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

> Remote attackers can crash the BIND DNS server by sending a message with an invalid TSIG algorithm, causing an assertion failure.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 May 2025
- **Updated:** 23 May 2025
- **Also known as:** ALPINE-CVE-2025-40775, AZL-61972, BELL-CVE-2025-40775, DEBIAN-CVE-2025-40775, GHSA-gqgx-hgvf-f75f, USN-7526-1, openSUSE-SU-2025:15156-1

## What is CVE-2025-40775?

CVE-2025-40775 is a high-severity denial of service vulnerability in ISC BIND 9, published on 21 May 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.
This issue affects BIND 9 versions 9.20.0 through 9.20.8 and 9.21.0 through 9.21.7.

Weakness: CWE-232 (Improper Handling of Undefined Values)

## Which versions are affected by CVE-2025-40775?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bind` | openSUSE:Tumbleweed | All versions | 9.20.9-1.1 |

## How do I fix CVE-2025-40775?

Upgrade to a fixed version.

## Is CVE-2025-40775 being exploited?

There is no known exploitation of CVE-2025-40775 in the wild and no public exploit code.

Public exploit code:

- Demonstrates a real-world zero-trust bypass by exploiting BIND CVE-2025-40775 to disrupt DNS, break secret rotation, and expose static credentials in a cloud-native lab. (github_poc): https://github.com/AlexSvobo/nhi-zero-trust-bypass

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40775 and fix it if it is affected.

What it is: CVE-2025-40775 is a high-severity denial of service vulnerability in ISC BIND 9, published on 21 May 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-40775 in the wild and no public exploit code.

Affected product: ISC BIND 9. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-40775/ (Markdown: https://insights.sbomify.com/CVE-2025-40775/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40775
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40775
- OSV: https://osv.dev/vulnerability/CVE-2025-40775

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40775/
Summaries are generated and may contain mistakes; the official description above is the record.