If you run your own DNS servers, you’ve probably heard of BIND. It’s the backbone of most DNS installations. Lately, a new vulnerability—CVE-2025-40778—has caught the attention of security pros because it could let bad actors inject fake data into your DNS cache.

Let’s break it down. I’ll explain exactly what went wrong, why it’s dangerous, who’s vulnerable, and how an attacker might exploit it, with snippets of code and references for a deeper dive.

What is CVE-2025-40778?

CVE-2025-40778 is a recently patched vulnerability in BIND 9—the Internet’s most popular DNS server. It occurs when BIND gets too lenient with processing records from incoming DNS answers. Under certain situations, this allows attackers to inject *forged* or *malicious* DNS records into the server’s cache.

This is classic DNS cache poisoning—where attackers make your DNS server save bogus IP addresses for websites, possibly redirecting your users to phishing sites, malware, or whatever else they want.

BIND 9.20.9-S1 through 9.20.13-S1

If you’re running BIND anywhere in those version ranges—update now.

Here’s how the vulnerability works in plain English

- When BIND gets a DNS answer, it sometimes accepts and caches extra records included in the answer, even if they don't truly “match” the original question or expected types.
- A clever attacker, if able to inject their own response (sometimes by racing the real server or exploiting network infrastructure), can smuggle in malicious records.
- These records can then live in your DNS cache, tricking your users until the records expire (called the TTL).

BIND asks upstream for the IP address.

3. Attacker manages to inject a fake answer that not only replies for bank.example.com, but also includes a forged record for login.example.com.

Let's see, in simplified Python, how an attacker might try to inject a forged record

import socket

# Attacker crafts a DNS response
FAKE_DNS_ANSWER = b'\x12\x34\x81\x80\x00\x01\x00\x02\x00\x00\x00\x00' \
    b'\x06bank\x07example\x03com\x00\x00\x01\x00\x01' \
    # Answer 1: Correct answer for original query
    b'\xc\xc\x00\x01\x00\x01\x00\x00\xe\x10\x00\x04\x7f\x00\x00\x02' \
    # Answer 2: Forged answer for unrelated query
    b'\x05login\xc\xc\x00\x01\x00\x01\x00\x00\xe\x10\x00\x04\x01\x02\x03\x04'

server_address = ('victim-bind-server', 53)
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(FAKE_DNS_ANSWER, server_address)

*Note:* This snippet does not exploit the race conditions or send proper DNS queries, but shows how a forged answer could be packed. Tools like Scapy are often used for real DNS attacks, allowing for more precise packet crafting.

Interception: Attacker sniffs sensitive traffic.

Anyone relying on your DNS for security (think firewalls, email servers) can be tricked.

Official Advisory and References

- ISC CVE Announcement
- Release Notes & Patch Details
- BIND 9 Security Notifications

How to Stay Safe

Patch immediately!

You can update from your OS package manager, or directly from ISC

sudo apt-get update && sudo apt-get install bind9
# or use your distro’s equivalent

Restart BIND after upgrading

sudo systemctl restart bind9

Limit who can send DNS requests to your server (use allow-query restrictions).

- Use DNSSEC.

Final Thoughts

CVE-2025-40778 is a big reminder that even mature, widely-trusted servers like BIND can have simple, old-school logic bugs with serious consequences. If your DNS server is open to the Internet, patch immediately and double-check your defenses. Attackers are always looking for these “soft spots” in core Internet infrastructure.

If you want to dig deeper or need help patching, check the official advisory or drop into the ISC support channels.

Timeline

Published on: 10/22/2025 16:15:42 UTC
Last modified on: 11/04/2025 22:16:11 UTC