CVE-2026-39409

Summary

ipRestriction() does not canonicalize IPv4-mapped IPv6 client addresses (e.g. ::ffff:127.0.0.1) before applying IPv4 allow or deny rules. In environments such as Node.js dual-stack, this can cause IPv4 rules to fail to match, leading to unintended authorization behavior.

Details

The middleware classifies client addresses based on their textual form. Addresses containing ":" are treated as IPv6, including IPv4-mapped IPv6 addresses such as ::ffff:127.0.0.1. These addresses are not normalized to IPv4 before matching.

As a result:

• IPv4 static rules (e.g. 127.0.0.1) do not match because the raw string differs
• IPv4 CIDR rules (e.g. 127.0.0.0/8, 10.0.0.0/8) are skipped because the address is treated as IPv6

For example, with:

denyList: ['127.0.0.1']

a request from 127.0.0.1 may be represented as ::ffff:127.0.0.1 and bypass the deny rule.

This behavior commonly occurs in Node.js environments where IPv4 clients are exposed as IPv4-mapped IPv6 addresses.

Impact

Applications that rely on IPv4-based ipRestriction() rules may incorrectly allow or deny requests.

In affected deployments, a denied IPv4 client may bypass access restrictions. Conversely, legitimate clients may be rejected when using IPv4 allow lists.