CVE-2026-39409

Description

## 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.

How to fix CVE-2026-39409

To remediate CVE-2026-39409, upgrade the affected package to a fixed version below.

• —upgrade to 4.12.12 or later

Is CVE-2026-39409 being exploited?

Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.

Affected packages (1)

• from 0, < 4.12.12

CVSS scores