Bug 2535484 (CVE-2026-92395)

Summary: CVE-2026-92395 @fastify/proxy-addr: @fastify/proxy-addr: Access control bypass via mishandled IPv4-mapped IPv6 subnets
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: bbrownin, sdawley
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in @fastify/proxy-addr. This flaw allows a remote, unauthenticated attacker to bypass access controls and security policies by spoofing client IP (Internet Protocol) addresses. When configured with certain IPv4-mapped IPv6 network ranges, the plugin improperly treats all incoming IPv4 connections as trusted reverse proxies. An attacker can exploit this issue by supplying forged proxy request headers, enabling them to circumvent IP-based restrictions, bypass rate limiting, and mislead audit logging.
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Description OSIDB Bzimport 2026-09-16 14:51:32 UTC 
@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. In versions 3.0.0 through 5.1.0, 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, which defeats 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. As a workaround, ensure any IPv4-mapped IPv6 trust subnet uses a prefix length of at least 97, or express the range in plain IPv4 notation.