---
title: "CVE-2025-40775: DNS message with invalid TSIG causes an assertion failure"
slug: "cve-2025-40775"
description: "When an incoming DNS protocol message includes a Transaction Signature (TSIG), BIND always checks it. If the TSIG contains an invalid value in the algorithm field, BIND immediately aborts with an assertion failure."
updated: 2025-05-21T12:21:14Z
published: 2025-05-21T12:21:14Z
canonical: "kb.isc.org/cve-2025-40775"
---

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# CVE-2025-40775: DNS message with invalid TSIG causes an assertion failure

**CVE:** [CVE-2025-40775](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-40775)

**Title:** DNS message with invalid TSIG causes an assertion failure

**Document version:** 2.0

**Posting date:** 21 May 2025

**Program impacted:** [BIND 9](https://www.isc.org/bind/)

**Versions affected:**

BIND

- 9.20.0 -> 9.20.8
- 9.21.0 -> 9.21.7

(Versions prior to 9.18.0 were not assessed.)

**Severity:** High

**Exploitable:** Remotely

**Description:**

When an incoming DNS protocol message includes a Transaction Signature (TSIG), BIND always checks it. If the TSIG contains an invalid value in the algorithm field, BIND immediately aborts with an assertion failure.

**Impact:**

Denial-of-service. By sending specific messages to the server, an attacker can cause `named` to terminate unexpectedly.

- Authoritative servers are affected by this vulnerability.
- Resolvers are affected by this vulnerability.

**CVSS Score:** 7.5

**CVSS Vector:** CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

For more information on the Common Vulnerability Scoring System and to obtain your specific environmental score please visit: [https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1](https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1).

**Workarounds:**

No workarounds known.

**Active exploits:**

This flaw was discovered in internal testing. We are not aware of any active exploits.

**Solution:**

Upgrade to the patched release most closely related to your current version of BIND 9:

- 9.20.9
- 9.21.8

**Document revision history:**

- 1.0 Early Notification, 14 May 2025
- 1.1 Clarified the list of affected versions, 19 May 2025
- 2.0 Public disclosure, 21 May 2025

**Related documents:**

See our [BIND 9 Security Vulnerability Matrix](https://kb.isc.org/docs/aa-00913) for a complete listing of security vulnerabilities and versions affected.

**Do you still have questions?** Questions regarding this advisory should be mailed to [bind-security@isc.org](mailto:bind-security@isc.org) or posted as confidential GitLab issues at [https://gitlab.isc.org/isc-projects/bind9/-/issues/new?issue[confidential]=true](https://gitlab.isc.org/isc-projects/bind9/-/issues/new?issue%5Bconfidential%5D=true).

**Note:**

ISC patches only currently supported versions. When possible we indicate EOL versions affected. For current information on which versions are actively supported, please see [https://www.isc.org/download/](https://www.isc.org/download/).

**ISC Security Vulnerability Disclosure Policy:**

Details of our current security advisory policy and practice can be found in the ISC Software Defect and Security Vulnerability Disclosure Policy at [https://kb.isc.org/docs/aa-00861](https://kb.isc.org/docs/aa-00861).

The Knowledgebase article [https://kb.isc.org/docs/cve-2025-40775](https://kb.isc.org/docs/cve-2025-40775) is the complete and official security advisory document.

**Legal Disclaimer:**

Internet Systems Consortium (ISC) is providing this notice on an "AS IS" basis. No warranty or guarantee of any kind is expressed in this notice and none should be implied. ISC expressly excludes and disclaims any warranties regarding this notice or materials referred to in this notice, including, without limitation, any implied warranty of merchantability, fitness for a particular purpose, absence of hidden defects, or of non-infringement. Your use or reliance on this notice or materials referred to in this notice is at your own risk. ISC may change this notice at any time. A stand-alone copy or paraphrase of the text of this document that omits the document URL is an uncontrolled copy. Uncontrolled copies may lack important information, be out of date, or contain factual errors.