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CVE-2026-77860

LOW 3.7

Published 2026-09-16 · Last modified 2026-09-16

In NLnetLabs Unbound 1.20.0 up to and including 1.26.0, a vulnerability on the 'serve-expired' code path can cause a double decrement on the 'wait-limit' counter per client IP essentially bypassing one of the counter measures that was introduced for DNSBomb (CVE-2024-33655). A malicious actor can exploit this by controlling an authoritative zone with short TTL, so cached entries expire quickly. Each 'slow' query, one the attacker's authoritative never answers, is followed by one query for an expired cached name, which is answered immediately via the 'serve-expired' path and decrements the counter twice. This second query was named 'pump'. By alternating slow queries and pumps, the attacker keeps the per-client counter at or below the configured 'wait-limit' indefinitely, and can hold an arbitrary number of pending queries from a single source IP, up to the global mesh quota (num-queries-per-thread); eventually bypassing one of the counter measures introduced for DNSBomb (CVE-2024-33655). This vulnerability is present on the 'serve-expired' code path.

NO EXPLOITATION SIGNALS

No known exploitation, public exploit, or elevated probability at this time. Track for changes.

Exploitation likelihood

0.3%chance of exploitation in 30 days · 23rd percentile

○ In CISA KEV ○ Catalogued PoC

Impact if exploited

3.7CVSS 3.1 · LOW

  • ConfidentialityNone
  • IntegrityNone
  • AvailabilityLow

What an attacker needs

  • ✓Access: Reachable over the network — no local access needed
  • ✓Privileges: No account or privileges required
  • ✓User interaction: No user interaction needed
  • ⚠Complexity: Needs a race window or specific setup

✓ lowers the bar for an attacker · ⚠ raises it

Proof of concept

No public PoC is currently catalogued.

Search for new PoCs

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Tracked sources are checked daily for recent CVEs, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.

Data snapshot:

Affected

Vendors Nlnet Labs Nlnetlabs

Products Unbound

Weakness (CWE)

  • CWE-675: : Multiple Operations on Resource in Single-Operation Context

CVSS vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References and research data (JSON)

Vendor product assessments

Showing 32 of 48 vendor product statements; 16 omitted. Read the full vendor advisory.

  • python3-unbound-0:1.26.1-1.hum1@aarch64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=aarch64&distro=hummingbird-20251124&repository_id=public-hummingbird-aarch64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-0:1.26.1-1.hum1@aarch64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=aarch64&distro=hummingbird-20251124&repository_id=public-hummingbird-aarch64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-0:1.26.1-1.hum1@src as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=src&distro=hummingbird-20251124&repository_id=public-hummingbird-source-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-anchor-0:1.26.1-1.hum1@aarch64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=aarch64&distro=hummingbird-20251124&repository_id=public-hummingbird-aarch64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-anchor-0:1.26.1-1.hum1@x86_64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=x86_64&distro=hummingbird-20251124&repository_id=public-hummingbird-x86_64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-devel-0:1.26.1-1.hum1@x86_64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=x86_64&distro=hummingbird-20251124&repository_id=public-hummingbird-x86_64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-dracut-0:1.26.1-1.hum1@x86_64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=x86_64&distro=hummingbird-20251124&repository_id=public-hummingbird-x86_64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-libs-0:1.26.1-1.hum1@aarch64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=aarch64&distro=hummingbird-20251124&repository_id=public-hummingbird-aarch64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-munin-0:1.26.1-1.hum1@noarch@public-hummingbird-aarch64-rpms as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=noarch&distro=hummingbird-20251124&repository_id=public-hummingbird-aarch64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-munin-0:1.26.1-1.hum1@noarch@public-hummingbird-x86_64-rpms as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=noarch&distro=hummingbird-20251124&repository_id=public-hummingbird-x86_64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-utils-0:1.26.1-1.hum1@x86_64 as a component of Red Hat Hardened Images · fixed

    Vendor assessment for this product only.

    pkg:rpm/redhat/[email protected]?arch=x86_64&distro=hummingbird-20251124&repository_id=public-hummingbird-x86_64-rpms

    • vendor fix: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/ Vendor guidance
    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • python3-unbound as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/python3-unbound

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-devel as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound-devel

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-dracut as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound-dracut

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-utils as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound-utils

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound.src as a component of Red Hat Enterprise Linux 10 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound?arch=src

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • python3-unbound as a component of Red Hat Enterprise Linux 9 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/python3-unbound

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-devel as a component of Red Hat Enterprise Linux 9 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound-devel

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-dracut as a component of Red Hat Enterprise Linux 9 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound-dracut

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound.src as a component of Red Hat Enterprise Linux 9 · known affected

    Vendor assessment for this product only.

    pkg:rpm/redhat/unbound?arch=src

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • openshift/ose-rhel-coreos-9 as a component of Red Hat OpenShift Container Platform 4 · known affected

    Vendor assessment for this product only.

    pkg:oci/[email protected]?repository_url=registry.redhat.io/openshift/ose-rhel-coreos-9

    • workaround: To mitigate this issue, disable the 'serve-expired' option in the Unbound configuration. This can be achieved by setting 'serve-expired: no' in the 'unbound.conf' file. A restart of the Unbound service is required for the changes to take effect. Disabling this feature may impact the availability of stale DNS records.
    • none available: Fix deferred
    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound as a component of Red Hat Enterprise Linux 6 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-libs as a component of Red Hat Enterprise Linux 6 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-libs

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-python as a component of Red Hat Enterprise Linux 6 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-python

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound as a component of Red Hat Enterprise Linux 7 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-libs as a component of Red Hat Enterprise Linux 7 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-libs

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-python as a component of Red Hat Enterprise Linux 7 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-python

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • python3-unbound as a component of Red Hat Enterprise Linux 8 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/python3-unbound

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-devel as a component of Red Hat Enterprise Linux 8 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-devel

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • unbound-libs as a component of Red Hat Enterprise Linux 8 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-libs

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00
  • openshift/ose-rhel-coreos-8 as a component of Red Hat OpenShift Container Platform 4 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:oci/[email protected]?repository_url=registry.redhat.io/openshift/ose-rhel-coreos-8

    Red Hat · CVE-2026-77860 · Updated 2026-09-21T11:54:38+00:00

Known Affected Software Configurations

VendorProductVersion range
NlnetlabsUnbound≥ 1.20.0 and < 1.26.1

All CVSS metrics

  • LOW 3.7 v3.1 · CNA Primary
    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  • LOW 3.7 v3.1 · NVD Secondary
    CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References

Patches & mitigations

Advisories

Technical & other

Sources: NVD · CVE.org · EPSS