance
Sources retain their own attribution and scoring. Follow the original record to confirm affected versions, fixed releases, and configuration conditions.
GitHub Reviewed Security Advisories · GHSA-jqcg-44mw-7w3h
Open original source · Updated Oct 05, 2026
proxy-addr vulnerable to IP spoofing via IPv4-mapped IPv6 trust subnet
Source severity: CRITICAL / 0
| Ecosystem | Package | Affected range | First patched |
|---|---|---|---|
| npm | proxy-addr | >= 1.1.0, < 2.0.8 | 2.0.8 |
Original records & references
- NIST NVD record
- CVE Program record
- github.com — Reviewed advisory
PUBLISHED 2026-10-05T19:30:26-04:00
MODIFIED 2026-10-05T19:30:27-04:00
INGESTED 2026-10-06T11:45:33-04:00