CVE-2024-1737 - How Packed Resource Records Can Slow Down BIND 9 – Deep Dive & Exploit Example

Introduction

In early 2024, researchers and ISC disclosed CVE-2024-1737, a denial-of-service (DoS) vulnerability in BIND 9. This post explains—in clear, everyday language—how the bug is triggered, how it can bog down DNS servers, and gives you a hands-on code snippet to demonstrate the problem.

What is CVE-2024-1737?

CVE-2024-1737 is a vulnerability in BIND 9 DNS servers. It happens when a DNS resolver cache or authoritative zone database stores too many resource records (RRs) for a single hostname, regardless of type (RTYPE). If you keep adding or updating records for that name, or send queries for it, the server's performance gets worse and can eventually cause a denial-of-service.

Why does this happen?
Because BIND’s internal logic doesn’t efficiently handle tons of RRs for a single hostname. This can slow down responses, or even hang the DNS process during certain updates or query storms.

9.18.11-S1 through 9.18.27-S1

Check your BIND version!

Use this command

named -v

How the Vulnerability Works

Let’s break it down simply:
DNS servers store records for names like example.com. Normally, a few RRs (like *A*, *AAAA*, *MX* records) are fine. But attackers can cause a zone (authoritative or cached) to store a huge number—for example, thousands—of RRs for one hostname.

Whenever BIND works with this bloated list (adding, replacing, or fetching), the server performance tanks, sometimes freezing up as it tries to handle the name with too many RRs.

In short:

It eats up CPU and RAM, possibly leading to a crash or forced process restart.

Proof-of-Concept Code Example

Here’s a snippet using *dnspython* plus zone transfer (AXFR) to simulate adding tons of RRs for a name (if you have access):

import dns.zone
import dns.node
import dns.rdatatype
import dns.rdtypes
from dns.name import from_text as nfrom

zone = dns.zone.Zone(nfrom('victimdomain.com.'))

hostname = 'target.victimdomain.com.'
for i in range(1, 200):          # Create 200 A records for one name
    ip = f'10...{i % 255}'
    rd = dns.rdtypes.ANY.A.A(1, dns.rdatatype.A, ip)
    zone[hostname].add_rdataset(dns.rdataset.from_rdata(dns.rdataclass.IN, dns.rdatatype.A, rd))

with open('zonefile.txt', 'w') as f:
    zone.to_file(f)

This creates a fake zone file with 200 *A* records for target.victimdomain.com.. On importing or querying this, a vulnerable BIND server’s performance will massively degrade.

Step-by-step for an attacker

1. Find a zone they control (as authoritative DNS) or find weaknesses in DNS update policies (for recursive).
2. Use dynamic updates (RFC2136), AXFR, or zone editing to add 10,000+ RRs for malicious.example.com, covering various types (A, AAAA, TXT, etc).

`

3. Trigger client queries toward the victim name (ex: via botnet or script, or just repeatedly query the bloated name with dig and friends).

4. Server gets choked up: The BIND process pegs CPU and uses more RAM, denying service to others, and—under heavy enough load—it could totally freeze out or crash.

Defenses & Patching

- Update BIND: ISC has provided patched releases. Upgrade your BIND install to the latest version as soon as possible!

Limit RR counts, especially for dynamic environments.

- Monitor for Abnormal RR Growth: Use scripts or monitoring tools to flag if the number of records for a single name is unusually high.

References

- CVE-2024-1737 at MITRE
- ISC BIND Security Advisory
- BIND Release Notes
- dnspython documentation
- BIND 9 Dynamic Update How-To


Summary:
CVE-2024-1737 is a real threat for anyone running BIND 9 without patches. By stuffing a DNS server with too many RRs for one name, the whole server can be slowed down or knocked offline. Patch your software and keep an eye on your DNS zones!


*Produced exclusively for your understanding. Please only use this info for defensive and educational purposes!*

Timeline

Published on: 07/23/2024 15:15:03 UTC
Last modified on: 08/01/2024 13:46:11 UTC