CVE-2026-55991
MEDIUM 5.9 ALL-YEARSIn NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream.
No known exploitation, public exploit, or elevated probability at this time. Track for changes.
Exploitation likelihood
0.4%chance of exploitation in 30 days · 30th percentile
Impact if exploited
5.9CVSS 3.1 · MEDIUM · 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: 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
Search GitHub now — results may be empty or unverified.
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.
Weakness (CWE)
- CWE-195: : Signed to Unsigned Conversion Error
CVSS vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References and research data (JSON)
Vendor product assessments
Showing 32 of 35 vendor product statements; 3 omitted. Read the full vendor advisory.
unbound-main@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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
unbound-main@noarch 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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
unbound-main@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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
unbound-main@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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
unbound as a component of Red Hat Enterprise Linux 10 · known affected
Vendor assessment for this product only.
pkg:rpm/redhat/unbound
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
unbound-anchor as a component of Red Hat Enterprise Linux 10 · known affected
Vendor assessment for this product only.
pkg:rpm/redhat/unbound-anchor
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
unbound-libs as a component of Red Hat Enterprise Linux 10 · known affected
Vendor assessment for this product only.
pkg:rpm/redhat/unbound-libs
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
unbound as a component of Red Hat Enterprise Linux 9 · known affected
Vendor assessment for this product only.
pkg:rpm/redhat/unbound
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
unbound-libs as a component of Red Hat Enterprise Linux 9 · known affected
Vendor assessment for this product only.
pkg:rpm/redhat/unbound-libs
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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 prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
openshift/ose-rhel-coreos-8 as a component of Red Hat OpenShift Container Platform 4 · known affected
Vendor assessment for this product only.
pkg:oci/ose-rhel-coreos-8?repository_url=registry.redhat.io/openshift/ose-rhel-coreos-8
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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/ose-rhel-coreos-9?repository_url=registry.redhat.io/openshift/ose-rhel-coreos-9
- workaround: To prevent exploitation, disable DNS-over-QUIC (DoQ) functionality within Unbound if it is not actively used. This typically involves adjusting Unbound's configuration to remove or comment out DoQ-related listening directives. After modifying the configuration, restart the Unbound service for the changes to take effect.
- none available: Fix deferred
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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-devel 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-devel
Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound.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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound 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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound.src 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?arch=src
Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00python3-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound 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
Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound-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-2026-55991 · Updated 2026-09-01T12:38:29+00:00unbound.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-2026-55991 · Updated 2026-09-01T12:38:29+00:00
Known Affected Software Configurations
| Vendor | Product | Version range |
|---|---|---|
| Nlnetlabs | Unbound | ≥ 1.22.0 and < 1.25.2 |
All CVSS metrics
- MEDIUM 5.9 v3.1 · CNA Primary
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H - MEDIUM 5.9 v3.1 · NVD Secondary
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
References
Advisories
- https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt CVE record · NLnet Labs · Updated 2026-09-16T14:24:57.805Z
NVD · CVE-2026-55991 · Updated 2026-10-24T13:10:00.147
Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00 - https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-55991.json Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
- https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-47683 EUVD · EUVD-2026-47683
Technical & other
- https://access.redhat.com/security/cve/CVE-2026-55991 Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
- https://bugzilla.redhat.com/show_bug.cgi?id=2506147 Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
- https://www.cve.org/CVERecord?id=CVE-2026-55991 Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
- https://nvd.nist.gov/vuln/detail/CVE-2026-55991 Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
- https://access.redhat.com/errata/RHSA-2026:43588 Red Hat · CVE-2026-55991 · Updated 2026-09-01T12:38:29+00:00
Sources: NVD · CVE.org · All sources and attribution · EPSS