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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-55991
StatePUBLISHED
AssignerNLnet Labs
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-22 14:17:21 UTC
Updated2026-07-22 20:33:11 UTC
DescriptionIn 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.

Risk And Classification

Primary CVSS: v3.1 5.9 MEDIUM from [email protected]

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

Problem Types: CWE-195 | CWE-195 CWE-195: Signed to Unsigned Conversion Error

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.9MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
3.1CNACVSS5.9MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA NLnet Labs Unbound affected 1.22.0 1.25.2 semver Not specified

References

ReferenceSourceLinkTags
www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt [email protected] www.nlnetlabs.nl
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: Qifan Zhang (Palo Alto Networks) (en)

CNA: Xuanchao Xie (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-05-27T00:00:00.000ZIssue reported by Qifan Zhang
CNA2026-06-15T00:00:00.000ZNLnet Labs shares patch with Qifan Zhang
CNA2026-06-17T00:00:00.000ZQifan Zhang verifies patch
CNA2026-07-11T00:00:00.000ZIssue reported by Xuanchao Xie
CNA2026-07-12T00:00:00.000ZNLnet Labs shares patch with Xuanchao Xie
CNA2026-06-13T00:00:00.000ZXuanchao Xie verifies patch
CNA2026-07-22T00:00:00.000ZFixes released with version 1.25.2

Solutions

CNA: This issue is fixed starting with version 1.25.2

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