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

Published May 20, 2026
·
Updated

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.

Other sources

Unbounded resend loop in BIND 9 resolver

— Microsoft

Affected Software

6 affected componentsFixes available
ISC BIND 9>=9.18.36<=9.18.48, >=9.20.8<=9.20.22, >=9.21.7<=9.21.21, >=9.18.36-S1<=9.18.48-S1, >=9.20.9-S1<=9.20.22-S1
ISC BIND>=9.18.36<9.18.49
ISC BIND>=9.20.8<9.20.23
ISC BIND>=9.21.7<9.21.21
Microsoft azl3 bind 9.20.21-1
Patch available
debian/bind9<=1:9.16.50-1~deb11u2, <=1:9.16.50-1~deb11u5, <=1:9.18.47-1~deb12u1, <=1:9.20.21-1~deb13u1
1:9.18.49-1~deb12u11:9.20.23-1~deb13u11:9.20.23-1

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade debian/bind9 to a version that resolves this vulnerability.

    Fixed in 1:9.18.49-1~deb12u1Fixed in 1:9.20.23-1~deb13u1Fixed in 1:9.20.23-1
  2. Upgrade

    Upgrade BIND 9 to a version that resolves this vulnerability.

    Fixed in 9.18.49
  3. Upgrade

    Upgrade BIND 9 to a version that resolves this vulnerability.

    Fixed in 9.20.23
  4. Upgrade

    Upgrade BIND 9 to a version that resolves this vulnerability.

    Fixed in 9.21.22
  5. Upgrade

    Upgrade BIND 9 to a version that resolves this vulnerability.

    Fixed in 9.18.49-S1
  6. Upgrade

    Upgrade BIND 9 to a version that resolves this vulnerability.

    Fixed in 9.20.23-S1

Event History

May 20, 2026
CVE Published
via MITRE·01:10 PM
Data Sourced
via MITRE·01:10 PM
RemedyDescriptionSeverityWeakness
Data Sourced
via NVD·01:16 PM
RemedyDescriptionSeverityWeaknessAffected Software
May 21, 2026
Data Sourced
via Launchpad·08:49 PM
Description
May 22, 2026
Data Sourced
via Ubuntu·08:48 PM
RemedyDescriptionSeverityAffected Software
May 23, 2026
Data Sourced
via Microsoft·08:01 AM
DescriptionSeverityWeaknessAffected Software
Updated
via Microsoft·08:01 AM
DescriptionSeverity
Data Sourced
via Debian·08:50 PM
DescriptionAffected Software
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Frequently Asked Questions

1

What is the severity of CVE-2026-5950?

CVE-2026-5950 is classified as a critical severity vulnerability that can lead to severe resource exhaustion.

2

How do I fix CVE-2026-5950?

To mitigate CVE-2026-5950, upgrade to the latest versions of BIND 9 that address this issue.

3

What software is affected by CVE-2026-5950?

CVE-2026-5950 affects specific versions of Internet Systems Consortium BIND 9, particularly between 9.18.36 and 9.18.48, among others.

4

What type of attack does CVE-2026-5950 enable?

CVE-2026-5950 allows remote unauthenticated attackers to exploit the BIND 9 resolver to create an unbounded resend loop, resulting in resource exhaustion.

5

How can CVE-2026-5950 impact my system?

CVE-2026-5950 can lead to denial of service on affected systems by overwhelming them with continuous query retries.

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