# CVE-2025-40776: Birthday Attack against Resolvers supporting ECS

> Attackers can poison the cache of a BIND 9 resolver configured with EDNS Client Subnet, leading to incorrect DNS responses.

- **Severity:** High 8.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:H/A:N`)
- **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:** 22 Jul 2025
- **Also known as:** ALPINE-CVE-2025-40776, BELL-CVE-2025-40776, GHSA-2hm8-9847-q7gc

## What is CVE-2025-40776?

CVE-2025-40776 is a high-severity 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

A `named` caching resolver that is configured to send ECS (EDNS Client Subnet) options may be vulnerable to a cache-poisoning attack.
This issue affects BIND 9 versions 9.11.3-S1 through 9.16.50-S1, 9.18.11-S1 through 9.18.37-S1, and 9.20.9-S1 through 9.20.10-S1.

Weakness: CWE-349 (Acceptance of Extraneous Untrusted Data With Trusted Data)

## Which versions are affected by CVE-2025-40776?


## How do I fix CVE-2025-40776?

Upgrade to a fixed version.

## Is CVE-2025-40776 being exploited?

There is no known exploitation of CVE-2025-40776 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40776 and fix it if it is affected.

What it is: CVE-2025-40776 is a high-severity 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-2025-40776 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-2025-40776/ (Markdown: https://insights.sbomify.com/CVE-2025-40776/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40776
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40776
- OSV: https://osv.dev/vulnerability/CVE-2025-40776

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40776/
Summaries are generated and may contain mistakes; the official description above is the record.