How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

326,895

Total vulnerabilities in the database

CVE-2026-30827

Summary

The default keyGenerator in express-rate-limit applies IPv6 subnet masking (/56 by default) to all addresses that net.isIPv6() returns true for. This includes IPv4-mapped IPv6 addresses (::ffff:x.x.x.x), which Node.js returns as request.ip on dual-stack servers.

Because the first 80 bits of all IPv4-mapped addresses are zero, a /56 (or any /32 to /80) subnet mask produces the same network key (::/56) for every IPv4 client. This collapses all IPv4 traffic into a single rate-limit bucket: one client exhausting the limit causes HTTP 429 for all other IPv4 clients.

Details

Root Cause

In source/ip-key-generator.ts:

export function ipKeyGenerator(ip: string, ipv6Subnet: number | false = 56) {
  if (ipv6Subnet &amp;&amp; isIPv6(ip)) {
    return `${new Address6(`${ip}/${ipv6Subnet}`).startAddress().correctForm()}/${ipv6Subnet}`
  }
  return ip
}

net.isIPv6('::ffff:192.168.1.1') returns true, so IPv4-mapped addresses enter the subnet masking path. With a /56 prefix, the start address for any ::ffff:x.x.x.x is ::, producing the key ::/56.

Proof of Concept

const { isIPv6 } = require(&#039;net&#039;);
const { Address6 } = require(&#039;ip-address&#039;);

function ipKeyGenerator(ip, ipv6Subnet = 56) {
  if (ipv6Subnet &amp;&amp; isIPv6(ip)) {
    return `${new Address6(`${ip}/${ipv6Subnet}`).startAddress().correctForm()}/${ipv6Subnet}`;
  }
  return ip;
}

console.log(ipKeyGenerator(&#039;::ffff:192.168.1.1&#039;, 56)); // ::/56
console.log(ipKeyGenerator(&#039;::ffff:10.0.0.1&#039;, 56));    // ::/56
console.log(ipKeyGenerator(&#039;::ffff:8.8.8.8&#039;, 56));     // ::/56
// ALL produce &#039;::/56&#039; — same bucket

End-to-End Validation

On a dual-stack Express server (app.listen(port, '::')), tested with Express 5.2.1:

request.ip for IPv4 clients is ::ffff:127.0.0.1
Rate limit key resolves to ::/56
After limit requests from any IPv4 client, all other IPv4 clients receive 429

When This Occurs

Node.js dual-stack servers (default on Linux when listening on ::)
Any environment where request.ip contains IPv4-mapped IPv6 addresses
Only affects the default keyGenerator (custom key generators are not affected)

Impact

Denial of Service: A single client can block all IPv4 traffic by exhausting the shared rate limit
Affects default configuration: No special options needed to trigger this

Affected Versions

All versions of express-rate-limit between v8.0.0 and v8.2.1.

Fix

This issue was fixed in commit 14e53888cdfd1b9798faf5b634c4206409e27fc4. This fix has been included in release v8.3.0, and backported to all affected minor versions in the form of releases v8.2.2, v8.1.1, and v8.0.2.

Published: Mar 6, 2026
Updated: Mar 7, 2026
CVE: CVE-2026-30827
GHSA: GHSA-46wh-pxpv-q5gq
Severity: High
Exploit:

CVSS v3:

Severity: Unknown
Score:
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CWEs:

CWE-770

Affected Software
References

Software	From	Fixed in
express-rate-limit	8.2.0	8.2.2
express-rate-limit	8.1.0	8.1.0.x
express-rate-limit	8.1.0	8.1.1
express-rate-limit	8.0.0	8.0.2

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.