April 30, 2024

CVE-2023-50868: Vulnerability in NSEC3 Iteration and Closest Encloser Proof

The security of our digital infrastructure is of utmost importance in the face of ever-evolving cyber threats. One such vulnerability, CVE-2023-50868, has recently come to light, impacting BIND 9, a popular DNS server software. In this blog, we will delve into the details of this vulnerability, its potential impact, and discuss effective measures to mitigate the risk.

Understanding CVE-2023-50868

This vulnerability takes advantage of DNSSEC-signed zones that are meticulously crafted by attackers, leveraging NSEC3 parameters configured beyond the bounds set by the Best Current Practice RFC9276. By incorporating additional iterations, the attacker sets the stage for a random subdomain assault on this zone.

What makes this vulnerability particularly intriguing is that resolvers, driven by the absence of universal adherence to the Best Practice document, accept these surplus iterations. As a result, they unwittingly devote significant CPU cycles to SHA1 hashing, unknowingly exhausting their resources. At its core, this vulnerability capitalizes on the unsuspecting acceptance of surplus SHA1 hash iterations, creating an avenue for a devastating denial-of-service attack.

What sets this vulnerability apart is the subtle oversight within the relevant standard, RFC 5155 section 8.3, which fails to provide a warning about this lurking risk. Adding to the complexity, multiple implementations have neglected to safeguard against this vulnerability, leaving an array of systems susceptible to exploitation.

The novelty of CVE-2023-50868 lies in its ability to manipulate not only the targeted zone but also the number of retries executed by the Closest Encloser Proof algorithm. This unprecedented level of control empowers the attacker, amplifying the effectiveness of the attack to a staggering 125 times greater than previously believed.

Mitigation Strategies

To protect your DNS resolution service from the risks posed by CVE-2023-50868, it is crucial to implement effective mitigation strategies. While disabling DNSSEC validation entirely is a possible workaround, it is not recommended. Instead, we strongly advise upgrading your BIND version to one of the following:

  • BIND 9.16.47 or later
  • BIND 9.18.23 or later
  • BIND 9.19.21 or later

These updated versions address the vulnerability by optimizing DNSSEC validation and minimizing the impact on server workload. Additionally, adhering to the Best Current Practice document RFC 9276, which provides guidance on NSEC3 parameter settings, is essential for authoritative server operators to avoid potential problems in the future.

Nexusguard Ensures Unaffected DNS Protection for Customers

Within the intricate web of the DNS infrastructure, Nexusguard's DNS Protection (DP) hosting service plays a vital role as an Authoritative Name Server. While resolvers handle the essential task of mapping domain names to IP addresses, it's the authoritative name server's responsibility to provide accurate and authoritative DNS information for specific domains.

Nexusguard's DP hosting service excels in managing DNS queries for the domains under its authority. With meticulous precision, it maintains up-to-date DNS records, ensuring the domains it hosts remain seamlessly connected. Consequently, when a DNS query targets a domain hosted on Nexusguard's DP hosting service, the response is swift and backed by authoritative DNS information.

Crucially, the vulnerability in question primarily affects resolvers, which constitute distinct components within the broader DNS ecosystem. However, thanks to Nexusguard's DP hosting service's focused architecture and design, prioritizing its role as an authoritative name server, it remains unaffected by the aforementioned vulnerability. 

For more information, please read about Nexusguard’s DNS Protection or reach out to us via our contact form.

As the digital landscape becomes increasingly complex, proactive measures are crucial to protect your infrastructure from DNSEC resource exhaustion vulnerabilities like CVE-2023-50868. Stay informed about security advisories, promptly apply patches and updates, and follow best practices to fortify your DNS resolution service.

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