# CVE-2025-40777: A possible assertion failure when 'stale-answer-client-timeout' is set to '0'

> Attackers can cause a denial of service in BIND caching resolvers by triggering an assertion failure under specific stale-answer configurations.

- **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:** 16 Jul 2025
- **Updated:** 4 Nov 2025
- **Also known as:** ALPINE-CVE-2025-40777, AZL-65553, BELL-CVE-2025-40777, DEBIAN-CVE-2025-40777, GHSA-4x4c-8qp9-8ggh, ROOT-OS-DEBIAN-11-CVE-2025-40777, SUSE-SU-2025:02349-1, USN-7641-1, openSUSE-SU-2025:15354-1

## What is CVE-2025-40777?

CVE-2025-40777 is a high-severity denial of service vulnerability in ISC BIND 9, published on 16 Jul 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

If a `named` caching resolver is configured with `serve-stale-enable` `yes`, and with `stale-answer-client-timeout` set to `0` (the only allowable value other than `disabled`), and if the resolver, in the process of resolving a query, encounters a CNAME chain involving a specific combination of cached or authoritative records, the daemon will abort with an assertion failure.
This issue affects BIND 9 versions 9.20.0 through 9.20.10, 9.21.0 through 9.21.9, and 9.20.9-S1 through 9.20.10-S1.

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2025-40777?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `bind` | SUSE:Linux Enterprise Module for Basesystem 15 SP7 | All versions | 9.20.11-150700.3.6.1 |
| `bind` | SUSE:Linux Enterprise Module for Server Applications 15 SP7 | All versions | 9.20.11-150700.3.6.1 |
| `bind` | openSUSE:Tumbleweed | All versions | 9.20.11-1.1 |

## How do I fix CVE-2025-40777?

Apply the vendor's patch.

## Is CVE-2025-40777 being exploited?

There is no known exploitation of CVE-2025-40777 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40777 and fix it if it is affected.

What it is: CVE-2025-40777 is a high-severity denial of service vulnerability in ISC BIND 9, published on 16 Jul 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-2025-40777 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-2025-40777/ (Markdown: https://insights.sbomify.com/CVE-2025-40777/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40777
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40777
- OSV: https://osv.dev/vulnerability/CVE-2025-40777

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40777/
Summaries are generated and may contain mistakes; the official description above is the record.