CVE-2026-90711: proxy-addr vulnerable to IP spoofing via IPv4-mapped IPv6 trust subnet
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 10.0.0.0/8). If IPv4-mapped IPv6 notation is required, use the full form so the prefix covers the mapped marker (for example ::ffff:10.0.0.0/104 for the 10.0.0.0/8 block).
Other sources
proxy-addr is a Node.js module that determines a request's client address behind trusted reverse proxies, and it backs Express req.ip and req.ips. In versions 1.1.0 through 2.0.7, 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. This is a fail-open regression introduced in version 1.1.0. The issue is fixed in proxy-addr 2.0.8, and users should upgrade to 2.0.8 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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/proxy-addrto a version that resolves this vulnerability.Fixed in 2.0.8 - Upgrade
Upgrade
proxy-addrto a version that resolves this vulnerability.Fixed in 2.0.8 - Configuration
Write IPv4 trust subnets in plain IPv4 notation, such as 10.0.0.0/8, or use an IPv4-mapped IPv6 prefix length of at least 97; for example, use ::ffff:10.0.0.0/104 for the 10.0.0.0/8 block instead of ::ffff:10.0.0.0/8.
proxy-addr IPv4-mapped IPv6 trust subnet prefix/notation = Use a prefix length of at least 97 or plain IPv4 notation
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Deployments using proxy-addr versions 1.1.0 through 2.0.7 are exposed if they configure a trusted proxy subnet in IPv4-mapped IPv6 notation with an IPv4-sized prefix, such as ::ffff:10.0.0.0/8. Plain IPv4 notation is not identified as affected.
What does an attacker need to exploit it?
An unauthenticated network client only needs to reach the application and send an arbitrary X-Forwarded-For header. The malformed trust subnet causes the socket peer to be trusted, allowing the attacker to control the client address read by the application.
How can this be mitigated before upgrading?
Upgrade to proxy-addr 2.0.8 or later when possible. Until then, ensure IPv4-mapped IPv6 trust subnets use a prefix length of at least 97, or express the trusted range in plain IPv4 notation.
What application controls can be bypassed?
Any control that relies on the address returned through proxy-addr, including Express req.ip and req.ips, may be affected. This includes IP-based access control, rate limiting, geolocation, and audit logging.