Fastify Proxy-Addr Plugin IP Trust Bypass Allows X-Forwarded-For Spoofing

@fastify/proxy-addr is a Fastify plugin that determines a request's client address behind trusted reverse proxies, and it backs Fastify request.ip and request.ips. An IP trust bypass vulnerability affects versions 3.0.0 through 5.1.0.

In affected versions, a trust subnet written in IPv4-mapped IPv6 notation with an IPv4-sized prefix, such as ::ffff:10.0.0.0/8 instead of the correct ::ffff:10.0.0.0/104, is accepted without error but trusts every IPv4 address on the internet rather than the block it names. Because the socket peer then becomes trusted at hop 0, any unauthenticated client can supply an arbitrary X-Forwarded-For header and control the address the application reads, defeating IP-based access control, rate limiting, geolocation, and audit logging.

The plugin inherited this defect from the upstream proxy-addr module (CVE-2026-90711). The issue is fixed in @fastify/proxy-addr 5.1.1, and users should upgrade to 5.1.1 or later.

• Affected versions: 3.0.0 through 5.1.0
• Fixed version: 5.1.1
• Workaround: ensure any IPv4-mapped IPv6 trust subnet uses a prefix length of at least 97, or express the range in plain IPv4 notation