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Fencer Intel
medium 5.9

CVE-2026-55991 json

Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2

published 3mo ago modified 18d ago also EUVD-2026-47683

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 30%
public exploits
0
none known

Summary

In 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.

Patch / fix

  • NLnet Labs Unbound 1.25.2
  • unbound Debian 13 1.26.1-0+deb13u1
  • unbound Debian 14 1.25.2-1

debian advisories DEBIAN-CVE-2026-55991

  • unbound hummingbird 1 1.25.2-0.1.hum1
  • unbound-libs hummingbird 1 1.25.2-0.1.hum1
  • unbound-munin hummingbird 1 1.25.2-0.1.hum1
  • unbound-utils hummingbird 1 1.25.2-0.1.hum1
  • unbound-anchor hummingbird 1 1.25.2-0.1.hum1
  • unbound-dracut hummingbird 1 1.25.2-0.1.hum1
  • python3-unbound hummingbird 1 1.25.2-0.1.hum1

red hat advisories RHSA-2026:43588

  • unbound Linux Micro 6.0 1.25.2-1.1
  • unbound Linux Micro 6.1 1.25.2-slfo.1.1_1.1
  • unbound SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.25.2-160000.1.1

suse advisories SUSE-SU-2026:23360-1SUSE-SU-2026:23349-1SUSE-SU-2026:23050-1SUSE-SU-2026:23215-1SUSE-SU-2026:23226-1

  • unbound openSUSE Leap 16.0 1.25.2-160000.1.1
  • unbound openSUSE Tumbleweed 1.25.2-1.1

opensuse advisories openSUSE-SU-2026:21550-1openSUSE-SU-2026:11380-1

  • unbound Azure Linux 3.0 1.25.2-1

azure linux advisories AZL-92976

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-55991 updated cvss 5.9

unbound: Unbound: Denial of Service via crafted DNS-over-QUIC connection

  • Red Hat Enterprise Linux 9 unbound, unbound-libs, unbound-dracut, python3-unbound Red Hat Enterprise Linux 10 unbound Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9 Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images unbound 1.25.2-0.1.hum1, unbound-libs 1.25.2-0.1.hum1, unbound-munin 1.25.2-0.1.hum1, unbound-utils 1.25.2-0.1.hum1, unbound-anchor 1.25.2-0.1.hum1, unbound-dracut 1.25.2-0.1.hum1, +1 more
    fixed: vendor fix RHSA-2026:43588
  • Red Hat Enterprise Linux 6 unbound, unbound-libs, unbound-python Red Hat Enterprise Linux 7 unbound, unbound-libs, unbound-python Red Hat Enterprise Linux 8 unbound, unbound-libs, python3-unbound
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
3mo ago

Exploit prediction

0.37% chance of exploitation in the next 30 days, higher than 30% of all scored CVEs (FIRST EPSS, 2026-11-11).

CVSS scores

assigned by version score vector
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

  • NLnet Labs Unbound

Classification

Weakness
  • CWE-195 Signed to Unsigned Conversion Error

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.