CVE-2023-50868: Critical Vulnerability in Closest Encloser Proof in DNS Protocol (NSEC3 Issue) Leading to DoS Attacks through High CPU Consumption

DNS protocols are an essential part of the internet as they resolve domain names into IP addresses, allowing users to connect to websites and servers. However, a critical vulnerability (CVE-2023-50868) has been discovered in the Closest Encloser Proof aspect of the DNS protocol, specifically in the RFC 5155 when RFC 9276 guidance is not followed. This vulnerability, commonly referred to as the "NSEC3" issue, allows remote attackers to cause a denial-of-service (DoS) attack. To achieve the attack, perpetrators can exploit the inefficiencies in the SHA-1 computations by sending malicious DNSSEC responses in random subdomain attacks. This article discusses how the attack works, shares code snippets, links to original references, and provides exploitation details.

Vulnerability: CVE-2023-50868 (NSEC3 Issue)

The NSEC3 issue arises from the RFC 5155 specification when it implies that an algorithm should perform thousands of iterations of a hash function in certain situations. When an attacker sends a series of DNSSEC responses with malicious subdomains, it forces the targeted DNS servers to conduct multiple instances of SHA-1 computations. Consequently, the server's CPU consumption rises drastically, ultimately causing a denial-of-service (DoS) attack.

Exploit Details

The exploit focuses on the Closest Encloser Proof aspect of the DNS protocol, particularly when RCS 9276 guidance is not followed for RFC 5155. The exploit takes advantage of the vulnerability, forcing targeted servers to consume significant CPU resources, making them unable to process legitimate requests. By continually feeding malicious DNSSEC responses, attackers can disable a DNS resolver temporarily or for an extended period.

Here's a simplified illustration of how this attack could be coded

import hashlib
import random
import string

# Function to generate random malicious subdomains
def generate_malicious_subdomains(domain):
    generated_subdomains = []

    for i in range(100): # or more iterations as desired by the attacker
        random_string = "".join(random.choices(string.ascii_letters + string.digits, k=10))
        subdomain = f"{random_string}.{domain}"
        generated_subdomains.append(subdomain)

    return generated_subdomains

# Function to simulate the DNS resolver processing the subdomains
def dns_resolver_simulation(subdomains):
    for subdomain in subdomains:
        # Simulate the SHA-1 computation that would occur within the DNS resolver
        sha1_hash = hashlib.sha1(subdomain.encode()).hexdigest()

# Example
domain = "example.com"
malicious_subdomains = generate_malicious_subdomains(domain)
dns_resolver_simulation(malicious_subdomains)

Original References

For more information on the closest encloser proof and DNS protocol specifications, refer to the following RFC documents:

- RFC 5155: DNS Security (DNSSEC) Hashed Authenticated Denial of Existence

- RFC 9276: Accommodating A Flag Day to Change DNSSEC Algorithms

Conclusion

CVE-2023-50868 (NSEC3 issue) is a critical vulnerability in the Closest Encloser Proof aspect of the DNS protocol, which can allow remote attackers to cause a denial-of-service (DoS) attack by exploiting the inefficiencies in SHA-1 computation. The vulnerability lies within RFC 5155 when RFC 9276 guidance is not followed. DNS servers need to be vigilant and adhere to the guidance provided to avoid the exploitation of this vulnerability.

Timeline

Published on: 02/14/2024 16:15:45 UTC
Last modified on: 06/10/2024 17:16:16 UTC