# CVE-2026-5947: SIG(0) validation during query flood may lead to undefined behavior

> A race condition in BIND's SIG(0) validation during query floods can cause a use-after-free, potentially leading to crashes or code execution.

- **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:** 20 May 2026
- **Updated:** 1 Sep 2026
- **Also known as:** ALPINE-CVE-2026-5947, AZL-87099, BELL-CVE-2026-5947, DEBIAN-CVE-2026-5947, ECHO-e0dd-6c9a-2e1c, GHSA-6mm6-m775-chpm, MINI-w957-5q6j-whqw, UBUNTU-CVE-2026-5947

## What is CVE-2026-5947?

CVE-2026-5947 is a high-severity use after free vulnerability in Isc Bind, published on 20 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Undefined behavior may result due to a race condition leading to a use-after-free violation.  If BIND receives an incoming DNS message signed with SIG(0), it begins work to validate that signature.  If, during that validation, the "recursive-clients" limit is reached (as would occur during a query flood), and that same DNS message is discarded per the limit, there is a brief window of time while the SIG(0) validation may attempt to read the now-discarded DNS message.
This issue affects BIND 9 versions 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, and 9.20.9-S1 through 9.20.22-S1.
BIND 9 versions 9.18.28 through 9.18.49 and 9.18.28-S1 through 9.18.49-S1 are NOT affected.

Weakness: CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition), CWE-416 (Use After Free), CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2026-5947?

It affects 1 product.


Products: Isc Bind.

## How do I fix CVE-2026-5947?

Apply the vendor's patch.

## Is CVE-2026-5947 being exploited?

There is no known exploitation of CVE-2026-5947 in the wild and no public exploit code.

## Other vulnerabilities in Isc Bind

- [CVE-2008-0122](https://insights.sbomify.com/CVE-2008-0122/): Memory corruption in n/a (critical 10.0)
- [CVE-2001-0010](https://insights.sbomify.com/CVE-2001-0010/): Memory corruption in n/a (critical 10.0)
- [CVE-2001-0011](https://insights.sbomify.com/CVE-2001-0011/): Memory corruption in n/a (critical 10.0)
- [CVE-2001-0013](https://insights.sbomify.com/CVE-2001-0013/): Memory corruption in n/a (critical 10.0)
- [CVE-2000-1029](https://insights.sbomify.com/CVE-2000-1029/): Memory corruption in n/a (critical 10.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-5947 and fix it if it is affected.

What it is: CVE-2026-5947 is a high-severity use after free vulnerability in Isc Bind, published on 20 May 2026. 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-2026-5947 in the wild and no public exploit code.

Affected product: Isc Bind. 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-2026-5947/ (Markdown: https://insights.sbomify.com/CVE-2026-5947/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-5947
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-5947
- OSV: https://osv.dev/vulnerability/CVE-2026-5947

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-5947/
Summaries are generated and may contain mistakes; the official description above is the record.