fastify proxy-addr up to 5.1.0 Address Determination access control

Summaryinfo

A vulnerability was found in fastify proxy-addr up to 5.1.0 and classified as critical. Affected is an unknown function of the component Address Determination. Executing a manipulation can lead to access control. This vulnerability is handled as CVE-2026-92395. The attack can be executed remotely. There is not any exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability has been found in fastify proxy-addr up to 5.1.0 and classified as critical. This vulnerability affects an unknown code of the component Address Determination. The manipulation with an unknown input leads to a access control vulnerability. The CWE definition for the vulnerability is CWE-284. The product does not restrict or incorrectly restricts access to a resource from an unauthorized actor. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

@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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-92395 since 09/16/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are neither technical details nor an exploit publicly available. The MITRE ATT&CK project declares the attack technique as T1068.

Upgrading to version 5.1.1 eliminates this vulnerability.

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Productinfo

Type

• Firewall Software

Vendor

• fastify

Name

• proxy-addr

Version

• 5.0
• 5.1.0

Website

• Product: https://github.com/fastify/proxy-addr/

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