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CVE-2026-40691

HIGH 7.5 ALL-YEARS

Published 2026-07-22 · Last modified 2026-07-22 · NLnet Labs · NVD: Analyzed

In Unbound 1.9.0 up to and including 1.25.1, when a DNSCrypt query is received over TCP, the routine that encrypts the reply in place fails to bound the reply length against the destination buffer size. The size clamp that protects the UDP path is not applied on the TCP path, so a reply larger than 65504 bytes is shifted forward by 48 bytes inside a buffer of capacity equal to 'msg-buffer-size', writing past the end of the heap allocation. A single malicious encrypted query crashes the resolver and lead to denial of service. This vulnerability needs Unbound to be compiled with DNSCrypt support ('--enable-dnscrypt') and the 'dnscrypt:' clause to be configured and enabled for the listening interfaces.

ELEVATED IMPACT

Severe if exploited (CVSS 7.5), but no confirmed exploitation. Patch on a normal cadence.

Exploitation likelihood

0.3%chance of exploitation in 30 days · 22nd percentile

○ In CISA KEV○ Catalogued PoC◆ SSVC: exploitation none, automatable yes

Impact if exploited

7.5CVSS 3.1 · HIGH · CNA

  • ConfidentialityNone
  • IntegrityNone
  • AvailabilityHigh

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: No special conditions — reliably repeatable

✓ 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 weekly for the full archive, including PoCs published after the original disclosure. A later release appears after a tracked source lists it and a refresh succeeds.

Affected

Vendors Nlnet Labs Nlnetlabs

Products Unbound

Weakness (CWE)

  • CWE-122: : Heap-based Buffer Overflow
  • CWE-787: : Out-of-bounds Write

CVSS vector

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

References and research data (JSON)

Vendor product assessments

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

  • python3-unbound-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • python3-unbound-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-anchor-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-anchor-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-devel-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-dracut-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-dracut-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-libs-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-libs-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-munin-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-munin-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-utils-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-utils-0:1.25.2-0.1.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
    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • python3-unbound as a component of Red Hat Enterprise Linux 10 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/python3-unbound

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-anchor as a component of Red Hat Enterprise Linux 10 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-anchor

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-dracut as a component of Red Hat Enterprise Linux 10 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-dracut

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-utils as a component of Red Hat Enterprise Linux 10 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-utils

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+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-2026-40691 · Updated 2026-10-19T17:35:23+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-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound.src 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?arch=src

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-devel 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-devel

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+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-2026-40691 · Updated 2026-10-19T17:35:23+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-2026-40691 · Updated 2026-10-19T17:35:23+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-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound.src 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?arch=src

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound as a component of Red Hat Enterprise Linux 9 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound-dracut as a component of Red Hat Enterprise Linux 9 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound-dracut

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • unbound.src as a component of Red Hat Enterprise Linux 9 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not present

    pkg:rpm/redhat/unbound?arch=src

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00
  • openshift/ose-rhel-coreos-9 as a component of Red Hat OpenShift Container Platform 4 · known not affected

    Vendor assessment for this product only.

    Reason: vulnerable code not in execute path

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

    Red Hat · CVE-2026-40691 · Updated 2026-10-19T17:35:23+00:00

Known Affected Software Configurations

VendorProductVersion range
NlnetlabsUnbound≥ 1.9.0 and < 1.25.2

All CVSS metrics

  • HIGH 7.5 v3.1 · CNA Primary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  • HIGH 7.5 v3.1 · NVD Secondary
    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

Advisories

Technical & other

Sources: NVD · CVE.org · All sources and attribution · EPSS